V. Temperatures are measured with thermometers, RTD's, or thermistors. ...... Simple fixtures may be used as guides during the removal or insertion of a ..... control. Dirty sensors or sensors that are out of calibration may lead to faulty readings,.
EFFICIENCY IMPROVEMENT OF COOLING TOWERS Project report By Mr. Abhishek Ramdev Sharma
(P.R.No.200908062)
Mr. Chodankar Abhijeet Deepak
(P.R.No.200908057)
Mr. Sarvesh Kadam
(P.R.No.200908095)
Under the guidance of Flt.Lt. B.R.Kulkarni (Goa College of Engineering)
Department of Mechanical Engineering Goa College of Engineering, Farmagudi. GOA- 403401
Project Approval Sheet
The project entitled “EFFICIENCY IMPROVEMENT OF COOLING TOWER"
By Mr. Abhishek Ramdev Sharma
(P.R.No.200908062)
Mr. Chodankar Abhijeet Deepak
(P.R.No.200908057)
Mr. Sarvesh Kadam
(P.R.No.200908095)
completed in the year 2012-2013 is approved as a partial fulfillment of the requirements for the degree of BACHELOR OF ENGINEERING in Mechanical Engineering and is a record of bonafide work carried outsuccessfully under our guidance.
________________
Flt.Lt. B.R.Kulkarni Project Guide
________________
Dr. Rajesh Prabhu Gaonkar (H.O.D. Mechanical Dept.)
(Goa College of Engineering) _______________ Principal Dr. R.B. Lohani Goa College of Engineering Date: ________________ Place: ________________
CERTIFICATE
This is to certify that the following students has been admitted to the candidacy of degree B.E. (Mechanical) in June 2013 and have undertaken the dissertation entitled “Efficiency improvement of cooling tower” which is approved for the degree of B.E.(Mechanical) under Goa University as it is found satisfactory.
Mr. Abhishek Ramdev Sharma
(P.R.No.200908062)
Mr. Chodankar Abhijeet Deepak
(P.R.No.200908057)
Mr. Sarvesh Kadam
(P.R.No.200908095)
________________
Internal Examiner
________________
Flt.Lt. B.R.Kulkarni Project Guide (Goa College of Engineering) Date:________________ Place: ________________
________________
External Examiner
ACKNOWLEDGEMENTS We are pleased to place the project entitled “EFFICIENCY IMPROVEMENT OF COOLING TOWERS" in the hands of the examiner. Apart from the efforts of ourgroup we take this opportunity to express our gratitude to all the people who have been helpfuland advising us from time to time for the successful completion of this project. We convey our earnest gratitude to our guide, Flt.Lt. B.R.Kulkarnifor his invaluable and tremendous support and help without which this project would not have materialized We also express our gratitude towards the help received from Mr. Felix FurtadoandMr. Anil kenkre for his valuable support and help in our project. We sincerely appreciate the encouragement extended to us byour head of department Rajesh Prabhu Gaonkarand our Principal Dr. R.B.Lohani.
Dr.
We also thank for the help received fromMr. Sulaksh Priolkar, Mrs.Zena Andrade, Mr. Pankaj Naik, Mr. Prem Kumar, Mr.Anthony Fernendes, Mr. Mayur Dessai, Mr. Sooraj Mohan, for his valuable support and help in our project. We sincerely thank Zuari Agro-Chemicals Ltd.for giving us an opportunity to work on this project. We would also like to thank our parents, friends along with the Almighty without whom this work would not have taken shape.
CONTENTS Abstract …………………………………………………………………………………………...i List of figures ……………………………………….……………………………………………ii List of tables ………………………………………...……………………………………………v Chapter 1.Introduction……………………………….…………………………………………1 1.1 General….......................................................................................................................1 1.2 Need of the cooling towers……………………..……………………………………..1 1.3 History of cooling towers ………………………..……………………………………2 Chapter 2.Literature review ……………………………………………………………………6 2.1 General……………………………………………………………………………...…6 2.2 Need of the project ……………………………………………………………………6 2.3 Objective of the project……………………………………………………………..…6 Chapter 3.Company profile …………………………………………………………………….8 Chapter 4.Cooling tower fundaments ………………………………………………………...10 4.1 Nomenclature ………………………………………………………………………..10 4.2 Types of cooling towers...............................................................................................14 4.3 Fluidized cooling tower………………………….………………………………..…23 4.4 Material and construction……………………………………………………………26 4.5 Common misconceptions………………………….…………………………………27 4.6 Structural components.................................................................................................30 4.6.1 General components……………………..……………………………….30 4.6.2 Mechanical components………………….………………………………41 Chapter 5.Factor affecting cooling tower efficiency…………...…………………………..…48 Chapter 6.Design methodology ………………………………..………………………………63 6.1Applications of quality function deployment method and fuzzylogic for improving the design characteristics in FRP cooling tower……………..………………………………63 6.1.1 General…………………………………………………………………………63 6.1.2 Problem description............................................................................................63 6.1.3 Problem solving method………………………………………………….……66 6.1.4 Implementation of QFD and Fuzzy QFD……..…………………………….…69 6.1.5 Results and conclusion…………………………..……………………………..72 Chapter 7.Mathematical Modeling ………………………………..………………………….73 7.1 Merkel theory ……………………………………………..…………………………74 7.2 Poppe Method…………………………………………….……………….…………81
7.3 Universal model for cooling towers………………………..……………..………….83 7.4 Critical investigation of Lewis factor in cooling tower analysis………….…………90 Chapter 8. Analysis of cooling tower in Zuari Agro Chemical Ltd……..……….………….98 8.1 Range and approach data over the year period ………………...……..……………..98 8.2 Water treatment data over a year period……………………………………………106 8.3 Calculation involved in cooling tower………………………….……..……………106 8.4 Energy audit for cooling towers.................................................................................110 8.5 Particulars of no.1 and no.2 cooling towers……………………………...…………112 8.6 Cost data…………………………………………………….………………………116 8.7 Zuari Industries fan data ………………………………………………….……….117 Chapter 9. Case Studies ……………………………………………………………...……….123 9.1 Case study on fibre filters…………………………………….…………….………123 9.2 Case study on natural draft cooling tower………………….……………...……….134 Chapter 10. Maintenance …………………………………………………………….………143 10.1 cooling tower maintenance………………………………………………..………143 10.2 Condition monitoring of cooling tower fan gearboxes……………............………145 10.3 Improving cooling tower fan system efficiency ………………………....………150 Chapter 11. Buying and replacing of cooling tower……………………………….……….156 11.1 Buying and replacing of cooling tower……………………………………………156 11.2 In situ tower performance testing…………………………………………………160 Chapter 12. Recommendations ………………………………………………………………165 12.1 LG SONIC………………………………………………………………...………165 12.2 BALDOR………………………………………………………………………….171 12.3 MIOX……………………………………………………………………...………183 12.4 Louvers …...………………………………………………………………………189 12.5 Lakos Filtration……………………………………………………………………191 12.6 Water conservation ……………………………………………………….………194 12.7 Tips for thermal energy conservation…………………………………….……….205 Chapter 13. Conclusion and Future scope ………………………………………….……….206 References ………………………………………………………………………..……………207 Appendix…………………………………………………………………………...………….208
ABSTRACT The project aims at improving the efficiency of cooling towers in Zuari industries. Proper operating and maintenance procedures are suggested. Recent trends and technologies are used to bring in an improvement in efficiency of cooling towers. QFD and fuzzy logic design methodology will be carried out for improving efficiency of FRP cooling towers. Scientific study on fibre filters will be of utmost importance for water conservation purpose. Cooling tower is going to be an eco-friendly equipment. Energy audit and thermal analysis of cooling tower is carried out for energy savings and monetary benefits to the industry. The payback analysis of the products suggested will be provided for future benefits.
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List of Figures Sr No. 1.1 1.2 1.3 1.4 1.5 1.6 1.7 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23 4.24 4.25 4.26 4.27 4.28 4.29 4.30 4.31 4.32 5.1 5.2
Name of the figure Cooling water system Cooling tower system Early cooling tower ACME cooling tower Details of ACME cooling trays An improved cooling tower The Worthington cooling tower Atmospheric towers Crossflow natural draft towers Crossflow natural draft towers Forced draft cooling towers Counterflow type design Crossflow type design Hybrid draft tower Induced draft counterflow tower Double flow cross flow tower Single flow tower Spray filled counterflow cooling tower Small factory assembled tower Multicell factory assembled tower Round mechanical draft crossflow cooling tower Octagonal vertical draft counterflow cooling tower Fluidized cooling tower Stages of fluidized cooling tower system Graph of bed pressure drop vs. superficial gas velocity picture of prototype FBCT in fluidized operation Different type of basins Typical cross section of concrete sump pit Frame work and joint details of cooling tower Seven cell tower with individual riser Single header cooling tower Electric probe FRP fan cylinder Splash type fill wood splash bar Splash type fill plastic splash bar Type of drift eliminators Type of louvers Grommet type drive shaft coupling Disc type drive shaft coupling interference Recirculation ii
5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 6.1 6.2 6.3 6.4 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 8.1 8.2 8.3 8.4 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 9.15 9.16 9.17 9.18
Effect of plume and discharge velocity Corrosion affected cell fouling Microbiological activity Graph of filtration Graph of softening Graph of chemical residual Desalination of plant using reverse osmosis technology Four phases of QFD Design methodology flow chart for QFD Graph for understanding customer Proposed structure of QFD Graph for counter flow cooling diagram Graph for cross flow cooling diagram Control volume of counterflow fill Air side control volume of fill Control volume of the fill representation of heat and mass transfer in the cooling tower Graph of saturation air enthalpy vs. temperature Effect of Lewis factor shown in graph Effect of Lewis factor shown in graph Effect of Lewis factor shown in graph Effect of Lewis factor shown in graph Graph of energy variation over a day for cooling tower1 Graph of energy variation over a day for cooling tower4 Graph of energy variation over a year for cooling tower1 Graph of energy variation over a year for cooling tower4 Satellite Plant Cooling Towers Warm water spray inside the central plant cooling tower Graph of vapour recovery experiment Water statics graph VICKS Ultrasonic Humidifier fibre filter Pleated air filter Single walled pleated air filter Multiple walled pleated air filter Graph for Vapour absorption versus time of fibre filter Graph for Vapour absorption versus time of fibre filters Graph for Vapour absorption versus time of fibre filters Graph for Vapour absorption versus time of fibre filters Graph for Vapour absorption versus time of fibre filters Schematic of wind tunnel and test section Graph of turbulence intensity Model for experiment Schematic of equipments used in the test iii
9.19 9.20 9.21 9.22 10.1 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10 12.11 12.12 12.13 12.14 12.15 12.16
Graph for outer pressure distribution Graph for inner pressure distribution Graph for Mean pressure distribution curve Graph for outer pressure distribution gear with the damaged tooth Baldor’s Direct Drive motor Graph of input power vs. rpm Graph of efficiency vs. load Cell 1 in original configuration PM motor installed in place of the gearbox in Cell 2 Graph of input power vs. speed Graph of motor starting performance Graph of motor speed variation Oxidative power of key oxidizers Pressure drop vs. velocity Basin cleaning protection devive Side stream protection device Full stream protection device Heat exchanger protection device Heat pump protection device Graph for water use and COC
iv
List of tables Sr No. 6.1 6.2 9.1 9.2 11.1 11.2 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10
Table content Questionnaires table for QFD Technical description table Amount of water emitted by the ultrasonic humidifier over different periods of time Internal mean pressure coefficient Estimated economic life common considerations for a standard tower installation Biocide used in Zuari industries ltd. Power consumption comparison Power consumption comparison Power consumption comparison Annual energy savings Sound pressure data Failure data Louvers specification table Overflow table Audit table
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CHAPTER 1 COOLING TOWERS Cooling towersare heat removal devices used to transfer process waste heat to the atmosphere. The cooling effect is achieved partly by an exchange of latent heat resulting from the evaporation of circulating water and partly by the transfer of sensible heat. The primary task of a cooling tower is to reject heat into the atmosphere. They represent a relatively inexpensive and dependable means of removing low-grade heat from cooling water.
Need for cooling tower Cooling towers form an integral part of power plant operations. They are used to remove excess heat that is generated in places such as power stations, chemical plants and even domestically in air conditioning units. In power stations, electricity is generated when steam drives a turbine. This steammust be condensed before it can be returned to the boiler to continue the cycle ofsteam and electricity generation. The condensation process happens in a heatexchanger. Cooling water is needed in the heat exchanger and it is this cooling water that iscycled through the cooling tower. In this way the water for the boilers and steamturbine is kept separate from the cooling water. This stops impurities getting into theturbine steam. In chemical processes excess heat can be generated. This heat is removed usingheat exchangers and cooling water which is cycled through a cooling tower.
Cooling tower removes the waste heat by principle of sensible heat transfer and evaporation. Therefore the water is circulated in a cycle and reused. Sources like rivers, lakes, ponds are not used again and again hence the ecological balance is maintained.
History of cooling towers Cooling towers originated out of the development in the 19th century of condensers for use with the steam engine. Condensers use relatively cool water, via various means, to condense the steam coming out of the pistons or turbines. This reduces the back pressure, which in turn reduces the steam consumption, and thus the fuel consumption, while at the same timeincreasing power and recycling boiler-water. However the condensers require an amplesupply of cooling water, without which they are impractical—the cost of the water exceeds the savings on fuel. While this was not an issue with marine engines, it formed a significant limitation for many land-based systems. By the turn of the 20th century, several evaporative methods of recycling cooling water were in use in areas without a suitable water supply, such as urban locations relying on municipal water mains. In areas with available land, the systems took the form of cooling ponds; in areas with limited land, such as in cities, it took the form of cooling towers. These early towers were positioned either on the rooftops of buildings or as free-standing structures, supplied with air by fans or relying on natural airflow.An American engineering textbook from 1911 described one design as "a circular or rectangular shell of light plate — in effect, a chimney stack much shortened vertically (20 to 40 ft. high) and very much enlarged laterally. At the top is a set of distributing troughs, to which the water from the condenser must be pumped; from these it trickles down over "mats" made of wooden slats or woven wire screens, which fill the space within the tower." A hyperboloid cooling tower was patented by the Dutch engineers Frederik vanIterson and Gerard Kuypers in 1918. The first hyperboloid cooling towers were built in
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1918 near Heerlen. The first ones in the United Kingdom were built prior to 1930 in Liverpool, England to cool water used at a coal-fired electrical power station.
The first mention of cooling towers contains ten articles from sources in Canada, Germany, and the United States, and names four manufacturers in the United States and two inGermany. Cooling tower was not something new but the result of applying former practices on a large scale to satisfy a suddenly emerging need. Bibbins considers cooling surface as a major point in cooling tower design, but this excluded drop surfaces and refers only to the wetted surface over which water flows. He says: “The development of efficient cooling surface has been gradual, beginning with the employment of no surface at all, viz. a series of fine sprays directed into o settling pond of large area. In mining districts a series of inclined baffles of rough sawed lumber is commonly employed or simply a series of brush heaps, piled upon the several platforms of a rough wooden tower, the prevailing winds being depended upon to effect the required evaporation.” Obert describes what he calls a make-shift cooling tower as follows: “A New Jersey electric light plant that runs engines condensing had been dependant for its condensing water upon a small pond of water covering about two acres, but after an increase in plant it was found that the water in the water in the ponddid not cool off fast enough to enable the necessary vaccum in the condensers to be maintained. A rough inexpensive tower with several platforms was erected in the middle of the pond and the air pump discharges 3
were piped upward to a point several feet above the top platform. The splashing of the water as it discharged and runs down over the platforms, a distance of about 40ft, causes sufficient evaporation to cool the water down about 19.4oC.” The Acme cooling tower, built by the Acme Cooling Tower Mfg. Co., Weehawken, N.J. is a commercially built platform tower.
ACME COOLING TOWER
Obert describes it as employing: “for its surface-exposing action a simple gravitational spray or sprinkler effect. The platform consists of a series of shallow pans arranged one above the other. Rows of perforations disposed symmetrically over the bottoms of the pans resemble large sprinklers. To prevent the water from flowing from the perforations in solid streams, deflecting plates are placed upon the bottom of the pans one under each row. Fine wire netting is wrapped around the tower frame so as to enclose the water course.
4
Worthington Tower Details: The cylindrical casing of forced draft tower may be steel or masonry. The filling consists of tile pipes set on end in tiers with sheets of wire netting interposed between successive tiers. Galvanized pipes are substituted for roof installations. The return water is pumped to a rotary distributor, consisting of radial pipes and functioning much like a lawn sprinkler, the reaction of the discharge jets causing the rotation. The “fanless” design has the casing extended as a chimney to provide natural draft.
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CHAPTER 2 LITERATURE REVIEW 2.1 General Most industrial production processes need cooling waters to operate efficiently and safely. Refineries, steel mills, petrochemical manufacturing plants, electric utilities and paper mills all rely heavily on equipment or processes that require efficient temperature control. Cooling water systems control these temperatures by transferring heat from hot process fluids into cooling water. As this happens, the cooling water itself gets hot. Before it can be used again, it must either be cooled or replaced by a fresh supply of cool water. Therefore literature review needs to be carried out to improve the cooling tower efficiency.
2.2 Need for project Cooling tower is considered as a simple equipment to cool water in an industry. As water is becoming costly and industry has to pay a huge sum, so we would like to recommend the company water conservation techniques. The main aim of the project is to provide monetary benefits to Zuari Industries and to improve the efficiency of their cooling towers. It also targets at bringing about an improvement in operational and maintenance procedures. The latest technologies related to cooling towers are going to be suggested. Cooling tower thermal and energy analysis is done to show the deviation from the design values.
2.3 Objective of project The project aim is to improve the efficiency of cooling towers in Zuari industries. We will study the fundamentals of cooling towers in detail. It includes nomenclature, classification of cooling towers, material and construction of cooling towers and the structural components used in it. The structural components include general, mechanical and electrical components which are studied in detail. The new concept of fluidised bed cooling tower is introduced. Common misconceptions about cooling tower are going to be explained. We are going to note down the factors affecting cooling towers. These factors are classified as controllable and uncontrollable. Their causes and effects are studied. QFD and fuzzy logic methodology is used to improve the efficiency of FRP cooling towers. Merkel, Poppe and universal mathematical models are used in implementation of cooling tower analysis. Critical investigation of Lewis factor is going to be carried out for heat and mass transfer analysis of cooling towers The actual and design values of performance characteristics are compared and recommendations are made for their improvement. Efficiency of a cooling tower is calculated over a year time and its variation is studied plotted over a graph paper. Energy audit is carried out for energy and monetary savings. Fan performance tests are also carried out for checking the fan system efficiencies. 6
Korean patents have suggested use of fibre filters to improve overall water efficiency significantly in cooling tower. It reduces the evaporation losses. Further research should be carried on to find optimal thickness, area, and orientation of filter – extensive parameter study. Efficient ways to extract condensed vapour from saturated filters and practical application to cooling towers are put forward for implementing in Zuari industries. The effect of wind loads on natural draft cooling towers is also going to be observed and studied. Cooling tower maintenance plays a pivotal role in improving its efficiency. The problems occurring due to old arrangement of cooling tower fans are studied and necessary measures are going to be suggested. The steps involved in buying and replacing cooling towers are also going to be studied. Insitu performance testing is also given importance. BALDOR adjustable speed direct drive cooling tower motor is been suggested for cooling tower fan for improving reliability, reducing maintenance, running quieter and saving energy. LG SONIC is a product initiated by LG to reduce the biocide usage in cold water basin to prevent the algae and microbial growth. MIOX product is been suggested for water treatment purpose at the site itself. Water conservation techniques are given prime importance and energy conservation tips are given for company’s benefit. All products are eco-friendly in nature and won’t harm the environment in any ways.
2.4 Summary Proper operating procedures will be carried out.Recent trends are used to bring in an improvement in efficiency of cooling tower. Scientific study on fibre filters should be of utmost importance for water conservation purpose. Cooling tower is going to be eco-friendly equipment henceforth.
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CHAPTER 3 COMPANY PROFILE ZUARI AGRO CHEMICALS LIMITED Promoted in 1967 in Goa, by pioneering industrialist Dr KK Birla, Zuari Agro Chemicals Limited is one of the leading fertilizer conglomerates in India. The company is also a significant importer of fertilizers and farm nutrients. The company produces high-quality complex fertilizers of various grades along with seeds, pesticides, micro nutrients, and specialty fertilizers. Zuari fertilizer plant has an annual installed capacity of 946000 metric tonnes of fertilizer. The entire manufacturing facility comprises of four separate plants, namely Ammonia, Urea, NPK A and NPK B. The plants employ the latest in pipe-reactor technology and are based on the slurry granulation process. Zuari Agro Chemicals Limited continues to nourish its core fertilizer manufacturing operations, with extensive renewal, revamping and expansion programme at its plants. The company has a manufacturing facility at Goa, with four plants, dedicated to provide farmers with urea, DAP and NPK based fertilizers. All products are marketed under the "Jai Kissan" brand. Zuari has marketing offices spread over a wide area covering Goa, Maharashtra, Andhra Pradesh, Karnataka and Tamil Nadu. Zuari has over 2,000 dealers and 5,000 sub dealers that market various brands of fertilizer and other agri-inputs. In 2010-11, Zuari Agrochemicals recorded the highest ever sales of 2.21 million metric tonnes of fertilizer of which over one million metric tonnes were traded fertilizers. In the last three years, traded fertilizers have not only boosted volumes in the market, but also given the company’s profitability a big fillip. Zuari Agro Chemicals Limited announced on 19th December, 2011 that it has formed a Joint Venture Company, MCA Phosphates Pte Ltd, with Mitsubishi Corporation, Japan for the for the purpose of investments in rock phosphate assets. Since then MCA Phosphates Pte Ltd has acquired 30% equity stake in Fosfatos del Pacifico (Fospac), of Peru for a consideration of USD 46.12 million. Fospac is a subsidiary of Cementos Pacasmayo S.A.A (Pacasmayo), a company listed on the Lima Stock Exchange. Additionally, the Zuari- Mitsubishi combine has entered into an Off-Take Agreement with Fospac, to purchase the entire production of concentrated rock phosphate after meeting local demand, if any, for a minimum term of 20 years. The company produces high-quality complex fertilizers of various grades along with seeds, pesticides, micro nutrients, and specialty fertilizers. All products are marketed under the "Jai Kisaan" brand, a household name among farmers. The Zuari fertiliser plant is a landmark in large-scale industry development in Goa. At the time of installation, the plant was the largest industrial undertaking in Goa, acting as a catalyst for economic growth in the neighbouring areas. 8
The plant was started as a financial and technical collaboration between the House of Birlas, US Steel Corporation (USX), International Finance Corporation, and the Bank of America. The design, engineering, and construction of the plant were carried out by Toyo Engineering Japan. The plant has an annual installed capacity of 9,46,000 metric tonnes of fertilisers.The entire manufacturing facility comprises four separate plants, namely Ammonia, Urea, NPK A and NPK B. The plants employ the latest in pipe-reactor technology and are based on the slurry granulation process. Zuari is rapidly transforming the core fertiliser operations to become an integrated agricultural services provider. In this new and improved avatar, Zuari is going beyond conventional manufacturing. Packaging its services for all farmer requirements, Zuari is making huge volumes of nutrient-rich fertilisers available in the market place through securing assets and long- term sourcing arrangements for traded fertilisers internationally, to become one of the country’s largest importers of fertilisers. In the last three years, traded fertilisers have not only boosted volumes in the market, but also given the company’s profitability a big fillip. India’s economy is heavily reliant on its agricultural output. Naturally, the demand for effective and affordable fertilisers is enormous. Zuari has marketing offices spread over a wide area covering Goa, Maharashtra, Karnataka, Andhra Pradesh and Tamil Nadu. Our marketing head-office is based at Zuarinagar, Goa. Zuari has over 2,000 dealers and 5,000 sub-dealers that market various brands of fertilisers and other agri-inputs. Zuari seeks to facilitate farming and farmers, guiding them through innovative, scientific agricultural practices that help in improving crop yields. The company has set up agricultural labs and toll-free help-lines, conducted training programmes and pioneered demonstrations for the farmer. An educated farmer is the first step towards bettering the quality of rural life. We have even adopted villages as part of the rural action scheme, ensuring that the community benefits, as a whole. Some of the welfare programmes that Zuari has conducted in the past include projects for rain-harvesting, bore-well recharging and crop specific projects catering to coconut, sugarcane, paddy and cashew.
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CHAPTER 4 COOLING TOWER FUNDAMENTALS NOMENCLATURE Air Horsepower: The power output developed by a fan in moving a given air rate against a given resistance. Air Inlet: Opening in a cooling tower through which air enters. Sometimes it is referred to as the louvered face on induced draft towers. Air Rate: It is the mass flow of dry air per square foot of cross sectional area in the tower's heat transfer region per hour. Air travel: Distance which air travels in its passage through the fill. It is measured vertically on counter-flow towers and horizontally on cross-flow towers. Air velocity:Velocity of air-vapour mixture through a specific region of the tower (i.e. the fan). Ambient Wet-Bulb Temperature: The wet-bulb temperature of the air encompassing a cooling tower, not including any temperature contribution by the tower itself. Generally it measured upwind of a tower in a number of locations sufficient to account for all extraneous sources of heat. Approach: Difference between the cold water temperature and either the ambient or enteringwet-bulb temperature. Atmospheric: refers to the movement of air through a cooling tower purely by natural means, or by the as pirating effect of water flow. Automatic Variable-Pitch Fan: A propeller type fan whose hub incorporates a mechanismwhich enables the fan blades to be re-pitched simultaneously and automatically. They are used on cooling towers and air-cooled heat exchangers to trim capacity and/or conserve energy. Basin: See "Collection Basin" and "Distribution Basin". Basin Curb: Top level of the cold water basin retaining wall; usually the datum from whichpumping head and various elevations of the tower are measured. Bleed off: See "Blow-down". Blowdown: Water discharged from the system to control concentrations of salts or other impurities in the circulating water. Blower: A squirrel-cage (centrifugal) type fan; usually applied for operation at higher than 10
normal static pressures. Blowout: See "Windage". Brake Horsepower: The actual power output of a motor, turbine, or engine. Btu (British thermal unit): The amount of heat gain (or loss) required to raise (or lower) the temperature of one pound of water 10 F. Capacity: The amount of water that a cooling tower will cool through a specified range, at a specified approach and wet-bulb temperature. Casing:Exterior enclosing wall of a tower, exclusive of the louvers. Cell: Smallest tower sub-division which can function as an independent unit with regard to air and water flow; it is bounded by either exterior walls or partition walls. Each cell may have one or more fans and one or more distribution systems. Circulating water rate: It is the quantity of hot water entering the cooling tower. Coldwater temperature: Temperature of the water leaving the collection basin, exclusiveof any temperature effects incurred by the addition of make-up and/or the removal of blow-down. CollectionBasin: Vessel below and integral with the tower transientlycollected and directed to the sump or pump suction line.
where
water
is
Counter-flow: Air flow direction through the fill is counter current to that of the falling water. Cross-flow: Air flow direction through the fill is essentially perpendicular to that of the falling water distribution. Basin: Shallow pan-type elevated basin used to distribute hot water over the tower fill by means of orifices in the basin floor. Application is normally limited to cross-flow towers. Distribution system: Those parts of a tower beginning with the inlet connection which distribute the hot circulating water within the tower to the points where it contacts the air for effective cooling. Many include headers, laterals, branch arms, nozzles, distribution basins, and flow-regulating devices. Drift: Circulating water lost from the tower as liquid droplets entrained in the exhaust air stream. Drift Eliminators: An assembly of baffles or labyrinth passages through which the air passes prior to its exit from the tower, for the purpose of removing entrained water droplets from the exhaust air. Dry-bulb temperature: The temperature of the entering or ambient air adjacent to the 11
cooling tower as measured with a dry-bulb thermometer. Entering wet-bulb temperature: The wet-bulb temperature of the air actually enteringthe tower, including any effects of recirculation. In testing, the average of multiple readings taken at the air inlets is calculated to establish a true entering wet-bulb temperature. Evaporation Loss: Water evaporated from the circulating water into the air stream in the cooling process. Exhaust (Exit) wet-bulb temperature: See "leaving wet-bulb temperature". Fan Deck: It is the surface enclosing the top structure of an induced draft cooling tower, exclusive of the distribution basins on a cross-flow tower. Fan Pitch: The angle which the blades of a propeller fan make with the plane of rotation, measured at a prescribed point on each blade. Fill: It is that portion of a cooling tower which constitutes its primary heat transfer surface and sometimes referred to as "packing". Forced draft: It refers to the movement of air under pressure through a cooling tower.Fans of forced draft towers are located at the air inlets to "force" air through the tower. Heat load: Total heat to be removed from the circulating water by the cooling tower per unit time. Height:On cooling towers erected over a concrete basin, height is measured from the elevation of the basin curb. "Nominal" heights are usually measured to the fan deck elevation, not including the height of the fan cylinder. Heights for towers on which a wood, steel, or plastic basin is included within the manufacturer's scope of supply are generally measured from the lowermost point of the basin, and are usually overall of the tower. Hot water temperature: Temperature of circulating water entering the cooling tower's distribution system. Induced draft: refers to the movement of air through a cooling tower by means of an induced partial vacuum. Fans of induced draft towers are located at the air discharges to "draw" air through the tower. Inlet wet-bulb temperature: See "Entering Wet-Bulb Temperature". Leaving wet-bulb temperature: Wet-bulb temperature of the air discharged from a cooling tower. Length: For cross-flow towers, length is always perpendicular to the direction of air flow through the fill (air travel), or from casing to casing. For counter-flow towers, length is always parallel to the long dimension of a multi-cell tower, and parallel to the intended direction of cellular extension on single-cell towers. Liquid-to-Gas Ratio: A ratio of the total mass flow of water and dry air in a cooling tower. 12
Louvers: Blade or passage type assemblies installed at the air inlet face of a cooling tower to control water splash out and/or promote uniform air flow through the fill. In the case of film-type cross-flow fill, they may be integrally moulded to the fill sheets. Make-up: Water added to the circulating water system to replace water lost by evaporation, drift, wind- age, blow-down, and leakage. Mechanical draft: refers to the movement of air through a cooling tower by means of a fan or other mechanical device. Module - A pre-assembled portion or section of a cooling tower cell. On larger factory assembled towers, two or more shipped modules may require joining to make a cell. Natural draft: refers to the movement of air through a cooling tower purely by natural means typically by the driving force of a density differential. Net effective volume: It is that portion of the total structural volume within which the circulating water is in intimate contact with the flowing air. Nozzle: A device used for controlled distribution of water in a cooling tower. Nozzles are designed to deliver water in a spray pattern either by pressure or by gravity flow. Partition: An interior wall sub-dividing the tower into cells or into separate fan plenum chambers. Partitions may also be selectively installed to reduce windage water loss. Psychrometer: An instrument incorporating both a dry-bulb and a wet-bulb thermometer,by which simultaneous dry-bulb and wet-bulb temperature readings can be taken. Range: It is the difference between the hot water temperature and the cold water temperature Recirculation: describes a condition in which a portion of the tower's discharge air re-enters the air inlets along with the fresh air. Its effect is an elevation of the average entering wetbulb temperature compared to the ambient. Riser: Piping which connects the circulating water supply line, from the level of the base of the tower or the supply header, to the tower's distribution system. Shell: The chimney-like structure, usually hyperbolic in cross-section, utilized to induce air flow through a natural draft tower, sometimes referred to as a "stack" or "veil". Speed reducer: A mechanical device, incorporated between the driver and the fan of a mechanical draft tower, designed to reduce the speed of the driver to an optimum speed for the fan. The use of geared reduction units predominates in the cooling tower industry, although smaller towers will utilize differential pulleys and V-belts for the transmission of relatively low power. Splash bar: One of a succession of equally-spaced horizontal bars comprising the splash surface of a fill deck in a splash-filled cooling tower. Splash bars may be flat, or may be 13
formed into a shaped cross- section for improved structural rigidity and/or improved heat transfer capability. When flat, they are sometimes referred to as "slats" or "lath". Splash-Filled: descriptive of a cooling tower in which splash-type fill is used for the primary heat transfer surface. Spray-filled: descriptive of a cooling tower which has no fill with water-to-air contactdepending entirely upon the water break-up and pattern afforded by pressure spray nozzles. Sump: A depressed chamber either below or along-side (but contiguous to) the collectionbasin, into which the water flows to facilitate pump suction. Sumps may also be designed as collection points for silt and sludge to aid in cleaning. Total air rate: Total mass flow of dry air per hour through the tower. Total water rate: Total mass flow of water per hour through the tower. Tower pumping head: The static lift from the elevation of the basin curb to the centre lineelevation of the distribution system inlet; plus the total pressure (converted to feet of water) necessary at that point to effect proper distribution of the water to its point of contact with the air. Water rate: Mass flow of water per square foot of fill plan area of the cooling tower per hour. Wet-bulb temperature: The temperature of the entering or ambient air adjacent to the cooling tower as measured with a wet-bulb thermometer. Wet-bulb thermometer: A thermometer whose bulb is encased within a wetted wick. Windage: Water lost from the tower because of the effects of wind, sometimes called "blowout". Wind load: The load imposed upon a structure by a wind blowing against its surface.
TYPES OF COOLING TOWERS Cooling towers are designed and manufacturedin several types,withnumeroussizes(models)available in each type. Not all types are suitable for application to every heat load configuration. Understanding the various types, along with theiradvantages and limitations,can be of vital importance to the user. 1.Atmospheric towers utilize no mechanical device (fan) to create air flow through the tower. The small atmospheric tower depicted in Figure derives its air flow from the natural induction (aspiration) provided by a pressure-spray type water distribution system. Althoughrelatively inexpensive, they are usually applied only in very small sizes, and are far
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more affected by adverse wind conditions than are other types. Their use on processes requiring accurate, dependable cold water temperatures is not recommended.
Conversely, the atmospheric type known as the hyperbolic natural draft tower is extremely dependable and predictable in its thermal performance. Air flow through this tower is produced by the density differential that exists between the heated (less dense) air inside the stack and the relatively cool (more dense) ambient air outside the tower. Typically, these towers tend to be quite large (250,000gpm and greater), and occasionally in excess of 500 feet in height. Their name, of course, derives from the geometric shape of the shell. Although hyperbolic towers are more expensive than other normal tower types, they are used extensively in the field of electric power generation, where large unified heat loads exist, and where long amortization periods allow sufficient timefor the absence of fan power (and mechanical equipment maintenance costs) to recoup the differential cost of the tower. The synfuels industry also generates heat loads warranting consideration of the use of hyperbolic towers. However, because natural draft towers operate most effectively in areas of higher relative humidity, many such plants located in arid and/or higher altitude regions find mechanical draft towers more applicable.
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2.MECHANICAL DRAFT TOWERS Mechanical draft cooling towers use power driven fan motors to force or draw air through the circulating water. These can be categorized as forced draft (air pushing) or induced draft (draw-through) arrangement by virtue of the location of fan.
Forced draft In forced draft cooling towers, air is "pushed" through the tower from an inlet to an exhaust. A forced draft or mechanical draft tower is a blow-through arrangement, where a blower type fan at the intake forces air through the tower. The forced draft cooling towers have certain disadvantages: 1) The blower forces outside air into the tower creating high entering and low exiting air velocities. The low exiting velocity of warm moisture laden air has the tendency to get re-absorbed by the blower fan. This increases the apparent wet bulb temperature, and the cooling tower ceases to give the desired approach. 2) A Forced draft Cooling Tower can only be square or rectangular shaped. A forced draft arrangement always has a fan on the side. As such, the cooling tower cannot be bottle shaped. Further, due to this characteristic, the water distribution system cannot be that of a sprinkler form. This results in inefficient water distribution. 3) It is difficult to maintain this type of a cooling tower because of the inaccessibility of the fills. The cold water basin is covered and difficult to access. 4) Pressurized upper casing is more susceptible to water leaks than the induced draft styles. 5) A forced draft design typically requires more motor horsepower, typically double that of a comparable induced draft counter-flow cooling tower. 6) With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. The forced draft benefit is its ability to work with high static pressure. They can be installed in more confined spaces and critical layout situations. These can be used for indoor applications and ducted to outside of the building. Induced draft An induced draft mechanical draft tower is a draw-through arrangement, where a fan located at the discharge end pulls air through tower. The fan induces hot moist air out of the discharge end. This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake. When compared to Forced draft cooling towers, induced draft towers have the following advantages: 1) Recirculation tendency is less of a problem. The air that is thrown out from the top of the Cooling Tower has no chance of getting back into the cooling tower. The push of the fan adds to the upward thrust of the warm air. 2) The induced draft can be square as well as round. The distribution system is that of a sprinkler which is considered to be the most efficient water distribution system. 3) Noise level is very low, because the fan and motor are placed on the top of the Cooling Tower. They are not in level with the observer. 16
4) A forced draft Cooling Tower cannot be a cross flow type model. An induced draft can be either cross flow or counter flow. 5) The parts of this type of a cooling tower are easily accessible and there is no problem in their maintenance. Types of Induced Draft Tower (Characterization by Air Flow) Induced draft cooling towers are characterized as Cross-flow and Counter-flow designs, by virtue of air-to-water flow arrangement. The difference lies in the FILL arrangement. Counter-flow Cooling Towers In a counter-flow induced draft cooling towers, air travels vertically across the fill sheet, opposite to the downward motion of the water. Air enters an open area beneath the fill media and is then drawn up vertically. The water is sprayed through pressurized nozzles and flows downward through the fill, opposite to the air flow. Cross-flow Cooling Towers In cross flow induced draft cooling towers, air enters one or more vertical faces of the cooling tower and moves horizontally through the fill material. Water drops by gravity and the air pass through the water flow into an open plenum area. A shallow pan type elevated basin is used to distribute hot water over the tower fill by means of orifices in the basin floor.
The application relies on gravity distribution and is normally limited to cross-flow towers. The surface enclosing the top structure of an induced draft cooling tower, exclusive of the distribution basins on a cross-flow tower, is called the Fan deck.
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Comparative Analysis (Counter-flow v/s Cross-flow) What is Common to both Designs? 1) Both are generally induced flow arrangement although counter-flow design is available in forced flow arrangement too. 2) The interaction of the air and water flow allows a partial equalization and evaporation of water. 3) Both are generally draw-through arrangement where a fan induces hot moist air out the discharge. 4) Both produce low entering and high exiting air velocities, reducing the possibility of recirculation. What is Different in Cross-flow and Counter-flow designs? The comparative analysis is made on the following distinctive parameters: 1. Fill Media Counter-flow cooling towers utilize a plastic film fill heat exchange media that reduces both pump head and horsepower costs; whereas cross-flow towers typically utilize a splash-type heat exchanger. However, it is possible to find either type of exchange media in both types of towers. 2. Space and Size Constraints Counter flow towers are compact and have a smaller footprint, but these tend to be taller than cross flow models resulting in increased pump head, which translates to higher pump energy as well as the requirement for taller architectural screens. Cross Flow Cooling Towers have to be large sized because of the cavity which is to be left between the fan and the fills. 3. Dimensional references For cross-flow towers, length is always perpendicular to the direction of air flow through the fill (air travel), or from casing to casing. For counter-flow towers, length is always parallel to the long dimension of a multi-cell tower, and parallel to the intended direction of cellular extensions on single-cell towers.
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4. Spray Pattern (Water Distribution) Counter flow towers use a pressurized spray system that is considered to be the most efficient method of water distribution in a cooling tower. No sprinkler distribution is possible in a cross flow cooling tower. 5. Operating Weight Counter flow towers have low operating weight and thus find greater acceptability at roof locations. Cross-flow operating weight is higher than the counter-flow tower. 6. Fill Arrangement For counter flow tower, the wet deck (fill media) is encased on all the four sides. Thishelps prevent icing in winter operations. The prevailing winds do not directly affect the fill. The entire working system is guarded from the sun's rays which helps reduce algae growth. Air inlet louvers serve as screens to prevent debris from the entering system. Cross-flow wet deck (fill) is encased on two sides only. The prevailing winds directly affect the fill and have problems of icing in 7. Fill Support In counter flow design, the wet deck (fill) is supported from structural supports underneath. This prevents sagging and creates a working platform on top of the fill for service. In crossflow design, the fill media is generally supported by rods. Icing and wear may deteriorate the fill, making it sag, which may affect performance. 8. Operating Efficiency Counter flow cooling towers are 25% more efficient than cross flow type. The reason being is that as the air is being sucked from the lower part of the cooling tower, it rises upwards, gets warmer and when it reaches the top, it is hottest at that point. Since the water is flowing in the downward direction, it is the hottest at the top. Since the hottest of air meets the hottest of water, evaporation is more, and thus, the cooling is more. In the case of a cross-flow tower, air that passes the water is not capable to pass such waters at different temperatures. Thus the level of cooling is less. 9. Safety Requirements Counter-flow towers are typically taller than other styles but do not require handrails or piping at the top of tower. Cross-flow towers frequently require handrail, safety cage, and service platform per the requirements of OSHA guidelines. It is difficult to service fan drive systems in cross-flow towers as these must have internal and external service platforms and ladders to reach the drive systems. 10.Maintenance Counter-flow towers are easy to maintain at the cold-water basin level because it is open on all sides with no restrictions from wet deck. Cross flow towers are difficult to clean at the cold water basin under wet deck because of limited access. 11. Balancing Requirements Counter-flow does not need balancing valves to even the flow. For cross-flow, open gravity hot water basins require balancing valves to ensure even flow and maximum performance.
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12.Limitations Counter-flow towers require airflow on all four sides for optimum performance. Care must be taken not to lay out more than two (2) towers side by side or middle cells will be difficult to access. Outer cells may have to be shut down to service inner cells. 13.Initial Cost Counter-flow towers are typically expensive to build and have higher initial cost than crossflow towers. 3. Hybrid draft towers can give the outward appearance of being natural draft towers with relatively short stacks. Internal inspection, however, reveals that they are also equipped withmechanical draft fans to augment air flow. Consequently, they are also referred to as fan-assisted natural draft towers. The intent of their design is to minimize the horsepower required for air movement, but to do so with the least possible stack cost impact. Properlydesigned, the fans may need to be operated only during periods of high ambient and peak loads. In localities where a low level discharge of the tower plume may prove to be unacceptable, the elevated discharge of a fan- assisted natural draft tower can become sufficient justification for it use.
5. SPRAY-FILLED TOWERS It has no heat transfer (fill) surface, depending only upon the water break-up afforded by the distribution system to promote maximumwater-to-aircontact. The use of such towers is normally limited to those processes where higher water temperatures are permissible. They are also utilized in those situations where excessive contaminants or solids in the circulating water would jeopardize a normal heat transfer surface. 6.CHARACTERIZATION BY CONSTRUCTION Field-erected towers are those on which the primary construction activity takes place at the site of ultimate use. All large towers, and many of the smaller towers, are prefabricated,piece-marked, and shipped to the site for final assembly. Labour and/or supervision for final assembly is usually provided by the cooling tower manufacturers. Factory-assembledtowers undergo virtually complete assembly at their point of manufacture, whereupon they are shipped to the site in as few sections as the mode of transportation will permit. The relatively small tower indicated in Figure would ship essentially intact. Larger, multi-cell towers are assembled as "cells" or "modules" at the 20
factory, and are shipped with appropriate hardware for ultimate joining by the user. Factoryassembled towers are also known as "packaged" or "unitary" towers.
7. Characterization by shape Rectilinear towers are constructed in cellular fashion increasing linearly to the length and number of cells necessary to accomplish a specified thermal performance.Round Mechanical Draft ("RMD") towers as the name implies, are essentially round in plan configuration with fans clustered as close as practicable around the centre point of the tower. Multi-faceted towers such as the Octagonal Mechanical Draft ("OMD"), also fall into the general classification of "round" towers. Such towers can handle enormous heat loads with considerably less site area impact than that required by multiple rectilinear towers in addition to which, they are significantly less affected by recirculation. 21
8. CHARACTERIZATION BY METHOD OF HEAT TRANSFER All of the cooling towers therefore described are evaporative type towers, in that they derive their primary cooling effect from the evaporation that takes place when air and water are brought into direct contact. At the other end of the spectrum is the dry tower whereby full utilization of dry surface coil sections, no direct contact (and no evaporation) occursbetweenairandwater.Hence the water is cooled totally by sensible heat transfer. In between these extremes are the Plume Abatementand Water Conservationtowers, whereinprogressivelygreater portions of dry surface coil sections are introduced into the overall heat transfer system to alleviate specific problems, or to accomplish specific requirements.
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DESIGN AND CHARACTERIZATION OF FLUIDIZED BED COOLING TOWERS The disadvantages of conventional cooling towers are: (1) Poor heat transfer properties because of the static nature of packings. (2) Conventional towers require a large land area to accommodate their size. (3) Large capital investment is required. (4) Static packing attracts microbes which may cause Legionnaire Disease. (5) Optimization is difficult because of differences in packing arrangements. (6) The enormous height calls for high pumping cost thus increasing the overall operating cost. These disadvantages provide fresh opportunities for new and superior design methods of water cooling to be explored. Fluidized bed cooling towers (FBCT) could replace conventional packed bed towers because of their superior heat and mass transfer characteristics and hence small size for a given cooling effect. The design involved the replacement of the conventional packing arrangement with low density spheres which undergo a process known as fluidization as the air moves upwards and the water is sprayed downwards as shown in Figure.
FBCT is a "novel gas-liquid contacting operation that holds huge potential for large flow volumes transfer". Although evaporative cooling within a fluidization process is relatively new, the fluidisation phenomenon is well understood and a large body of experimental work has been reported.Fluidization is describes as a state where granular particles possess fluidity when an ascending fluid is passed through the bed at an increasing velocity so that at one instance, theforce resisting the flow of fluid is equal to the bed weight after which there is no 23
increase in hydraulic resistance of the bed. This results in the bed expanding and suspending beyond the stability limit of a fixed bed, the stability limit marking the transition point to the fluidised state. At low fluid velocities or flow rates the solid particles lie on one another on a porous plate or retaining grid at bottom of the column. This is the fixed or statte State. If the velocity of the upward flowing fluid is increased still further, fluidisation occurs, bubbles are formed and intensive mixing of the bed is realised with a turbulent action similar to a boiling fluid. This is the fluidised State. Further increase of the fluid velocity, will eventually cause entrainment of the solid particles from the column into the upward moving fluid. The contact and close proximity of the particles to one another ceases as the solid particles become mobile. This is the pneumatic or hydraulic transport State.
Figures show the relationship between the upward gas and the downward liquid throughputs and the behaviour of the whole bed of material. The quantity of material over the porous retaining grid remains constant. Figure (a) corresponds to a fixed bed of particles and pressure drop increases as velocity of the gas increases at a constant liquid flow rate. The gas velocity at which fluidisation begins is called the minimum or incipient fluidisation velocity as shown in the next figure. This is displayed as line MM in Figure. As the gas flow rate increases, the pressure drop over the whole of the bed remains constant and is equal to the total weights of the fluidised solid material and the bed fluids, the height of the bed on the other hand increases as displayed in Figure(c). This is the expansion phenomenon characteristic of fluidised bed and corresponds to the limit of existence of the fluidised bed. At a particular gas flow rate, the liquid droplets together with the solid material are entrained out of the contacting zone. In an ideal case of uniform expansion the bed will cease to exist at this gas velocity. This is the flooding ormaximum fiuidisationstate
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The advantages of the fluidised bed cooling technique are principally derived fromthe very large particle area exposed to the fluid, the ease with which solids canbe handled in the fluidised state and the excellent heat and sometimes masstransfer transport due to the bubbling of the bed. Moreover, the constant agitation ensures self-washing of spherical packing and hence reduction of the possibilityof the build-up of microbes. In addition, the operational cost may be reduced since untreated water may be used. The disadvantages of fluidisation are that operating rates are limited to within the range over which the bed can exist and in addition, the cost of power required to fluidise the bed may be excessive especially with dense and deep beds. Also, there are limits of size and depth of particles that may be handled by the system. Moreover, it is difficult to characterise the particles themselves and there can be a wide range of behaviour in accordance to the conditions under which a particular fluidised bed is being operated. The most outstanding advantage of a fluidised bed cooling tower is its small size for a given cooling duty, due to its high heat transfer rate.
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Cooling Tower Materials Cooling tower structures are constructed using a variety of materials. While package cooling towers are generally constructed with fiberglass, galvanized steel (or stainless steel in special situations), many possibilities exist for field-erected structures. Field-erected towers can be constructed of Douglas fir, redwood, fiberglass, steel or concrete. Each material has advantages and disadvantages. Wood In early days, towers were constructed primarily of Redwood because of its natural tendency to inhibit decay. As the Redwood resources diminished, Douglas-Fir come into existence. Douglas-Fir however supports the growth and proliferation of micro-organisms causing rapid diglinification (eating of wood). Various methods of pressure treatment and incising are used to prevent micro-organisms attack to wood, which includes CCA and ACC treatment. Chromate Copper Arsenate (CCA) was initially used as a preservative but because of its arsenic content, Acid Copper Chromate (ACC) has replaced it. Irrespective of any treatment, the leaching of chemicals is still a concern to the environment and sometimes extensive additional water treatment of blow-down and tower sediment is needed. Some drawbacks of wooden towers are stated below: 1) The wooden structure is less durable and its life expectancy is low. Delignification (eating of wood) is controlled by adjusting the pH strictly between 7 and 7.5 2) The drift losses are over 1%. 3) The tower has a larger footprint and needs more space when compared to other alternatives. 4) Algae formation is a continuous problem in this type of Cooling Tower. 5) The wooden structure is less durable. 6) The wooden tower usually requires a large concrete tank that involves more cost, time and labour. 7) Since this type of Cooling Tower is extremely heavy, it has to be installed on ground only. 8) The nozzles on the wooden tower consume a significant amount of pressure head, which results in pressure drop.
Galvanized Steel The most cost-effective material of construction for packaged towers is G-235 hot dip galvanized steel, from both structural and corrosion resistance standpoint. G-235 is the heaviest galvanizing mill commercially available, and offers a substantial amount of protection as compared to the lighter zinc thicknesses used several decades ago, providing reliable corrosion protection for most HVAC and industrial system water chemistries. The most common upgrade from G-235 galvanized steel is Type 304 stainless steel. Parts that are submerged during operation and/or at shutdown can benefit the most by upgrading to stainless steel.Note that the G-235 designation refers to 2.35 ounces of zinc per square foot (717 g per m2) of the steel sheet.
Stainless Steel Type 304 stainless steel construction is recommended for cooling towers that are to be used in a highly corrosive environment.
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Concrete Towers Larger field erected towers for power plant and refinery applications are constructed of concrete. Concrete towers will last more than 40 years, but they are themost expensive to build. Because of their cost, they represent only 2 to 3% of all field-erected towers. Sometimes concrete construction is also used for architectural reasons (where the tower is disguised to look like or blend in with a building), or the cooling tower is designed as a structure with a life expectancy equal to the facility it serves. Fibre-Reinforced Plastic (FRP) Towers Currently, the fastest growing segment of the cooling tower market is structures built withpultruded FRP sections. This inert inorganic material is strong, lightweight, chemically resistant and able to handle a range of pH values. Fire-retardant FRP can eliminate the cost of a fire protection system, which can equal 5 to 12% of the cost of a cooling tower. Note that for the cooling towers erected over a concrete basin, height is measured from the elevation of the basin curb. "Nominal" heights are usually measured to the fan deck elevation, not including the height of the fan cylinder. Heights for towers, on which a wood, steel, or plastic basin is included within the manufacturer's scope of supply, are generally measured from the lowermost point of the basin.
Common Misconceptions of cooling towers ʺMy tower is cooling the water 20°ʺ Many people have been guilty at one time or another of defininga cooling tower is doing in terms of range (HWT‐CWT). Often, this is accepted as a level of performance of the cooling tower. Nothing could be further from the truth. The equation for heat load is as follows: Heat Load [Btu/hr] = 500 x Water Flow Rate [GPM] x Range [oF] Now, heat load, of course, is supplied by the unit being served by the cooling tower. The tower itself is neither a heat source nor a heat sink. In the usual circulating system the heat ‐load is independent of the cooling tower. The number 500 is a constant, therefore is independent of the cooling tower. The circulating water flow is determined by the number of pumps running and the pressure drop in the overall circulating water system. Therefore, it likewise is independent of the cooling tower. If heat load, the constant, and the circulating water flow are all independent of the cooling tower, then by mathematical deduction the range is likewise completely independent of the cooling tower. Therefore, the range is the same whether there is a two‐cell tower or a four‐cell tower. The range would be the same if the fans were on full‐speed, half‐speed, or turned off. Consequently, such a statement as ʺMy tower is not performing because I bought it to cool 20°, and it is only cooling the water 10°ʺ has no validity whatsoever. Likewise, the converse is true. Someone who has a cooling tower which is ʺcooling the water 30°ʺ whereas it was only designed to cool 20°, may not be in such a fortunate position as he might think. Both these cases show no indication whatsoever of actual thermal capability of the cooling tower. What then is a measure of the thermal capability of the cooling tower? It is not the amount of heat being rejected, rather it is the levelat which this heat is rejected. The measure of performance of thecooling tower is the resultant cold water temperature or even more specifically, the approach (CWT‐WBT) under given conditions. Cold water temperature is the primary dependent variable, and vividly indicatescooling tower capability. 27
Approach Approach is CWT minus WBT. Another misconception that constantly crops up in cooling tower work is concerning approach. One will say, ʺI bought a tower to make a 10° approach, yet I have a 15°approach today. Therefore, the cooling tower is not working in accordance with its design.ʺ As the WBT goes down, the CWT also goes down. However, this is not a one‐to‐one relationship. It is more nearly a two‐to‐one relationship. That is, for each 2°F drop in WBT, the CWT will drop approximately 1°F. Therefore, if the WBT is 10° below design WBT, then by definition the approach will be increased 5° above that specified as design. Of course, the converse is also true: as wet bulb increases above design, then the cold water temperature will increase roughly one degree for each two degree increase in wet bulb. These are rough approximations and can be changed somewhat by different L/G ratios. Therefore, when attempting to determine whether or not performance appears to be satisfactory at other than design conditions, it is wiser to utilize the cooling tower performance curves or the CTIBlue Book [with cooling tower characteristic curves] rather than to just compare the actual approach to the design.
Drift Loss Today, we see more and more interest in low drift loss cooling towers. This increased interest has beenbrought about by several factors. The major factor is continual insistence by the regulatory bodies thatdrift loss be minimized to reduce ecological effects.A second factor is an attempt to reduce water costs and treating costs by reducing water consumption. However, let us now look at the basic concept “reducing drift loss reduces water consumption.” Water is lost out of a cooling tower in the following ways: drift, evaporation, blow-down and windage. Drift loss is a loss of water due to physical entrainment of liquid droplets in the air stream. This places water, with its attendant dissolved solids, in liquid form in the atmosphere. Windage is of a similar nature but usually much smaller in quantity. Windage is the sometimes loss of water through the louvered area of the tower due to wind blowing through the tower. This occurs more often with operation of the fans at half‐speed or off. For the purposes of this discussion, we will consider windage a portion of drift loss. The fact that drift leaves the cooling tower as liquid water, thereby containing its proportionate share of dissolved solids, is the important factor to remember. In order to maintain a proper total dissolved solids content level in a cooling tower, it is necessary to continuously blow down or throwaway a portion of circulating water. This is done to prevent the tower from accumulating excessive amounts of total dissolved solids which would cause severe scale and/or corrosion problems. This loss of water from the system is absolutely essential andcannot be avoided for the successful operation of the cooling tower. Therefore, drift is more properly defined as ʺinvoluntary blow-down.ʺ It now becomes readily apparent that any decrease in the drift loss from the cooling tower will result in an equivalent increase necessary in the blow-down in order to prevent excessive solids buildup. When viewed in this manner, it is evident that lowering drift loss does not reduce water consumption and consequently does not save anything in basic water cost or chemical treating costs. Another common misconception occurs when large clouds of ʺfogʺ emanate from cooling towers under certain atmospheric conditions. Many people assume that drift is a major contributing factor. However, this ʺfogʺ is quite different from drift as this water leaves the system in the vapour state and recondenses to small liquid droplets after encountering the relatively colder ambient air. The fog itself does not carry with it the dissolved solids and consequently does not fit into the category of involuntary blow down. Furthermore, it cannot be stopped by reducing drift loss. Therefore, one should not specify more stringent drift loss requirements to 28
reduce fogging. Energy and money would be expended for absolutely no benefit.The only real justification for reduction of drift loss is based on the nature of the area where the drift falls and its effects on the mechanical equipment of the cooling tower. If the dissolved solids in the drift are being deposited on sensitive areas then it is important to keep drift loss as low as practicable. If a cooling tower is of induced draft design, excessive drift will certainly erode the mechanical equipment. Therefore, one should be certain as to the desired effect to be accomplished prior to making very restrictive and costly requirements in reduction of drift loss.
Evaporation The most common misconception concerning evaporation is that competing tower manufacturers and designs can accomplish widespread differences in the amount of water being evaporated in a cooling tower. The truth of the matter is that the large majority of heat exchange in a cooling tower is accomplished by the evaporation of a portion of the circulating water. This removes heat from the remainder of the circulating water by removing the latent heat of vaporization necessary to accomplish this phase change. Inasmuch as evaporating water is the basic function of the cooling tower, one is mislead to believe that a cooling tower can operate successfully without the proper evaporation. There are a few design features that can have a minor effect on the evaporation rate. For example, cooling towers with identical duties, but operating at different L/G ratios, will have slightly different evaporation rates. The normal ʺrule of thumbʺ for determination of evaporation is 0.1 % 1oF the circulating water flow for every 1.0 °F range.
The ʺMore Waterʺ Syndrome The ʺmore waterʺ fallacy is important and worthy of inclusion here. This situation commonly occurs in an operating cooling tower when, in the heat of summer, the temperature begins to rise on, say, a shellandtube exchanger in a refinery unit. Operation calls for more water in order to hold the temperature of the cooled process stream. Additional water is pumped over the tower, and this will result in a decrease in the towerʹs performance capabilities, thereby raising the CWT. The raising of the CWT going to the exchanger in some cases more than offsets the effect of increased heat transfer coefficient in the exchanger due to increased water flow. Therefore, the situation becomes worse and again ʺmore waterʺ is demanded. It takes a well‐trained operator to recognize the fact that, when trouble is occurring temperature‐wise, less water rather than ʺmore waterʺ is often necessary to bring the temperatures down. Of course, situations can exist where the increase in velocity through an exchanger occasions a better overall heat transfer, even though the temperature of the water through the unit is rising..
Power Considerations Often the effect of fan power on cooling tower thermal capability is misunderstood. For instance, it isquite often thought that a 10% increase in power by increasing fan pitch will accomplish a 10% increase in the capacity of the cooling tower. A quick check of the fan laws will reveal that this is far from the truth. If we are near design fan power and are operating at an essentially constant fan efficiency, air flow rate (and hence, cooling tower capability), will increase in proportion to the cube root of the power increase. For example, if the horsepower were increased to 110% of its original value, the air flow and thermal performance capability of the tower would only be increased approximately 3.2%. 29
STRUCTURALCOMPONENTS A. GENERAL The structure of a cooling tower must accommodate long duration dead loads imposed by the weight of the tower components, circulating water, snow and ice; plus short term loads caused by wind, maintenance and, in some areas, seismic activity.Wide-ranging temperatures must be accepted, as well as the corrosive effects of high humidity and constant oxygenation. That requirement plus the constant vibratory forces imposed by mechanical equipment operation, dictate structural considerations, and variations which are unique to the cooling tower industry. The components to be considered are the cold water basin, framework, water distribution system, fan deck, fan cylinders, mechanical equipment supports, fill, drifteliminators, casing, and louvers. B. COLD WATER BASIN The coolingtower basin serves the two fundamentallyimportantfunctionsof1) collectingthecold water followingits transitof the tower,and 2) acting as thetower'sprimaryfoundation.Becauseit also functionsas a collectingpointfor foreignmaterial washedoutoftheairby thecirculatingwater,it mustbe accessible,cleanable,have adequatedraining facilities,and be equippedwithsuitablescreening toprevententryofdebrisintothesuction-side piping. Basin Types Ground level installations, typical of virtually all large industrial towers, utilize concrete basins whereas elevated or rooftop installations are normally equipped withbasins provided by the cooling tower manufacturer compatible with the cooling tower framework. Typical materials include wood, steel and, occasionally, plastic. In those cases, the cooling tower manufacturer usually includes drain and overflow fittings,makeup valve(s), sumps and screens, as well as provisions for anchorage. Concrete basins for wood or steel framed, field erected towers are usually designed and built by the purchaser, utilizing dimensional and load information provided by the manufacturer.
Figure 4.20 To insure proper functioning of the tower, the basin must provide a stable, level foundation. Generally, a well-drained soil with moderate bearing capacity will support mechanical draft towers of wood or steel construction. Concrete towers impose heavier loads on the soil and, in 30
some cases, may require the use of piles or caissons. The soil should have a uniform bearing capacity under the basin to prevent uneven settlement. Footings must be below the prevailing frostline and construction practices should always conform to local codes. Wood towers may be equipped with either wood or steel basins, with tongue and groove, treated Douglas fir plywood flooring being the predominant choice because of its high strength and dimensional stability. Wood basins are normally flat, less than 2' deep and equipped with depressed sumps to facilitate pump suction. Joints are sealed to prevent leakage. Plywoodbasins typically require considerably less maintenance than do carbon steel basins. Steel basins may be of carbon steel (galvanized or painted), or stainless steel, and of either bolted or welded construction. If bolted, joints must be gasketed and sealed leak-tight. If welded, theweld vicinity should be suitably coated for corrosion protection. Steel basins also are normally flat, except for those under certain factory- assembled towers which incorporate a depressed section to facilitate cleaning and improve outflow characteristics.Being subject to oxidation, steel basins require more maintenance, and are more sensitive to water quality, than are wood basins. Basin Support A grillage of steel or concrete is normally utilized for support of a tower installed over a wood or steel basin. (Fig) Grillages must be designed to withstand the total wet operating weight of the tower and attendant piping, as well as the dead loads contributed by stairways, catwalks, etc. It must also accepttransient loads attributable to wind, earthquake, and maintenance traffic. Grillage membersmust be level, and of sufficient strength to precludeexcessdeflectionunderload.In designing the grillage, the possibility of future extension of the tower should be considered as a means of minimizing future cost impact. Basin Depth As indicated previously, wood and steel basins are of relatively shallowconstruction, typically 14" to 20" deep. Although greater depths are possible, they are seldom required or recommended. Sufficient freeboard above the operating water level is included to accommodate the normal amount of transient water that collects in the basin at shutdown.Greater design flexibility is afforded with the concrete basins typically utilized for larger towers, and adaptable for smaller towers. Once the load points are accommodated at the proper elevation, the basin floor (slab) may be as far below the top of the basin wall (curb) as required to satisfy design criteria. The basin must be deep enough toprovide sufficient hydraulic head for proper water flow into the sump(s), and to acceptthe transient water and potential back- flow at pump shutdown. Beyond this, the basin may be made deep enough to hold a reserve in case of interrupted make-up water supply; to stabilize watertemperatures under highly variable loads; or to act as a reservoir to supply the plant fire protection system."Dry basins" are minimum depth basins which drain by gravity into adjacent flumes, vessels, collection ponds, or streams. They are so designated because they are intended to drain completely uponpump shutdown. Typical applications of this principle are the "indoor tank", and the "helper" tower. Sufficiently low water levels in dry-basin towers may necessitate air seals to prevent the reduction in tower performance associated with air by-passing beneath the fill.
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Basin Sumps Sumps for towers with wood or steel basins are normally designed and furnished by the manufacturer. Concrete sumps (Fig), provided by the purchaser, should be de- signed for water entrance velocities of less than3'/sec., and should be of sufficient depth to satisfy pump suction head requirements. Screens are usually vertical, of½` square mesh, sized for1'/sec. net velocity through the open area of the screen, and held in place by channels imbedded in the sump walls to allow for easy removal. Screens may be installed in duplicate to permit cleaning during continued operation.
Basin Cleaning Facilities Because it is an area ofrelatively low flow velocity, any water borne or air borne particulates entering the circulating water system will tend to settle in the basin, where the resultant silt can be either periodically or continuously removed from the system. Periodic sludge removal usually takes place during normal shutdown intervals. Where towers are expected to operate continuously, strategically located basin partitions can permit partial shutdown for sectional cleaning and maintenance. Where possible, large capacity cleanout drains should be provided. Concrete basin floors should slope toward the sumps or drains at a rate of l' per 100', to permit flushing of the sediment. Where drains cannot be provided, basins should slope toward a cleanout sump from which sludge can be pumped, or removed manually.Side-stream filtration has been found to be an effective means of maintaining suspended solids at acceptable levels in the circulating water system, and of reducing the costsassociated with periodic silt removal. For most effective filtration, discharge flow from the filter should be returned to areas of lowvelocity in the basin in order to help maintain particulate suspension.
TOWER FRAMEWORK The most commonly used materials for the frame- work of field-erected towers are wood and concrete, with steel utilized infrequently to conform to a local building code, or to satisfy a specific preference. Factory-assembled towers predominate in steel construction, although wood construction is increasingly utilized in locations (or for processes) that tend 32
to promote corrosion. A uniform wind load design of30poundsper squarefootisstandard,withhighervalueseither dictatedoradvisableinsomeareas.Earthquake loads, if applicable,are in accordancewith zones definedin theUniformBuildingCodeoftheInternationalConferenceofBuildingOfficials.Design stressvaluesforwoodmembersand fastenersare basedon theNationalDesignSpecificationofthe NationalForestProductsAssociation.Steelmembers are governedby the AmericanInstituteof Steel Constructionmanual,andconcreteisbasedon BuildingCodeRequirementsforReinforcedConcrete of the AmericanConcreteInstitute. In largewoodtowers,thecolumnsare normally spacedon 4' x 8' or 6' x 6' centers.These baysizeshaveevolvedovertheyears,andhave proved best to properlysupportthe fill,drifteliminator,andlouvermodules,as wellas tokeep lumber sizes to thosethatare readilyavailable. Diagonalbracingin theplaneofthecolumnsisusuallyofcolumnsize(Fig), withloadstransmittedthroughfiberreinforcedplasticstructural connectorsat the joints,Horizontalgirts in the trans- verse andlongitudinaldirectionscarry the fillmodules, and keep the un braced columnlengthsto short verticalspans. In order to achieve a determinatedefinitionoflateralbracingofthecolumnsagainst buckling,transverseandlongitudinalgirtlines shouldbe at the same plane.
WATER DISTRIBUTION SYSTEM In a generalsense,pipingand distributionof the water withinthe envelope of the towerare responsibilitiesofthetowermanufacturer.Sitepiping,as wellas attendantrisers,valvesand controls,which occuroutsidethe confinesof the coolingtowerare providedand installedby others.Magnitudeandroutingofthecirculatingwater lines between the heat source and the tower location areusuallydictatedby typeoftower,topography and sitelayout.Lines may be buriedtominimizeproblemsofthrustloading, thermalexpansionand freezing; or elevated to minimize costofinstallationandrepair.In eithercase, the risers tothetowerinletmustbe externally sup-ported, independent of the tower structureand piping
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TypesandArrangements Cross-flowtowerconfigurationpermitstheuse of a gravity- flow distributionsystem wherein the supply water is elevated to hot water distributionbasins above the fill,from whichit flowsover the fill(by gravity) throughmeteringorificeslocatedin the distributionbasinfloor.Conversely,counter-flowconfiguration normallynecessitatestheuse of a pressure-type systemof closedpipe and spraynozzles. Gravitysystemsare readilyinspected,cleanedand maintained,and easilybalanced;but contribute negligiblyto overallheat transfer,tend to reo quireasomewhathigherpumpheadinlarger towers,and may promotethe formationof algae unlesstheopenbasinsarecovered.Pressure spraysystemsare moresusceptibletocloggingand more difficultto balance, clean, maintainand replace;butcontributeSignificantlytooverall heat transfer,tend toward lower pump heads in towersoflargersize, and are lessconduciveto algae growth. A typicalsupplypipingarrangement,applicable to multi-cellcross-flowor counterflowtowers, positionsthe supplyline adjacentto the long side of the towerand runningthe fulllength. Verticalrisers(one per cell)connectthesupply linetothemanufacturer'sinletconnectionsat the elevationof thetower'sdistributionsystem. Valves are usuallyinstalledin these risers to en- able individualcellsto be takenout of service. The cross-flowdesignpermitsmany piping variationsthatcanbe adaptedtomulticelltowers. Two risersat one end of the towerconnectingtothemanufacturer'sheaderpiping,is one method used for large circulatingwater rates. Whereflowratespermit,a singleriser at the end of the tower, or somewherealong the louveredface,can be utilized,connectingto the manufacturer'smanifoldheader at the top of the tower.In eithercase, the manufacturer'sheader pipingrunsthefulllengthofthetower,serving eachhalf-celldistributionbasinthroughflowcontrolvalves,andcrossoverpipingasnecessary. Concretecross-flowroundtowerstypically utilize an open concreteflume, fed by one or more concreteinternalrisers, for primary distributionof the hot water.Radialflowfromthe flume into the open distributionbasinis throughadjust- ableweirsorgates.Properplacementofstop logs in the flumepermitsthe opportunityfor major maintenanceof a sector of an operatingtower, shouldthe need arise. Concretecounter-flowroundoroctagonal towersalsomake use of one or more internal concreterisers feedingan elevated system of closedflumesor conduits(usuallyof concrete) which,in turn,supplyan arrayofbranchpiping andcloselyspacednozzlestoachieveuniform water distributionover the fill.
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DistributionSystemMaterials Distributionsystemsare subjectedto a combinationof hot water and maximumoxygenation.Therefore, the materials utilizedshouldbe highlyresistantto both corrosionand erosion.Historicallyproven materials arehotdipgalvanizedsteel,castiron,andredwoodstavepipe.Becauseoftherelativelylow pressurestowhichcoolingtowerpipingissubjected,theuse ofvarioustypesofplasticpipe and nozzles has alsobecomea mark of qualityconstruction.Exceptforrelativelysmall diameters,the plasticpipe utilizedis usuallyfibre reinforced.Precastandpre-stressedconcrete pipeandflumesarealsoutilizedonconcrete towers. RiserSwayBraces Windand/orearthquakeconsiderationswilloccasionallyinfluencespecifying engineerstocallforswaybraceswhichtietheupperendofafoundation-cantileveredrisertothe largertowerstructure,whichisassumedbythe specificationwritertohavethegreaterrigidity. Thisisnotgoodpracticewhentheriserisofa materialhavingahighmodulusofelasticity,such assteel.Thecoolingtowerstructurewillreact quitedifferentlyfromtheriserunderanimposed loadcondition.Forexample,underearthquake accelerationtheriserwillrespondathighfrequencyandlowamplitude,whereasthetower structure(oflowermodulusmaterial)willrespond atlowerfrequencyandgreateramplitude.The resultisthatanyconnectionbetweenthetwowill beattemptingtotransmittheseismicresponseof thetowerintothemorerigidrisers,anddamage totheend wallframingorthepipingconnections mayfollow.Ariserbracecapableoftransmittingthehighloadsgeneratedbyadifferentialresponseisa costlyauxiliarystructurewhichimposessignificantloadingsatanchoragepointsinthecold waterbasin,anditsutilizationinsuchcases shouldbeavoided.Properlydesignedcooling towerheaderpipingwillaccommodatetypical horizontalandverticalmovementwithoutdistress,andtheflangedjointbetweentheriserand thedistributionheadershouldneverbeconsideredasarisersupport.Incasesofdoubt,anexpansionjoi ntorflexiblecouplingshouldbeprovidedbetweentheriserandheadertoallowrelativemovementtoo ccur. RisersofFRPplasticpipehaveamodulusof elasticityclosetothatofthetowerstructureand willexperiencesimilarseismicresponse.Therefore,destructivetransferofopposedloadsisunlikely, andtheflexibilityoftheFRPpiperisermay requirelateralsupportatthetop.In suchcasesa riserswaybracemaybeadesirablesolution.
AncillarySystems 35
Provisionmustbemadeformake-up,overflowandblow-down,andshouldbe madeforbypass. Theamountofmake-upwaterrequiredconsistsof thetotalwaterlossesaccruedthrough evaporation, drift,blow-downandsystemleakage.Onrelatively smalltowers,makeupiscontrolledbyamechanicalfloatvalverespondingto thebasinwaterlevel. Floatswitchesorelectricprobesystems arenormallyused onlargertowerstoopenand closeamake-upvalve,ortostartandstopa make-up pump. These may be locatedin a stillingchamberdesignedtosuppresstheeffect of normalwave actionin the basin.Make-up lines frompotablesourcesare usuallybroughttothe coldwaterbasin,installedwiththeirpoint of dischargedownturned,and sufficientlyabove the basinwaterleveltoprecludecontaminationof thatsupplyby the circulatingwater.Nonpotable water,of course,may be connectedforinjection ofthemake-upsupplyat any pointin thewater circuit.Overflowlinesmaybesizedlarge enough to facilitateflush-outcleaningof a sump, but mustbe large enough to handlefullmake-up flowin case thatdevice malfunctions.
FAN DECK The fan deckisconsidereda partofthetower structure,actingasadiaphragmfortransmitting dead and live loads to the towerframing.It also provides a platformfor the supportof the fan cylinders, as wellas an access wayto themechanicalequipment and waterdistributionsystems. Fandeckmaterialsarecustomarilycompatible withthetowerframework.Woodtowersnormally utilizetongue-andgroovefirplywood;galvanized steel on steel towers; and pre-stresseddouble-tee sectionson concretetowers. Uniform live loading design on larger towers is normally60 poundsper squarefoot,reducingto 40 poundsper squarefooton the smallertowers.
FAN CYLINDERS Considerable thought calculation, modelling, and testinggoesintothedesignandconstructionofa fan cylinderbecauseit so directlyaffectsthe proper flowofair throughthetower.Fan efficienciescanbe severelyreducedby apoorlydesignedfan cylinder,or significantlyenhancedby a well-designedone.The essenceof a well-designedfan cylinder(Fig 1) incorporates;an easedinletto 36
promotesmooth flowofair to the fan;minimumfanblade tipclearance; a smoothprofilebelow and above the fan; sufficientstructuralstrengthto maintaina stableplan and profile; and either sufficientheight to protect operatingpersonnel,or a removablemesh guard, structurallyreinforced. Allof thesephysicalrequirementshave practical limitations,generallycontrolledby the materialsof construction.Fibre-reinforced plastic,because of its formability,strength,relativelylightweight,stability,andresistancetowaterandweathering,i s the preferredmaterialfor this application.Cylindersare formedover moulds whichaccuratelycontrolcontour and dimensions,resultingin a fan cylinderthatapproachesideal air movement, coupledwith minimum noise.Goodfancylindersarealsoconstructedof wood or steel.However, shape factorsusuallyresult in lowerfan efficiencies. Fan cylindersofan extendedheight(sometimes called"fanstacks")promotedischargeof the saturated air streamat higherelevations,minimizingthe effectsof recirculationand interference.One type of fan stackis in the formof a flareddiffuser thatprovidesa gradualincreasein crosssectionalarea beyond the fan with a resultant decreasein leaving air velocity.This effectivelyconvertsvelocitypressureto staticpressure,resulting inasignificantincreaseinairdeliveryoverwhat couldbe accomplishedwitha straightstackat the samefanhorsepower.Thesevelocity-recovery stacksare particularlyapplicableto large industrial towers.
MECHANICAL EQUIPMENT SUPPORTS The frameworkof a coolingtoweris not totallyin- flexible,even on concretetowerswhichutilizestructural members of relativelymassive cross section. Consideringthe tremendoustorsionalforces encounteredin the operationof large fans at high horsepower,it becomesapparentthatsome means ofassuringa constantplanerelationshipthrough- out the motor-gear reducer-fan drive trainmust be providedin ordertomaintainproperalignmentofthe mechanicalequipment. For smallerfan units,unitizedsteel weldmentsof structuralcrosssectionservewell.However,the forcesimposedby the operationoflargerfansdictate the use of unitizedsupportsof greatersophistication.Theseusuallyconsistoflarge,heavy-wall torque tubes welded to 37
outriggersof structuralsteel.Customarymaterialfor theseunitizedsupportsis carbonsteel,hot-dipgalvanizedafterfabrication, with stainlesssteel constructionavailableat significantadditionalcost. The combinationof heavy construction,plus galvanization,generallymakes stain- less steelconstructionunnecessary.
FILL(HEAT TRANSFER SURFACE) The single most importantcomponentof a cooling toweris the fill.Upon its abilityto promoteboth the maximumcontactsurface,andthemaximumcontacttime,betweenairand waterdependstheefficiencyof thetower.And,it mustpromotethisair- water contact while imposing the least possible restrictionto air flow. Mostreputablecoolingtowermanufacturersdesignand producefillspecificallysuitedto theirdistribution,fan, and supportsystems; developingall in concertto avoid the performance-degradingeffects of a misapplieddistributionsystem, or an airimpedingsupportstructure. The twobasicfillclassificationsare splashtype (Fig) and filmtype. (Fig) Althougheither type can be appliedin cross-flowor counter-flowconfiguration,counterflowtowersare tendingtowardalmost exclusiveuse of the filmfills.Cross-flowtowers,on the otherhand, makeuse ofeithertypewithequal facility,occasionallyin concert. Splash type fillbreaks up the water, and interrupts itsverticalprogress,bycausingittocascade throughsuccessiveoffsetlevelsofparallelsplash bars. Maximumexposureof the water surfaceto the passingair is thusobtainedby repeatedlyarresting the water'sfalland splashingit into smalldroplets, as wellas by wettingthesurfaceoftheindividual splashbars.Splashfillischaracterizedby reducedairpressure losses,and is not conduciveto clogging.How- ever, it is very sensitivetoinadequatesupport.The splashbars mustremainhorizontal.If saggingoccurs, the water and air will"channel"throughthe fill in separate flow paths, and thermalperformancewill be severelyimpaired.Also,if the toweris not level, waterwillgravitatetothelowendsofthesplash bars and produce thischannellingeffect.Longtermperformancereliabilityrequiresthat the splashbars be supportedon closecentres,and thatthe supportmaterialbe as inert as practicable. Of the various supportmechanismspresentlyin use, fibrereinforcedplasticgridhangersare recognized as havingthelongesthistoryof success,withPVC coatedwire grids also enjoyingconsiderableuse. In utilizing coated carbon steel grids, however, care must be exercisedto assurethatthe splashbars willnot abrade thecoating,exposingthe wire to corrosion.Treatedwoodlath (primarilyDouglasFir) predominatedformany yearsas splashbar material,and continuestobeextensivelyusedbecauseofits strength,durability,availability,andrelativelylow cost. Currently,however,plasticshave gainedpredominance.They may be injectionmouldings of polypropylene,or similarmaterialswhich can be compoundedfor resistanceto fire; or they may, be extrusionsof PVC, whichinherentlyhas a low flamespreadrate. Stainlesssteelor aluminum splashbarsare occasionallyusedin steelframed towerswheretotallyfireproofconstructionmay be mandatory. Film type fill causes the water to spread into a thin film,flowingoverlargeverticalareas,topromote maximumexposure to the air flow.It has the capabilityto provide more effectivecoolingcapacity withinthesame amountof space,butis extremely sensitiveto poor water distribution,as well as the air blockageand turbulencethata poorly designedsupport systemcan perpetuate. The overall tower design mustassureuniformair and waterflowthroughout the entire fillarea. Uniformspacingof the fillsheets is 38
alsoofprimeimportancedue to the tendencyof air to take the path of leastresistance. Because the fill sheets are closelyspaced, the useof filmfillshouldbe avoidedin situationswhere the circulatingwatercanbecomecontaminatedwith debris. Filmfillcan be made of any materialthatis capableofbeingfabricatedormoldedintoshaped sheets,witha surfaceformedasrequiredby the designto directtheflowof air and water.Because PVCisinerttomostchemicalattack,hasgood strengthcharacteristics,is lightin weight,has a low flamespreadrate,and can easilybe formedto the shaperequired,itiscurrentlythemostpopular material.
DRIFT ELLIMINATORS As a by-productof thecoolingtowerhavingpromoted the most intimatecontactbetween water and air in the fill, water dropletsbecome entrainedin the leavingairstream.Collectively,thesesolidwater dropletsare called"drift"and are not to be confused with the pure water vapour withwhichthe effluentair streamis saturated,nor withany dropletsformedby condensationofthatvapour.Thecompositionand qualityof driftis that of the circulatingwater flowing throughthe tower. Withthe towerlocatedupwindof powerlines, substations,and other criticalareas, its potentialas an operatinghazard can be significant.Drifteliminatorsremove entrainedwater from thedischargeair by causingit to make sudden changes indirection.Theresultingcentrifugalforceseparates the drops of water from the air, depositingthem on the eliminatorsurface,from which they flow back into the tower. Althoughdesignersstrive to avoid excess pressurelosses in the movementof air through the eliminators,a certainamountof pressuredifferentialis beneficialbecauseit assistsin promoting uniformair flowthroughthe towerfill. Eliminatorsare normallyclassifiedby the numberofdirectionalchanges,or"passes",withanincreaseinthenumberofpassesusuallyacco mpaniedby an increasein pressuredrop.They may consistof two or more passes of spacedslatspositionedin frames or may be mouldedintoa honeycombconfigurationwithlabyrinthpassages. Some towersthatutilizefilmtypefillhave drifteliminatorsmouldedintegrallywiththefill sheets. Since drift eliminators should be as corrosion resistant as the fill, materials acceptable for fill are usually incorporatedinto eliminator design, with treated wood and various plastics (predominantly PVC) being most widely used. 0.02% of the circulating water rate. In larger towers, affording more room and opportunity for drift-limiting techniques, drift levels will normally be in the region of 0.008%, with levels of 0.001% attainable. 39
CASING A cooling tower casing acts to contain water within the tower, provide an air plenum for the fan, and transmit wind loads to the tower framework. It must have diaphragm strength, be watertight and corrosion resistant, and have fire retardant qualities. It must also resist weathering, and should present a pleasing appearance.Currently, wood or steel framed, field-erected towers are similarly cased with fire-retardant fibre-reinforced polyester corrugated panels, overlapped and sealed to prevent leakage. Factory assembled steel towers utilize galvanized steel panels, and concrete towers are cased with precast concrete panels. If required for appearance purposes, the casing can be extended to the height the handrail.
LOUVERS Every well-designed cross-flow tower is equipped with inlet louvers, whereas counter-flow towers are only occasionally required to have louvers. Their purpose is to retain circulating water within the confines of the tower, as well as to equalize air flow into the fill. They must be capable of supporting snow and ice loads and, properly designed, will contribute to good operation in cold weather by retaining the increase in water flow adjacent to the air inlets that is so necessary for ice control. Closely spaced, steeply sloped louvers afford maximum water containment, but are the antithesis of free air flow, and can contribute to icing problems. Increasing the horizontal depth (width) of the louvers significantly increases their cost, but it permits wider spacing, lesser slope and improved horizontal overlap, and is the design direction 40
taken by most reputable manufacturers. The most-utilized louver materials are corrugated fire-retardant fibre reinforced polyester and treated Douglas Fir plywood on field-erected towers, galvanized steel on factory-assembled steel towers, and precast, pre-stressed concrete on concrete towers. The evolution of louver design began in the early era of splash type fill, more than a half century ago, at which time their primary function was to control the multitude of random water droplets produced by the splashing action. Because of the width and spacing necessary to accomplish this magnitude of water recovery, louvers became a highly visible, accented part of the cooling tower’s appearance. With the advent of acceptable film type fills, with their inherently better water management characteristics, louver design was reassessed. Ultimately, the highly visible type louversdisappearedfromcertain cooling towers specifically designedforoperation onlywithfilmtypefill. However, the louvers remain they have merely becomean integralpart of theleadingedge of the fillsheets One shouldnotanticipatefromthisthatlouvers are obsolescent.Splash type fillis stillwidelyused, especiallyin contaminated waterservice, and is expectedto remain so for the foreseeablefuture.Furthermore,the use of film-filledtowerswithoutexternal louversin certainoperatingconditions,suchas excessivelyhigh waterloadings,is ill advised.
MECHANICAL COMPONENTS GENERAL Coolingtowermechanicalequipmentis required to operatewithina highlycorrosive,moistureladen atmospherethatisuniqueto thecoolingtowerindustry,and the historicalfailurerate of commerciallyavailablecomponentscausedreputabletower manufacturersto undertaketheir own productionof specificcomponentssome years ago. Currently,the lowfailurerateofmanufacturer-producedcomponentsreinforcesthatdecision.Purchasersalso benefitfrom the advantageof single-sourceresponsibilityfor warranty,and replacementparts.Themechanicalcomponentsbasicto the operationofthecoolingtowerare fans,speedreducers, drive shafts,and water flowcontrolvalves.
FANS Propeller Fans Propeller type fans predominate in the cooling tower industry because of their ability to move vast quantities of air at the relatively low static pressures encountered. They are comparatively inexpensive, may be used on any size tower, and can develop high overall efficiencies when "system designed" to complement a specific tower structure - fill - fan 41
cylinder configuration. Most utilized diameters range from 24 inches to 10 meters (Fig 1), operating at horse-powers from 1/4 to 250 +. Fans larger in diameter are equipped with adjustable pitch blades, enabling the fans to be applied over a wide range of operating horsepowers. Thus the fan can be adjusted to deliver the precise required amount of air at the least power consumption. The rotationalspeed at whicha propellerfan is applied typicallyvaries in inverse proportionto its diameter.The smallerfans turnat relativelyhigh speeds,whereasthelargerones turnsomewhat slower.Thisspeed-diameterrelationship,however, is by no means a constantone. If it were, the blade tip speed of all coolingtower fans would be equal.The appliedrotationalspeedofpropeller fans usuallydependsupon best ultimateefficiency. However, sincehigher tip speeds are associatedwith highersoundlevels, it is sometimesnecessaryto selectfansturningat slowerspeeds to satisfya criticalrequirement.Theincreasedemphasisonreducingcooling toweroperatingcostshas resultedin theuse of largerfanstomove greatervolumesofair more efficiently. The new generationsof fans are lightin weightto reduce parasiticenergylosses, and have fewer,but wider, blades to reduce aerodynamicdrag. Moreover, the characteristicsof air flowthroughthetower,fromInlettodischarge, are analysedand appropriateadjustmentsto the structureare made to minimizeobstructions;fill anddistributionsystemsaredesignedandarranged to promotemaximumuniformityof air and waterflow;and drifteliminatorsare arrangedto directthe finalpass of air towardthe fan. This is recognizedas the "systems"approachto fan design,withoutwhichthebestpossibleefficiency cannotbe obtained. Theintentofgoodpropellerfandesignis to achieveair velocitiesacrossthe effectivearea of thefan,fromhub tobladetips thatare as uniformas possible.The mosteffectiveway toaccomplishthisis withtaperedand twistedblades havingan airfoilcrosssection. Historically, castaluminiumalloyshave been the classic materialsused for productionof thisblade type. Cast aluminiumbladescontinueto be utilizedbecauseof theirrelativelow cost,goodinternalvibrationdampingcharacteristics,andresistance to corrosionin most coolingtowerenvironments. Currently,lighterbladesofexceptionalcorrosionresistanceare made of fiberglassreinforced plastic(FRP),castinprecisionmoulds.Theseblades may be solid; formedaround a permanentcore; or formedhollowby theuse of a temporary core, (Fig. 2). In all cases, theyhave proved to be both efficientand durable. Fan hubs must be of a materialthatis structurallycompatiblewithbladeweightandloading, andmusthave goodcorrosionresistance.Galvanizedsteelweldments, greyand ductileiron castings,and wroughtor castaluminiumare in generaluse as hubmaterials.Wherehub and blades are of dissimilarmetals,they must be insulatedfromeach othertopreventelectrolytic corrosion. Smallerdiameter fans are customarily of galvanized sheet metal constructionwith fixed-pitch, non-adjustableblades. These fans are matchedto differingairflowrequirementsby changingthe designspeed.
Automatic Variable-Pitch Fans Centrifugal Fans These are usuallyof the doubleinlettype, used predominantlyon coolingtowers designedfor indoorinstallations.Their capability to operateagainstrelativelyhigh staticpressures makes themparticularlysuitablefor thattype of 42
application.However,theirinabilitytohandle large volumesof air, and theircharacteristically high input horsepower requirement (approximately twicethatof a propellerfan) limitstheiruse to relativelysmallapplications. Three types of centrifugalfans are available; 1) Forwardcurve blade fans 2) Radial blade fans 3) Backwardcurve blade fans. The characteristics of the forwardcurvebladefanmakeit themost appropriatetype for coolingtowerservice.By virtue of the directionand velocityof the air leaving thefanwheel,thefancanbe equippedwitha comparative smallsize housing,whichis desirable froma structuralstandpoint.Also,because therequiredvelocityis generatedat a comparativelylow speed, forwardcurve blade fans tend to operatequieterthan othercentrifugaltypes. Centrifugalfans are usuallyof sheet metal construction,withthemostpopularprotectivecoating beinghot-dipgalvanization.Damper mechanisms are alsoavailableto facilitatecapacitycontrolof the coolingtower.
Fan Laws For a given fan and coolingtowersystem,the followingis true: 1) The capacity (cfm) varies directly as the speed (rpm) ratio, and directly as the pitch angleof the blades relative to the plane of rotation. 2) The static pressure (hs) varies as the square of the capacity ratio. 3) The fan horsepower varies as the cube of the capacity ratio. 4) At constant cfm, the fan horsepower and static pressure vary directly with air density. If, for example, the capacity(cfm) of a given fan were decreasedby 50 percent (either by a reductiontohalfor designrpm,or by a reductionin blade pitch angle at constantspeed), the capacity ratiowouldbe 0.5. Concurrently,the staticpressure wouldbecome 25 percentof before,and the fan horsepower would become 12.5 percent of before. These characteristicsafford unique opportunitiestocombine cold water temperature control withsignificantenergy savings. Selectedformulas,derivedfromthesebasic laws, may be utilizedto determinethe efficacyof any particularfan application: Q= Volumeofair handled (cfm). A = Net flow area. V= Averageairvelocity at plane of measurement. g= Accelerationdue to gravity. D=Density of water at gauge fluid temperature. d= Air densityat point of flow hs = Staticpressuredropthroughsystem. hv = Velocitypressureat point of measurement. ht= Totalpressuredifferential(=hs + hv). vr=Fancylindervelocityrecoverycapability. Thermalperformanceofacoolingtowerdependsuponaspecificmassflowrateofair throughthefill, whereas the fan does its job purely in terms of volume.Sincethespecific volumeofairincreases with temperature, it can be seen thata larger volumeof air leaves the tower than entersit. The actualcfm handledby the fan is the productof mass flow rate times the specificvolumeof dry aircorrespondingto the temperatureat whichtheair leaves the tower. This volumetricflowrate is the "Q" used in the following formulas,and it must be sufficientto 43
produce thecorrectmassflowrate or the towerwillbe short of thermalcapacity. Utilizingappropriatecross-sectionalflow areas,velocitythroughthefanandfancylindercan calculatedas follows V=(Q/ A X 60)
be
It must be understoodthat"A"will change with theplaneat whichvelocityisbeingcalculated. Downstream ofthefan,"A"is thegrosscross- sectionalarea of the fan cylinder.At the fan, "A" is the area of the fanless the areaofthe hub or hub cover. Velocity pressure is calculated as follows -
hv =V2 x12 xd-------------------------------(10) 2x gxD
If "V"inFormula(10) representsthevelocity throughthefan,thenhv representsthevelocity pressurefor thefanitself(hvf). Moreover,if the fan is operatingwithina non-flared-discharge fan cylinder,thiseffectivelyrepresentsthetotalvelocitypressurebecauseofnorecoveryhaving takenplace.However, if the fan is operatingwithina flared, velocityrecoverytypefancylinder(Fig.68), hv mustberecalculatedforthefancylinderexit (hve), at theappropriatevelocity,andappliedin the followingformulatodeterminetotalvelocity pressure: hv =hvf-[(hvf-hve) x vr---------------------------------(11) Althoughthevalueofvr willvary withdesign expertise,and is empiricallyestablished,a value of 0.75 (75 percentrecovery) is normallyassigned forpurposesofanticipatingfanperformance withina reasonablywell-designed velocity-recovery cylinder. The power output of thefan is expressed in terms of air horsepower (ahp) and represents work done by the fan: aph = Q X ht X D-----------------------------------(12) 33000 X 12 Staticair horsepoweris obtainedby substituting staticpressure(hs) for totalpressure(ht) in Formula(12). A great deal of researchand development goes intotheimprovementoffanefficiencies,and thosemanufacturersthathave takena systems approachto thisR&D efforthave achievedresults which, althoughincrementally small, are highly significantin thelightofcurrentenergycosts. Staticefficienciesand overallmechanical(total) efficienciesare consideredin theselectionofa particularfaninaspecificsituation,withthe choiceusuallygoing to the fan whichdeliversthe requiredvolumeofairattheleastinputhorse- power: StaticEfficiency= TotalEfficiency=
static ahp -------------------------(13) inputhp ahp-------------------(14) inputhp
Itmustbeunderstoodthatinputhpismeasuredat thefan shaft,and doesnotincludethe trainlossesreflectedin actualmotorbrake horsepower(bhp). Input hp normallyapproximately95 percentof motorbhp on larger fan applications.
SPEED REDUCERS 44
drivewill
Speedreductionincoolingtowersisaccomplishedeitherby differentialgearsofpositive engagement,orby differentialpulleys(sheaves) connectedthroughVbelts.Typically,gearreduction unitsareappliedthrougha widerangeofhorsepower ratings,from the very large down to as littleas5 hp. V-belt drives, on the other hand, are usually appliedat ratingsof 50 hp or less. Gear Reduction Units Gear reducersareavailable in a variety of designs and reductionratios to accommodate the fan speeds and horsepower encounteredin coolingtowers.(Fig.1) Because of theirabilityto transmitpower at minimalloss, spiralbevel and helicalgear sets are most widelyutilized,althoughwormgearsarealsousedin somedesigns.Dependinguponthereduction ratiorequired,andtheinputhp,aGear reducer may use a singletypegear, or a combinationof typestoachieve"staged"reduction.Generally, two-stagereductionunitsareutilizedforthe large, slower-turningfansrequiringinputhorse- powers exceeding75 bhp.The servicelifeof a Gear reduceris directlyrelated to the surface durabilityof the gears, as well as the type of service imposed(i.e. intermittentvs. continuousduty). Long, trouble-freelifeisalsodependentuponthe qualityof bearingsused. Bearings are normally selectedfora calculatedlifecompatiblewith the expectedtype of service.Bearingsfor industrialcoolingtowerGear reducers(consideredas continuousduty) shouldbe selectedon the basis of a 100,000 hourL-10 life.L-10 lifeis definedas the lifeexpectancyin hours duringwhich90 per- cent or more of a given group of bearingsunder a specificloadingconditionwillstillbe in service. Intermittentdutyapplicationsprovidesatisfactorylifewitha lowerL-10 rating.AnL-10 lifeof35,000 hours is satisfactoryfor an 8 to 10 hour perday application.It is equivalent,in terms of years of service, to a 100,000 hourL-10 life for continuous duty. Lubricationaspects of a Gear reducer, of course, are as importantto longevityand reliabilityas are thecomponentsthatcomprisetheGear reducer. Thelubricationsystemshouldbeofasimple, non-complexdesign,capableoflubricating equallywellin bothforwardandreverse operation. Remote oil level indicators, and convenientlocationoffillanddrainlines,simplify andencouragepreventivemaintenance. Lubricantsandlubricatingproceduresrecommended by the manufacturershouldbe adhered to closely.
V-Belt Drives These are an acceptedstandardfor thesmallerfactoryassembledcoolingtowers,althoughmostofthelargerunitary towers are equippedwithGear reducers. Correctlydesignedand installed,and wellmaintained,V-belt drives can provide very dependable service.The drive consistsof themotorand fan sheaves, thebearinghousingassemblysupporting the fan, and the V-belts. V-belts (as opposed to cog belts) are used most commonlyfor coolingtowerservice. A varietyofV-belt designsis available,offeringa wide assortment of features.Most of these designsare suitable forcoolingtoweruse. In many cases,more thanone beltis requiredto transmitpower fromthe motorto the fan.Multiplebeltsmustbe supplied either as matchedsets, measured and pack- aged togetherat the factory,or as a banded belt having more than one Vsectionon a common backing. Various types of bearings,and bearing housingassemblies,are utilizedin conjunctionwith Vbelt drives.Generally,sleevebearingsareusedon smallerunitsandballorrollerbearingson the largerunits, withoil being the most commonlubricant.In all cases, water slingerseals are recommended to prevent moisture from entering the bearing. Beltswear and stretchand belt tensionmust be periodicallyadjusted.Meansfor suchadjustment shouldbe incorporatedas part 45
of the motor mount assembly. Stabilityand strengthof the mountingassemblyis of primeimportancein order to maintainproperalignmentbetweendriver and driven sheaves.Misalignmentis one of the most commoncauses of excessivebelt and sheave wear. Manuallyadjustablepitchsheavesareoccasionallyprovidedto allowa changein fan speed. These are of advantageon indoortowers,where theabilitytoadjustfanspeedcansometimes compensatefor unforeseenstaticpressure.
DRIVE SHAFTS The driveshafttransmitspowerfromthe output shaft of the motor to the inputshaft of the Gear reducer.Becausethedriveshaftoperateswithinthe tower,it mustbe highlycorrosionresistant.Turning at fullmotorspeed, it mustbe wellbalanced-and capableofbeingre-balanced.Transmittingfull motorpowerover significantdistances,it mustaccepttremendoustorquewithoutdeformation.Subjectedto long term cyclicaloperation,and occasionalhumanerror,itmustbecapableofaccepting some degree of misalignment. Driveshaftsaredescribedas"floating"shafts, equippedwithflexiblecouplingsatbothends. Where only normal corrosionis anticipatedand cost is of primaryconsideration,shaftsare fabricatedof carbonsteel,hotdipgalvanizedafterfabrication. (Fig.1)Shaftsforlargerindustrialtowers,and thosethatwillbe operatingin atmospheresmore conduciveto corrosion,are usuallyfabricatedof tubular stainlesssteel.The yokes and flanges whichconnectto themotorand gear reducershafts are of cast or welded construction,in a variety of materialscompatiblewith thatutilizedfor the shaft. Flexiblecouplingstransmittheload betweenthe driveshaftandthemotoror gear reducer,andcompensatefor minor misalignment.A suitablematerial foruseina coolingtower'ssaturatedeffluentair streamisneoprene,eitherinsolidgrommetform (Fig.2), or as neoprene-impregnatedfabric(Fig.3), designedto require no lubricationand relativelylittle maintenance.Excellentservicerecordshave been establishedby the neoprene flexiblecouplings,both as bonded bushingsand as impregnatedfabricdisc assemblies.Thesecouplingsarevirtuallyimpervioustocorrosion,andprovideexcellentflexing characteristics. It is very importantthat driveshaftbe properly balanced.Imbalancenotonlycausestower vibration,butalsoinduceshigherloadsandexcessive wearon themechanicalequipmentcoupledto the shaft.Mostcoolingtowerdriveshaftwill operateat speeds approaching1800 rpm. At these speeds, it is necessarythattheshaftsbe dynamicallybalanced to reduce vibrational forcesto a minimum.
46
VALVES Valvesareusedtocontrolandregulateflow throughthewaterlinesservingthetower.Valves utilizedforcoolingtowerapplicationincludestop valves,flow-controlvalves,andmakeupregulator valves.The typesofvalves,quantityrequired,and complexityofdesignare dictatedby thetypeand size of the tower,and the requirementsof theuser.
Stop Valves These are usuallyof the gate or butterflytype. They are used on both counter-flowand cross-flowtowersto regulateflow in multiple-riser towers and tostopflowin a particularriserfor cellmaintenance.Becauseflow-controlvalvesarecustomarilysuppliedwith cross-flowtowers,stopvalvesarenotnormally consideredmandatoryintheircase.Asarule, stop valves are locatedin a portionof sitepiping forwhichtheuserisresponsible.In morecomplexconcretetowerdesigns,stop valvesmay be incorporatedinto the internaldistributionsystem and providedby thecoolingtowermanufacturer.In thesecases,slidegatetypevalvesare used successfullywhen relatively low pressures are involved.
Flow-Control Valves Intherealmofcooling towers,these are consideredto be valves that dischargeto 47
atmosphere.Essentially,theyare end- of-linevalves,as opposedtoin-linevalves.They are used on cross-flowtowersto equalizeflow betweendistributionbasinsof a towercell, as well as betweencellsof a multi-celltower.(Fig 1) Properlydesigned,theymay be used to shut offflowto selecteddistributionbasins,for interim cleaningandmaintenance,whiletheremainder of the towercontinuesto operate.
Make-Up Valves These are valves utilizedto automaticallyreplenishthe normalwaterlossesfrom the system. They are normallyprovided by the manufacturerwhere the cold water collection basin is part of his scope of work.
CHAPTER 5 FACTORS AFFECTING COOLING TOWER EFFICIENCY The atmosphere from which a cooling tower draws its supply of air incorporatesinfinitely variable psychrometric properties and the tower reacts thermally or physically to each of those properties. The tower accelerates that air; passes it through a maze of structure and fill; heats it; expands it; saturates it with moisture; scrubs it; compresses it; and responds to all of the thermal and aerodynamic effects that such treatment can produce.Finally,the cooling towerreturnsthat"usedup" stream of air to the nearby atmosphere,with the fervent intentionthat atmosphericwindswillnot find a way to reintroduce it backinto the tower.
Factors affecting cooling tower performance: 1. Non controllable 2. Controllable Non controllable performance factors: 1. Ambient wet bulb temperature 2. Air density 3. Wind direction and intensity Controllable performance factors: 1. 2. 3. 4.
Hot water temperature Water flow rate and distribution Air flow Water quality 48
5. 6. 7. 8.
Fill design and condition Drift eliminator design and condition Fan design and condition Tower structural condition
HOT WATER TEMPERATURE Consequences of excessive hot water temperature: 1. 2. 3. 4.
Melted fill Distribution system damage Wood structure damage Galvanized steel corrosion
FILL MATERIAL LIMITS 1. PVC 120OF TO 140OF LIMIT 2. CPVC 150OF TO 170OF LIMIT 3. Wood Lath Fill 170OF LIMIT
IMPROVING AIR FLOW RATE 1. 2. 3. 4.
More efficient fan Pitch fan blades for full motor HP Minimize total tower pressure drop Velocity recovery stacks
COOLING WATER QUALITY 1. Water quality and fill design 2. Bio-fouling and new low clog fills
FILL DESIGN AND CONDITION SPLASH FILL UPGRADES 1. Wood lath to high performance fill bars 2. Flash fills –Omega bar with film fill
TOWER STRUCTURAL CONDITION STRUCTURAL MATERIAL ALTERNATIVES 1. 2. 3. 4.
CCA treated douglas fir and redwood Pultruded fiberglass Galvanized and stainless steel Concrete
Performance influencing factors are:
1.Wet-Bulb Temperature The primary basis for thermal design of any evaporative type cooling tower is the wetbulb temperature of the air entering the tower. Wet-bulb temperatures are measured by 49
causing air to move across a thermometer whose bulb (properly shielded) is encased in a wetted muslin "sock". As the air moves across the wetted bulb, moisture is evaporated and sensible heat is transferred to the wick, cooling the mercury and causing equilibrium to be reached at the wet-bulb temperature. When a wet-bulb thermometer and a dry-bulb thermometer are combined in a common device, simultaneous coincident readings can be taken, and the device is called a "psychrometer".If the actual wet bulb is higher than anticipated by design, then warmer than desired average water temperatures will result. Conversely, if the actual wet-bulb is lower than expected, then the owner will probably have purchased a cooling tower larger than he needs.
2. Dry bulb Temperature Although it is always good practice to establish an accurate design dry-bulb temperature (coincident with the design wet bulb temperature) it is absolutely required only when types of towers are being considered whose thermal performance is affected by that parameter. These would include the hyperbolic natural draft, the fan assisted natural draft, the dry tower, the plume abatement tower, and the water conservation tower. It is also required where there is a need to know the absolute rate of evaporation at design conditions for any type tower. Where required, the same thought process and concern should prevail in the establishment of a design dry-bulb temperature as occurred in determining the design wetbulb temperature.
3. HEAT LOAD Although appropriate selection of the cooling tower size establishes the equilibriumtemperatures at which the tower will reject a given heat load, the actual heat load itself is determined by the process being served. All else being equal, the size and cost of a cooling tower is proportional to the heat load. Therefore, it is of primary importance that a reasonably accurate heat load determination be made in all cases. If heat load calculations are low, the cooling tower purchased will probably be too small. If the calculations are high, oversized, more costly equipment will result.Since volumes of reliable data are readily available, air-conditioning and refrigeration heat loads can be determined with considerable accuracy. However, significant variations exist in the realm of industrial process heat loads, each very specific to the process involved. In every case, it is advisable to determine from the manufacturer of each item of equipment involved with, or affected by, the cooling water system the amount of heat that their equipment will contribute to the total.
4. CIRCULATING WATER RATE, RANGE AND APPROACH The heat load imposed on a cooling tower (KW) is determined by the kg of water per second being circulated through the process, multiplied by the number of degrees Celsius that the process elevates the circulating water temperature. In cooling tower parlance, this becomes Heat Load =circulating water rate x R = KW
(1)
R ="Range"=Difference between hot water temperature entering tower and cold watertemperature leaving tower, in degrees Celsius .
50
Figure graphically shows the relationship of range and approach as the heat load is applied to the tower. Although the combination of range and circulation rate is fixed by the heat load in accordance with Formula (1), approach (difference between cold water temperature and entering air wet-bulb temperature) is fixed by the size and efficiency of the cooling tower. A large tower of average efficiency will deliver cold water at a temperature which "approaches" a given wet-bulb temperature no closer than a somewhat smaller tower having significantly better efficiency. Given two towers of reasonably equal efficiencies, operating with proportionate fill configurations and air rates, the larger tower will produce colder water.
5.INTERFERENCE As previously indicated, local heat sources upwind of the cooling tower can elevate thewet bulb temperature of the air entering the tower, thereby affecting its performance. One such heat source might be a previously installed cooling tower on site, or in the immediate vicinity. Figure depicts a phenomenon called "interference", wherein a portion of the saturated effluent of an upwind tower contaminates the ambient of a downwind tower.Although proper cooling tower placement and orientation can minimize the effect of interference, many existing installations reflect some lack of long range planning, requiring that design adjustments be made in preparation for the installation of a new tower.
6. RECIRCULATION The important difference between ambient and entering wet bulb temperatures is described previously. The latter can be, and usually is, affected by some portion of the 51
saturated air leaving the tower being induced back into the tower air inlets. This undesirable situation is called "recirculation". So much time is devoted for both to determining the potential for recirculation under various wind conditions, and to designing their towers in such a way as to minimize its effect. The potential for recirculation is primarily related to wind force and direction, with recirculationtending to increase as wind velocity increases.Although wind is the primary cause of recirculation, several other aspects of cooling tower design and orientation play important parts in its reduction and control.
A.TOWER SHAPE When flowing wind encounters an obstruction of any sort, the normal path of the wind is disrupted and a reduced pressure zone, or "wake", forms on the lee side (downwind) of that obstruction. Quite naturally, the wind will try to fill this "void" by means of the shortest possible route. If the obstruction is tall and narrow, the wind easily compensates by flowing around the vertical sides.
B.ORIENTATION WITH PREVAILING WIND If the wind is blowing in its prevailing direction, the owner would have been well advised to turn the tower 90 degrees from its indicated orientation. With this orientation, the wind first encounters the relatively high, narrow end of the tower, and the small negative pressure zone at the far end is easily filled by wind flowing around the vertical sides.Furthermore, wind moving parallel to the line of fans causes the separate effluents from each fan cylinder to "stack up” one on another, forming a concentrated plume of greater buoyancy.
C. AIR DISCHARGE VELOCITY At any given atmospheric condition, the velocity at which the discharge plume from a towerwill rise depends upon the kinetic energy imparted by the fan, and the buoyant energy (decreasein density) imparted to the effluent plume by the tower heat load, both of whichare changed to potential energy by virtue of ultimate elevation of the plume. The directionthat a plume will travel depends upon the speed, direction, and psychrometric characteristicsof the wind it encounters upon leaving the fan cylinder.
52
Low wind velocities will permit an almost vertical plume rise, barring retardation of that rise by unusual atmospheric conditions. (For an induced draft tower operatingunder calm conditions, with a vertically rising plume, entering and ambient wet bulb temperatures can be considered to be equal.)Higher wind velocities will bend the plume toward the horizontal, where a portion of it can become entrapped in the aforementioned lee-side low pressure zone for re-entry into the tower. As can be seen, lower velocity ratios (higher wind velocities) result in greater recirculation. Since the velocity ratio is also a function of plume discharge velocity, ambient wind force cannot accept all of the blame for recirculation. At any given wind condition, the velocity ratio will decrease if the plume velocity is decreased, resulting in an increase in the recirculation ratio. This is what makes forced draft towers so susceptible to recirculation.
D.FANCYLINDERHEIGHTANDSPACING Within structural limitations, discharge heights of fan cylinders can be increased. Also, the fan cylinders can be spaced somewhat farther apart to allow for a less restricted flow of wind between them. Both of these stratagems, usually done in concert, can measurably diminish the potential for recirculation in most operating situations, although not without some impact upon tower cost.
7. TOWER SITING AND ORIENTATION It is the responsibility of the ownerspecified to situate the tower such that these and otherthermal performance influencing effects will be minimized. Since the long term capability of a cooling tower is determined by its proper placement on site, the importance of such placement cannot be overemphasized. Every effort should be made to provide the least possible restriction to the free flow of air to the tower. In addition to this primaryconsideration, the owner must give attention to the distance of the tower from theheat load,and the effect of that distance on piping and wiring costs; noise or vibration may create a problem, which can be expensive to correct after the fact; drift or fogging may be objectionable if the tower is located too close to an area that is sensitive to dampness or spotting; also easy access and adequate working space should be provided on all sides of the tower to facilitate repair and maintenance work. The performance of cooling tower is dependent upon the quantity and thermal quality of the entering air. External influences which raise the entering wet bulb temperature or restrict air flow to the tower will reduce its 53
effective capacity. Air restriction , recirculation and interferences can be minimized possibly elliminated by careful planning of tower placement. OTHER PARAMETERS A primary consideration for the operation of the cooling tower system is the water quality of the make-up source. Differing sources present differing challenges. Surface water sources include lakes, rivers, and streams, while groundwater sources consist of wells or aquifers. Depending on the location, surface water sources will have seasonal variations and can carry high levels of suspended silt and debris that cause fouling if not removed by prefiltration systems. Groundwater sources don’t have the seasonal variations that surface water sources have, but depending on the geology of the region, they can have high levels of dissolved minerals that contribute to scale formation or corrosion in the cooling system. Recently, water reuse has become popular and many cooling systems are being supplied reclaimed effluent or discharge water from other processes. While water reuse is a wise resource option, consideration should be made regarding the quality of the water and how that will impact the efficient operation of the cooling system, and the system’s ability to meet the required cooling demand. Whether the source water is surface, ground, or reuse, in nature there are a few basic water quality considerations that should be understood.
pH It is a measurement of how acidic or how alkaline a substance is on a scale of 0 to 14. A pH of 7.0 is neutral (the concentration of hydrogen ions is equal to the concentration of hydroxide ions), while measurements below 7.0 indicate acidic conditions, and measurements above 7.0 indicate basic or alkaline conditions. The pH scale is logarithmic (each incremental change corresponds to a ten-fold change in the concentration of hydrogen ions), so a pH of 4.0 is ten times more acidic than a pH of 5.0 and one hundred times more acidic than a pH of 6.0. Similarly, a pH of 9.0 is ten times more basic or alkaline than a pH of 8.0 and one hundred times more alkaline than a pH of 7.0.
Hardness Hardness refers to the presence of dissolved calcium and magnesium in the water. These two minerals are particularly troublesome in heat exchange applications because they are inversely soluble meaning they come out of solution at elevated temperatures and remain soluble at cooler temperatures. For this reason calcium and magnesium-related deposits will be evident in the warmest areas of any cooling system, such as the tubes or plates of heat exchangers, or in the warm top regions of the cooling tower fill where most of the evaporation occurs.
Alkalinity Alkalinity is the presence of acid neutralizing, or acid buffering minerals, in the water. Primary contributors to alkalinity are carbonate (CO3-2), bicarbonate (HCO3-), and hydroxide(OH). Additional alkaline components may include phosphate (PO4-3), ammonia (NH3), and silica (SiO2), though contributions from these ions are usually relatively small.
Conductivity
54
Conductivity is a measurement of the water’s ability to conduct electricity. It is a relative indication of the total dissolved mineral content of the water as higher conductivity levels correlate to more dissolved salts in solution. Conversely, purified water has very little dissolved minerals present meaning the conductivity will be very low.
System Concerns Cooling towers are dynamic systems because of the nature of their operation and the environment they function within. Tower systems sit outside, open to the elements, which makes them susceptible to dirt and debris carried by the wind. Their structure is also popular for birds and bugs to live in or around, because of the warm, wet environment. These factors present a wide range of operational concerns that must be understood and managed to ensure optimal thermal performance and asset reliability. Below is a brief discussion on the four primary cooling system treatment concerns encountered in most open-recirculating cooling systems.
Corrosion Corrosion is an electro-chemical or chemical process that leads to the destruction of the system metallurgy. Figure 7 illustrates the nature of a corrosion cell that may be encountered throughout the cooling system metallurgy. Metal is lost at the anodeand deposited at the cathode.The process is enhanced by elevated dissolved mineral content in the water and the presence of oxygen, both of which are typical of most cooling tower systems. There are different types of corrosion encountered in cooling tower systems including pitting, galvanic, microbiologically influenced, and erosion corrosion. Loss of system metallurgy, if pervasive enough, can result in failed heat exchangers, piping, or portions of cooling tower itself
Figure 5.4
55
Microbiologically influenced corrosion Scaling Scaling is the precipitation of dissolved minerals components that have become saturated in solution. Figure 9 illustrates calcium carbonate scale collecting on a faucet head. Factors that contribute to scaling tendencies include water quality, pH, and temperature. Scale formation reduces the heat exchange ability of the system because of the insulating properties of scale, making the entire system work harder to meet the cooling demand.
Fouling Fouling occurs when suspended particles fall out of solution forming deposits. Common foulants include organic matter, process oils, and silt (fine dirt particles that blow into the tower system, or enter in the make-up water supply). Factors that lead to fouling are low water velocities, corrosion, and process leaks. Fouling deposits, similar to scale deposits, impede the heat exchange capabilities of the system by providing an insulating barrier to the system metallurgy. Fouling in the tower fill can plug film fill reducing the evaporative surface area, leading to lower thermal efficiency of the system.
Figure 5.5
Microbiological Activity Microbiological activity is micro-organisms that live and grow in the cooling tower and cooling system. Cooling towers present the perfect environment for biological activity due to the warm, moist environment. There are two distinct categories of biological activity in the tower system. The first being planktonic, which is bioactivity suspended, or floating in 56
solution. The other is sessile bio-growth, which is a category given to all biological activity, biofilms, or bio-fouling that stick to a surface in the cooling system. Biofilms are problematic for multiple reasons. They have strong insulating properties, they contribute to fouling and corrosion, and the bi-products they create that contribute to further micro-biological activity. They can be found in and around the tower structure, or they can be found in chiller bundles, on heat exchangers surfaces, (see Figure 10), and in the system piping. Additionally, biofilms and algae mats are problematic because they are difficult to kill. Careful monitoring of biocide treatments, along with routine measurements of biological activity are important to ensure bio-activity is controlled and limited throughout the cooling system.
Figure 5.6
Treatment Options Traditional water treatment programs are designed and implemented to account for the system concerns outlined above. This ensures the tower system operates optimally and achieves the needed cooling requirement. These programs consist of chemical additives including corrosion inhibitors, dispersants, scale inhibitors, and biocides that function to protect the cooling system and keep heat exchange surfaces clean and free of deposits or biofilms. When this is accomplished, maximum cycles of concentration can be achieved, and the cooling system can be operated at peak efficiency both in terms of water use and energy use. Beyond traditional water treatment programs there are options to build upon the current program, improve the current program, or replace the current program.
Water Modelling Software platforms provide the ability to model the system’s scaling tendencies, corrosion characteristics, and view optimal chemical application (also referred to as “dosing”). This can be a powerful tool to better understand if the system is operating at the maximum cycles of concentration possible, to see problem points in the system (low flow velocity, high temperature heat exchangers, for example), and to see impacts of varying water characteristics. Additionally, these modelling platforms allow a facility to review the potential impact of integrating water reuse resources, altering chemical treatment programs, or variations to certain operational parameters such as pH set-point. In some cases, local and regional water treatment professionals may have the capability to supply similar modelling results as a part of your existing water treatment program or for a consulting fee.
Green Chemistries “Green” chemistry programs exist primarily to replace traditional treatments that have been deemed harmful for environmental reasons. “Green” chemistries often don’t result in improved thermal efficiency or reduced water consumption, but provide environmental compliance and reduced discharge of harmful or illegal substances. Examples of “green” chemistry programs include poly-silicate corrosion inhibitors (used for many years in potable water systems), poly-aspartic acid dispersants, and hydrogen peroxide for biocide applications. Another biocide that received the 1997 Environmental Protection Agency (EPA) Green Chemistry Award is THPS – tetrakis (hydroymethyl) phosphonium sulphate. 57
Automation Automation systems are available providing a broad range of capacities to control single or multiple parameters in the cooling system such as conductivity and blow-down control, pH control, and real-time chemical monitoring and dosing. Blow-down controllers are available from several different commercial suppliers and offer a range of control points from simple conductivity/blow-down control, to timed or meter relay chemical dosing. Many of them incorporate water meter inputs and alarm relays if threshold measurements are exceeded. Blow-down controllers offer continuous monitoring and control of the blow-down of the tower system. This ensures high conductivity is avoided minimizing scaling and corrosive conditions and minimizes excessive blow-down which wastes water. Figures 11 and 12 provide two opposing trends – one showing manual blow-down control and the other showing blow-down controlled with a conductivity controller. The charts illustrate the impact of implementing blow-down controllers, revealing conductivity rates that stay much closer to the ideal set point compared to manual control. More robust automation platforms are also available from several manufacturers that provide system-wide monitoring and dosing. These platforms are scalable depending on the need, but offer conductivity/blow-down control, pH control, real-time chemical monitoring and dosing, continuous corrosion monitoring, web-enabled reporting, and alarm relays. The benefit of these systems is tightened control of the various control points of the water treatment program, not only eliminating excessive water use and high cycle conditions, but also controlling chemical residuals and treatment dosing based on real-time corrosion and scaling indices. In trend terms, similar results to the conductivity improvements shown by Figures 11 and 12 can be achieved on chemical treatment residuals, pH set point and acid feed, biocide dosing, and corrosion monitoring. Figures 13 and 14 illustrate the performance improvement real-time dosing achieves on chemical residuals, ensuring the proper dosage of corrosion and scale inhibitors at all times and eliminating overfeed or underfeed of view tower performance remotely. Additionally, alarms for out-of-compliance measurements can be sent via email or text message alerting maintenance or operations personnel so corrective actions can be taken.
Filtration Filter systems are nothing new to industrial water systems, and have been used as pretreatment in many different applications for many years. In recent years, side-stream filtration systems have become popular among many water treatment professionals. They function to remove suspended solids, organics, and silt particles down to 0.45 microns from a portion or all of the system water on a continual basis, thereby reducing fouling, scaling and microbiological activity. This allows the cooling system to work more efficiently and often reduces the amount of water blown down. However, the net impact on water consumption must consider the fact that these platforms require back-washing to clean the filter system. The amount of water used to regenerate the filter system should be added to the water lost due to evaporation and blow-down.
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Figure 5.7
Softening Softeners function to remove hardness (calcium and magnesium) from the make-up water, or can be incorporated as a side-stream system to soften a portion of the water continuously. This effectively manages (or eliminates) the amount of calcium and magnesium in the tower bulk water, thereby reducing the scaling potential of calcium and magnesium related deposits. By reducing or eliminating the scaling potential of calcium and magnesium, higher attention should be given to corrosion monitoring and the corrosive characteristics of the water. Calcium and magnesium function naturally as corrosion inhibitors, so if they are minimized or removed from the water, the corrosive conditions will increase putting more importance on the corrosion management program and tighter requirements on pH and alkalinity control.
59
Figure 5.8
Figure 5.10
60
Figure 5.10
Chemical-free Platforms In recent years many innovations have been made in systems that provide chemical-free treatment replacements to traditional treatment platforms. These include systems that are singular in their target treatment, such as ozone and ultraviolet biocide systems. These systems are very effective disrupting and eliminating biological activity in the treatment zone, are flexible across a broad pH range, have low operating costs, and no undesirable byproducts. These systems treat an isolated point in the system, so far-reaching parts of the cooling system may have bioactivity due to extended distances from the treatment application point. Additionally, sessile bioactivity in the system beyond the point treatment may not be affected by the ozone and ultraviolet treatments. There are also other types of comprehensive chemical-free platforms and the market has evolved, offering many different types of platforms, which currently have varying degrees of success in the commercial marketplace. Several have been available for many years, but the evolution of the differing technologies has allowed for many more platforms to be developed. The exact mechanisms used to alter the water’s characteristics vary from platform to platform, including electromagnetic and hydrodynamic principles as point treatments to control scaling and corrosion along with microbiological activity. Systems that employ electromagnetic principles feature a treatment zone where an electric field courses through the flow of the recirculating water creating a “seeding” mechanism for dissolved minerals, principally calcium carbonate (also known as calcite). The seeding process creates a location for other dissolved minerals to agglomerate, or come out of solution, and then be removed via in-line filtration systems that follow the treatment system. The pulses of electricity also behave to counter bioactivity by damaging cell walls of potential microbes that pass through the treatment zone. Corrosion is controlled naturally by these systems as they allow the water to run at elevated pH levels, or alkaline conditions. Similarly, systems employing hydrodynamic cavitation impact the water’s characteristics to control scaling, corrosion, and biological activity. Instead of imparting electromagnetic principles to accomplish these ends, hydrodynamic cavitation impacts the 61
water mechanically by rapid changes in pressure and mechanical collision in the treatment zone. Again, scaling is controlled through the seeding mechanism of calcium carbonate (calcite) formation, which in turn collects any other dissolved minerals nearing their respective saturation points and removes them through in-line filtration. Low pressure zones strip the passing water of CO2, maintaining naturally alkaline conditions to control corrosion. Lastly, the cavitation process ruptures cell walls of microbes passing through the treatment zone, thereby managing bioactivity in the cooling systems. These systems and many others like them have case studies and testimonials posted to demonstrate successful replacement of standard treatment options, including energy savings and water efficiency gains. They offer the advantage of reduced or eliminated need for treatment chemicals and have had many successful installations with documented improvements. However, some installations have had difficulty depending on the water quality, especially where the natural tendencies of the source water lean toward corrosive conditions. Furthermore, point treatments may have some difficulty with systems that are extensive and have far reaching-pipe runs or heat exchangers that are a substantial distance from the treatment system itself.
Water Reuse Water reuse options vary depending on the nature of water uses from site to site along with a broad range of other considerations, including local and regional water reuse laws and the availability of an adequate reuse resource. The concept is relatively simple: take discharge water from one application, apply sufficient treatment to it if needed, and then use it as a make-up water resource for the cooling tower system. Two examples of reuse are collecting the condensation from air-handlers and then pumping them back to the tower system makeup, or implementing a reverse osmosis system on the cooling tower blow-down. The condensed water that collects in air handlers is an excellent resource of high quality make-up water for the tower system. Because it is condensation, by nature it will be relatively pure, and therefore will not require much additional treatment to make it sufficient for using as tower make-up water. These systems are not difficult to engineer, nor are they typically very capital intensive.Reverse osmosis (RO) systems (similar to Figure 15) are common in many applications including desalination, industrial pre-treatment platforms, and applications where the purity requirements for water are very high. In the case of water reuse from a tower system, the blow-down water would be sent directly into an RO system, which would purify a portion of the water (the permeate flow) and concentrate the bulk of the dissolved minerals into a smaller waste stream (the concentrate flow) that would be discharged from the system. These systems can be difficult to operate correctly, and are expensive to purchase and operate. However, locations where water resources are limited or discharge permits prohibit excessive cooling tower blow-down, RO platforms are a consideration to minimize the overall losses from a cooling system.
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CHAPTER` 6 DESIGN METHODOLOGY Introduction In the world market, especially in the present industrial sector, continuous improvement on existingproduct is very risky and also a very long process, and to satisfy the customer is difficult. QFD is used to solveall these problems at ease. The industrial sectors generally require good quality, good performing products invery shorter and predictable development cycle times and at a lower cost. QFD is not only technical tool butalso managerial philosophy that can enhance the organizational and managing effects. S.A.Oke et al. (2009) improved the design characteristics, to satisfy the customer needs and to reduce the material wastage by usingQFD tool and VA tool, to get the exact solution. Quality function deployment and AHP is used to SatisfyCustomer requirements and to improve the quality attributes (Onder Enkarslan et.al. 2011). In this study, the questionnaires are used to collect data from customers. According to Marvin E.Gonzalez(2003), QFD is usedby the product development team to take good decision and to satisfy the customer requirements by limitedresources. Thequestionnaires preparation is an important process in QFD. Improvement of the product quality characteristics and reduction of the cost is based on the most customer expectation. Multiple objectiveprogramming helps to prioritize design requirements in quality function deployment.The ultimate aim is to improve the design characteristics and customer expectation in the Cooling tower, improve the quality characteristics, reduce costs, reduce the production time and increasethe production.QFD is a best tool, to satisfy the customer requirements and to improvethe products quality; it also helps in generating the new idea from design team, existing process problem areeases to identify and rectify by the use of QFD. So in this article the QFD and fuzzy QFD method is used tosatisfy the customer requirements of cooling tower. The feedback is collected from customers by themarketing department.
2. Four Phases of QFD The QFD process mainly consists of four phases, product planning, product design, process planningand process control phase. Each phase of QFD is released by a matrix consisting of a set of input (WHAT) andoutput (HOW), the output of each phase is deployed in its next phase as input.
2.1 Problem Description This study uses QFD methodology to transfer customer requirements into the quality characteristics toimprove the efficiency of cooling tower. Based on the literature survey the QFD methodology is used toanalyses the quality development in cooling tower. QFD is used to collect the data in matrix format tocapture a number of issues pertinent and vital to the planning process.
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Fig 6.1 Four phases of QFD The Cooling tower has recorded a maximum of 60 percentage cooling efficiency which iscomparatively very low with regard to other products in the market. The main design flaw which produces suchlow efficiency is found due to the defect in the fan blade design and the inlet mesh height. A fan blade is one ofthe major components present in a cooling tower which helps to drive in the air inside. The main reason for thepoor air intake is because of the unproductive fan blade angle and the tip clearance. The fan blade angle is theangle at which the blade is bending. When the fan blade is made to rotate, a low pressure is created inside, andthis low pressure sucks the air inside the cooling tower. If the blade angle is improved the quantity of air takeninside will increase which in turn improves the efficiency of the cooling tower. Improper workings of the floatvalve due to the spring malfunction, damage in solenoid valve cause the damage of water out of the basin. Thetip clearance is one of the major design considerations. The tip clearance is the distance between the fan bladetip and the outer casing. The tip clearance should be the lowest possible value in order to get more efficiency. Ifthe tip clearance is high the low pressure created will be a least value and the air flow rate will also be very less.It is seen that in cooling towers the power consumption is more. The major reason for this is because singlefan is used to drive out the exhaust air present in the cooling tower. The motor is placed straight above the fanwhich makes the exhaust air flow turbulent. This turbulent flow of air is the major reason why the motorconsumes more power. The sprinkler is another component which plays an important role in the cooling tower.The sprinkler works fully on the thrust given by the inlet water. There is greater possibility of the sprinkler poresgetting closed by the regular usage of hard water. The thrust bearings can get damaged by the impurities formeddue to hard water usage. The next major component is the basin which is also made of FRP. Highly reinforcedresin is coated on the surface of the basin to improve its strength. Improper resin coating can create crack in thebasin which may lead to the leakage of water. Incorrect fitting of bolts and other accessories can also createleakage in the basin. Because of the regular usage of hard water the fills get blocked by the salt crystals formedby the process of crystallization. Rust formation and water leakage in pump are the other problems associatedwith this product. These defects should be improved to get the required efficiency and also satisfy the customerrequirements.
65
Figure 6.2 66
Understand the Customer Voice The process of questioning people will not completely reveal everything involved in understanding thecustomers wants and needs. The work of Nortek Kano provides a model that helps us understand the overallspectrum of customer expectations and satisfaction. Figure 4. Illustrate the Kano observations. The horizontalaxis shows how well the customers think the company’s product or service met their expectation. The verticalaxis shows the degree of actual customer satisfaction with the product or service. The lower curve will serve asan example for explanation. The arrow tip at the extreme right of this curve represents customers who feel thatthe manufacturer of the product fully met their expectations. Note, that the level of satisfaction of the customersdoes not reach the maximum represented point of the vertical axis. This is because this lower curve representsissues that are basic functions or “givens” for the product or service. These are things that customers havelearned to expect. Their presence does a little to promote major satisfaction. Their absence, on the other hand,will lead to dissatisfaction.
Figure 6.3
3. Problem Solving Methods 3.1. Feedback Questionnaire, Cooling Tower The customer complaints have been collected by using below questionnaires, table.1.show the feedback questions. Table 1: Feedback Questionnaire 67
Mark in the appropriate box S.NO Questionnaires
1
1
Performance of fan
2
Operating condition of sprinkler
3
Life of casing
4
Vibration of Fan motor
5
Power consumption of fan motor
6
Quality of Product
7
Efficiency of cooling tower
8
Performance of fills
9
Life of the cooling tower
10
Colour of cooling tower
11
Function of Float valve
12
Formation of rust in the cooling tower
13
Overall Noise and vibration of cooling tower
14
Performance of Cooling tower
2
3
4
5
Scale Rating: 1.Poor 2.Fair 3.Average 4.Good 5.Excellent Table 6.1
3.2. Higher Customer Accuracy Table 6.2 is to prepare the technical descriptions based on major customer complaints. Table 6.2:Higher Customer Accuracy
S.NO
Major customer complaints
Technical descriptions
1
Power Consumption is high
a. Repair or replacement of motor b. Check the supply voltage c. Change bearing or supplement grease
2
Fan Blade Damages
3
Sprinkler problem
a. Adjust fan blade angle b. Modify the fan design c. Replace the fan d. Tighten the Blade in Hub a. Clean sprinkler pipes(weekly)
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4
Water temperature is high
a. Repair Sprinkler and Distribution System. b. Replace the mesh with louver
5
Low Pressure of water
6
Basin leakages
a. Tighten the bolts in feed pump b. Replace the feed pump with proper capacity c. Clean Water Filters in feed pump a. Tighten the (inlet/outlet) mounting bolts & replace if needed. b.Proper coating of resin inside the basin
7
Fills problem
a. Redesign the fills b. Proper arrangement of fills c. Change the fills angle
8
Rust formation
Coat the metal surface with an appropriate coating(Galvanizing +Aluminium)
9
Water leakages in feed pump
10
Improper function of float valve
a. Tighten the loose bolts in feed pump b. Proper sealing in between inlet of cold water and feed pump. a. Replace the spring b. Check the solenoid valve
3.3. Proposed QFD The QFD (quality function deployment) is a method used to find various factors involved in a product to satisfy the customer requirements. It is also used as a tool to identify defects of a product, to improve thedesign features, for competitive analysis of the product, and to analyse the cost of the product. Most of theindustries use QFD as a benchmarking tool. There are certain steps that are to be followed when working with QFD. The primary step is to preparethe questionnaire. The questionnaire is prepared based on the suggestion and ideas given by the marketingdepartment .The preparation of questionnaire can be of different types. The basic methods are webbing based,field work, direct interview and group discussion.The individual analysis ofthe component helps to find out whether the component is working good or satisfactory. 1) The first step in QFD is the preparation of customer requirement matrix based on the questionnaires.The major customer requirements are taken and studied. The high priority requirements are given more weightage, and are placed on the top order in the matrix and the low priority requirements take the lower order in the matrix. 2) The second step is to prepare the customer important matrix. In this step the importance is given as (1.very low 2.low 3.medium 4.high 5.very high). The important rating is given by the marketing department based on customer’s complaints.
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3) The third step is the preparation of technical requirement matrix. This matrix is prepared by the design team. The design team studies the customer requirements and designs the product carefully so that the customer requirements are met. Various technical data is required to design the product to put up in a matrix form. The high priority technical data gets the top order whereas the low priority data gets the low order. 4) The fourth step is the preparation of interrelationship matrix. It is also prepared based on the weightage given to each components say (1.very low 2.weak 3.more or less weak 4.medium 5.more or less strong 6.strong 7.very strong). The customer requirement matrix and the technical requirement matrix are interrelated to prepare the interrelationship matrix. 5) The fifth step is to find out the absolute weights by using formula (i). This is done by comparing interrelationship matrix and customer importance matrix. 6) The rank based on the absolute weight is found out in the sixth step. 7) The seventh step is to find the normalized individual weight by using the formula (ii). This is done by taking the ratio of absolute individual weight and maximum absolute individual weight. Normalized individual weight is calculated to compare it with the fuzzy QFD. 8) The eighth step is to find out the market score. The market score is prepared by the marketing department based on the customer requirements. 9) After finding the market score the absolute weight of the customer had to be found out in ninth steps by using the formula (iii). The product of ‘important to customer’ value and the ‘market score’ value gives absolute weight of customer. The high weightage component gets the top rank. 10) In the tenth step absolute weight in percentage is found out by taking the ratio of individual resultant weights and the resultant weight by using the formula (iv) 11) The eleventh step is to find out the relative weight of the customer by using the formula (v) The product of ‘interrelationship matrix’ value and ‘absolute weight of customer’ value give the ‘relative weight’ values. 12) The twelfth step is to find out the relative element in percentage by using the formula (vi) This is done by taking the ratio of individual relative weight and the total relative weight. 13) The final step is the preparation of customer competitive assessments and the technical competitive assessments. This step deals with the comparison of all the matrix parameters with regard to the product of other organizations. This is purely a benchmarking step. Now, after finding out all the individual matrices and assessments the QFD chart is prepared. Figure 5 shows the proposed structure of QFD.
4. Implementation of QFD Cooling tower is being designed and manufactured for the past 28 years. The pace of redesigning cooling tower to enhance performance and lower price had been accelerated indue course of time by implementing altered designs. Thus, the better design features integrated customerrequirements and product developments. Based on market survey, sales records, interviews and marketingengineering of the whole team, customer requirements are considered to prepare the QFD table. The results yield the following technical requirements to improve the designcharacteristics of cooling tower. A. Implement low power motor. B. To select the suitable fan for proper working condition. C. To select the suitable feed pump for proper working pressure. 70
D. Check the solenoid valve. E. Proper arrangement of fills inside the cooling tower. F. Fan balancing. G. Proper coating of resin inside the basin. H. Change the fills angle. I. Coat the metal surface with an appropriate coating. J. Proper sealing in between inlet of cold water and feed pump.
Fig: 6.4
Scale rating for Interrelationship matrix 1- Very Weak 2- Weak 3- More or Less Weak 4- Medium 5- More or Less Strong 71
6- Strong 7- Very Strong
Scale rating for Importance to customer Customers Complaints Scale factor(Importance)
1%-30%
31%-50%
51%-70%
71%-80%
81%-100%
1-Very Low
2-Low
3-Medium
4-High
5-Very High
Scale rating for Market score 1. Weak 2. Fair 3.Strong
Scale rating for competitive evaluations 1. Poor 2.Fair 3.Good 4.Excellent 5.Perfect
Formula Used Absolute weight W = (C1j X I1) + (C2j X I2) +…., + (Cnj X In) Let, W = Absolute weight, C = Correlation of the customer and technical requirements, I = Importance of customer requirements.
(i)
Normalized individual weight =Absolute individual weight (ii) Maximum absolute individual weight Absolute weight of Customer = Importance to customer x Market score (iii) Absolute weight in percentage =
Individual resultant weights Absolute weight
(iv)
Relative Weight of Customer = Interrelationship matrix x Absolute weight of customer (v) Relative element in percentage =Individual Relative weight of the customer Relative weight of customer
(vi)
5. Implementation of Fuzzy QFD Fuzzy logic system is a type of probability logic which deals with the analysis of approximate,fixed and exact values, with respect to the traditional QFD theory. The fuzzy logic system uses a principle thatinvolves true value that range between 0 and 1. The results associated with the values between the ranges 0 to 1are termed as partial truth values which can be taken as completely true and completely false. Fuzzy logicsystem uses linguistic variables (VW,W,MW,M,MS,S,VS,VL,L,MI,H,VH,W,F,S) that are managed by specific function. Fuzzy logic system was introduced by Professor L.A.Zadeh in 1965. Fuzzy logic 72
system ismainly used to solve under defined or ill-defined problems to obtain accurate solutions. It is also used indecision making problems. The fuzzy logic system uses certain rules, concepts, theories, procedure andprinciples to solve the problems with respect to the input parameters. The logic principle used in this system isIF THEN rules. Fuzzy logic and fuzzy set uses linguistic variables in problem solving. Fuzzy logic systems inherit certain characteristics features which make it better than other logic systems. 1) The decision making is simple with incomplete and unknown information. 2) It is widely used in reasoning problem. 3) It is used in interpretations and easy to understand. 4) It is used in variable input and variable output systems.
6. Results and Discussion From the web based questionnaires, the QFD was prepared. The above customer requirements are incorporated in thedesign of the Cooling tower in the descending order of their absolute weight and relative weight. It isobserved that all the technical requirements belonging to the Cooling tower has to be analyzed andappropriate design characteristics has to be improved based on customer requirements. The results obtained from the fuzzy analysis are plotted in graph. The parameters whichshow very strong preference must be given higher priority. The higher priority problems are analyzed andredesigned to solve the problems. The problems solving steps must start from very strong preference to veryweak preference. Thus by using the fuzzy QFD method the design characteristics of Cooling tower isimproved based on the customer requirements.
7. Conclusions In this study, the customer requirements and technical requirements of the cooling tower was analyzed using QFD and FUZZY QFD, which are very efficient way of making the attributes of an Cooling tower. This technique demonstrates the application of QFD tool in the design of acooling tower with the aimto achieve customer satisfaction. The procedures developed in this article provide result in a QFD matrix thatwas constructed based on the customer and the technical requirements. The correlation between the customersand the technical requirements were used to identify the most important of customer requirement. It is hoped that the technique and methodology demonstratedin this study will promote the application of fuzzy QFD in the cooling tower industry extensively. This studyestablishes that the final outcome of a cooling tower design depends on the customer behavior during thequestioning phase.
CHAPTER 7 73
MATHEMATICAL MODELLING OF COOLING TOWERS Analysis of cooling towers is carried out to optimize the cooling tower efficiency. Research work has continued over decades in this field. It aims at energy conservation and monetary savings for the industry. The first serious in cooling tower analysis was made by I .V .Robinson on natural draught cooling towers. The performance was reduced to a figure of merit. His calculations for driving force were based upon the assumption that heat transfer depended upon the mean temperature difference instead of total heat content of air. He also tried to determine a friction constant for the tower which combined the shell resistance and the water lift weight. In 1922, a namesake, P. Robinson and C. S. Roll presented a thesis on cooling tower performance, followed by a theoretical analysis by Walker, Lewis and McAdams in 1923. Both the authors developed the basic equations for total mass and energy transfer and considered each process separately. Coffey and Horne examined the thermal equilibrium of wet bulb temperature and its relation to heat transfer. In 1925 Merkel combined the coefficient of sensible heat and mass transfer into a single over-all co-efficient based upon the enthalpy difference as a driving force. A more rigorous analysis of cooling tower model that relaxed Merkel’s restrictions was given by Sutherland in 1983. In 1989, Braun developed effectiveness models for cooling towers, which utilized the assumption of a linearized air saturation enthalpy and the modified definition of number of transfer units. However, Braun’s model needs iterative computation to obtain the output results and is not suitable for online optimization.Bernier (1994) reviewed the heat and mass transfer process in cooling towers at water droplet level and analysed an idealized spray-type tower in one-dimension, which is useful for cooling tower designers, but not much information is provided to plant operators. Austin (1997) recommended regression methods to create the models of each component in air conditioning systems for predicting and optimizing the system performance. Flake (1997) utilized a different regression technique to determine parameters of the cooling tower model developed by Braun (1989) and to build a predictive model for optimal supervisory control strategies. In 1999, Soylemezpresented a quick method for estimating the size and performance of forced draft counter-current cooling towers and experimental results were used to validate the prediction formulation. Unfortunately, this model also need iterative computation and not suitable for online optimization.
MERKEL METHOD 74
Introduction Merkel developed the theory for the evaluation of the thermal performance of cooling towers in 1925. This analysis is very popular and widely applied. The Merkel theory relies on several critical assumptions to reduce the solution to a simple hand calculation. Because of these assumptions, however, the Merkel method does not accurately represent the physics of the heat and mass transfer process in the cooling tower fill. The simplifying assumptions of the Merkel theory are: Assumption l: The Lewis factor relating heat and mass transfer is equal to 1. Assumption 2: The reduction of water flow rate by evaporation is neglected in the energy balance. Assumption 3: The air exiting the cooling tower is saturated with water vapor and it is characterized only by its enthalpy. The more rigorous method to evaluate cooling tower performance was developed by Poppe and Rogener in the early seventies. The Poppe method does not make the simplifying assumptions made by Merkel. The critical differences between the Merkel and Poppe methods are investigated by Kloppers and Kroger. Procedures to improve the accuracy of the Merkel method, and the cooling tower operating conditions under which they are valid, are discussed in the present study. The analysis combines the sensible and latent heat transfer into an over-all process based on enthalpy potential as the driving force. The process is shown schematically in Figure 1 where each particle ofbulk water in the cooling tower is assumed to be surrounded by an interface to which heat is transferred from the water.This heat is then transferred from the interface to the main air mass by (a)Transfer of sensible heat, and (b)By the latent heat equivalent of the mass transfer resulting from the evaporation of aportion of the bulk water. The two processes are combined, ingeniously, into a single equation
This is accomplished in part by ignoring any resistance to mass transfer from bulk water to interface; by ignoring the temperature differential between the bulk water and interface; and by ignoring the effect of evaporation. The analysis considers an increment of a cooling tower having one square feet of plan area, and a cooling volume V containing a sq ft of exposed water surface per cubic foot of volume. The flow rates are L lb of water and G Ib of dry air per hour. 75
Two errors are introduced when the evaporation loss is ignored. The water rate varies from L at the water inlet to (L – LE) at the outlet
Gdh = Ldt,
(4)
Gdh = Ldt + Gdh(t2 – 32)
(5)
Equation (2) or (3) conforms to the transfer-unit concept in which a transfer-unit represents the size or extent of the equipment that allows the transfer to come to equilibrium. The integrated value corresponding to a given set of conditions is called the Number of Transfer Units (NTH), which is a measure of the degree-of-difficulty of the problem. The equation is not self-sufficient so does not lend itself to direct mathematical solution. The usual procedure is to integrate it in connection with the heat balance expressed by equation (4). The basic equation reflects mass and energy balances at any point within a cooling tower, but without regard to the relative motion of the two streams. It is solved by some means of mechanical integration that considers the relative motion involved in counter-flow or crossflow cooling, as the case may be.
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Figure 7.1 Water entering the top of the cooling tower at t, is surrounded by an interfacial film that is assumed to be saturated with water vapour at the bulk water temperature. This corresponds to point A on the saturation curve. As the water is cooled to t2, the film enthalpy follows the saturation curve to point B. Air entering the base of the cooling tower at wet-bulb temperature TWB has an enthalpy corresponding to C' on the saturation curve .The driving force at the base of the cooling tower is represented by the vertical distance BC. Heat removed from the water is added to the air so its enthalpy increases along the straight line CD, having a slope equalling the L/G ratio and terminating at a point vertically below point A. Air and water conditions are constant across any horizontal section of a counter-flow cooling tower. Both conditions vary horizontally and vertically in a cross-flow cooling tower as shown in Figure 3. Hot water enters across the OX axis and is cooled as it falls downward. The solid lines show constant water temperatures. Air entering from the left across the OY axis is heated as it moves to the right, and the dotted lines represent constant enthalpies. Because of the horizontal and vertical variation, the cross section must be divided into unitvolumes having a width dx and a height dy, so that dV in equation (1) is replaced with dxdy and it becomes Ldtdx = Gdhdy = Kadxdy(h' -h)
(6)
Cross-sectional shape is taken into account by considering dx/dy = w/z so that dL/dG = L/G. The ratio of the overall flow rates thus apply to the incremental volumes and the integration considers an equal number of horizontal and vertical increments. 77
Figure 7.2
Development of Basic Equations Heat is removed from the water by a transfer of sensible heat due to a difference in temperature levels, and by the latent heat equivalent of the mass transfer resulting from the evaporation of a portion ofthe circulating water. Merkel combined these into a single process based on enthalpy potential differences as the driving force. The analysis considers an increment of a cooling tower having one square feet of plan area, and a cooling volume V, containing a square feet of exposed water surface per cubic foot of volume. Flowing through the cooling tower are L lb of water and G lb of dry air per hour .
Transfer Rate Equations The air at any point has a dry bulb temperature T, an absolute humidity (lb water vapour per lb dry air) H, and a corresponding enthalpy h. The water, having a bulk temperature t, Figure 1 is surrounded by an interfacial film having a temperature T'. The temperature gradients are such that T < T' < t. The specific heat of water is assumed to be unity and a constant, so the symbol will be omitted from the equations for simplicity. The rate of heat transfer from the bulk water to the interface is: dqW = Ldt = KLasV(t – T')
(11)
A portion of this heat is transferred as sensible heat from the interface of the main air stream. This rate is: dqS = KGadV(T' – T)
(12)
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The interfacial air film is assumed to be saturated with water vapour at temperature T', having a corresponding absolute humidity H". The procedure is to ignore any resistance to mass transfer from the water to the interface, but to consider the mass transfer of vapour from the film to the air, as dm = K'adV(H' – H) (13) Considering the latent heat of evaporation as a constant, r, the mass rate in equation (13) is converted to heat rate by multiplying by r rdm = dqS = rK'adV(H' – H) (14)
Mass and Energy Balances Under steady state, the rate of mass leaving the water by evaporation equals the rate of humidity increase of the air, so dm =GdH
(15)
The heat lost by the water equals the heat gained by the air. The usual practice is to ignore the slight reduction in L due to evaporation, in which case GdH = Ldt
(16a)
A more rigorous analysis considers evaporation loss, so L lb enters but (L – LE) lb of water leaves the cooling tower, and the heat balance is G(h2 -h1) = L(t1 – 32) – (L – LE)(t2 – 32)
(a)
G(h2 – h1) = L(t1 – t2) + LE(t2– 32)
(b)
LE = G(H2 – H1) G(h2 – h1) = L(t1 – t2) + G(H2 – H1) (t2 – 32) (c) The enthalpy of moist air is defined as h = cpa (T -T0) + H[r + cpv(T -T0)] Both H and T are variables, so the differential is dh = cpadT + H cpv dT + [r + cpv (T -T0)]dH or 79
dh = (cpa + H cpv) dT + [r + cpv (T -T0)]dH Humid heat is defined as s = cpa + Hcpv dh = sdT + [r + cpv(T – T0)]dH (17) in which the first term on the right represents sensible and the second latent heat. Equating dh in equation (16a) and (17) Ldt = GsdT + [r + cpv(T – T0)]GdH
(18)
Fundamental Equations The sensible heat relationship dqS = GsdT is used to convert equation (12) to dqS = KGadV(T' – T) = GsdT
(19)
The mass-transfer relationship dm = GdH is used to convert equation (13) to dm = K'adV(H' – H) = GdH
(20)
Lewis found that, for a mixture of air and water vapour .The ratio differs for other gases and vapours, but it fortuitously approaches unity for moist air. The relationship expressed in equation (21) incidentally, explains why the wet-bulb approximates the temperature of adiabatic saturation for an air-water mixture. Substituting KG = K's in equation (19) dqS = K'sadV(T' – T) = GsdT
(22)
Substituting equation (22) for GsdT' and equation (20) for GdH in equation (18) Ldt = K'sadV(T' – T) + [r + cpv(T – T0)]K'adV(H" – H) Collecting Ldt = K'adV{s(T' – T) + [r + cpv(T – T0)](H" – H)} (23) From the enthalpy equation, we get for the air stream h = cpa (T – T0) + H[r + cpv (T – T0)] 80
h = cpaT – cpaT0 + Hr + HcpvT – HcpvT0 h = cpaT + HcpvT – cpaT0 + H(r – cpvT0) Since s = cpa + Hcpv h = sT – cpaT0 + H(r – cpvT0)
(24)
Similarly, the enthalpy of the interface is h" = aT' – cpaT0 + H'(r – cpvT0)
(25)
Solving equations (24) and (25) for T and T', substituting the results in equation (23) and collecting Ldt = K'adV [(h' – h) + cpvT(H" -H)] (26) The second term on the right is relatively small so, following the example of Merkel, it is customarily dropped. Doing this and equating to equation (16a) Ldt = K'adV(h" -h) = Gdh
(27)
This final equation relates the air stream to the interfacial film, the conditions of which are indeterminate for all practical purposes. This difficulty is overcome by a final approximation in which T' is assumed to equal T. The coefficients KG and K' are then replaced by overall coefficients Kg and K, respectively. Assuming the Lewis relationship still applies Kg/Ks≅ 1
(28)
There is no fundamental reason why this should be so, and Koch reports the ratio is more nearly equal to 0.9 but common practice assumes it to apply. Using equation (28) instead of equation (21), the development from equation (22) on yields Ldt = KadV(h' – h) = Gdh
(29)
Integrating
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Equations (30a) and (30b) are convertible into one another and are two forms of the basic equation. The Poppe Method Without the simplifying assumptions of Merkel, the mass and energy balances from Fig. I and Fig. 2, yield after manipulation for unsaturated air3,
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Wherethe Lewis factor is defined as Bosnjakovic proposed the following relation to express the Lewis factor for air-water vapour systems:
The transfer coefficient or Merkel number according to the Poppe method is given:
The varying mass flow rate ratio in Eq. (4) and Eq. (5) can be determined by considering the control volume in the fill of Fig.3.A mass balance of the control volume in Fig. 3 yields,
83
Equations (4) to (7) are only valid if the air is unsaturated. If the air is supersaturated, the governing equations are,
The Merkel number according to the Poppe method is given by
The equations according to the Poppe method must be solved by an iterative procedure because wo in Eq. (8) is not known a priori.
A UNIVERSAL ENGINEERING MODEL FOR COOLING TOWERS This part is going to presents a universal engineering model which can be used to formulate both counter-flow and cross-flow cooling towers. By using fundamental laws of mass and energy balance, the effectiveness of heat exchange is approximated by a second order polynomial equation. Gauss-Newton and Levenberg-Marquardt methods are then used to determine the coefficients from manufactures data. Compared with the existing models, the new model has two main advantages: (1) As the engineering model is derived from engineering perspective, it involves fewer input variables and has better description of the cooling tower operation; (2) There is no iterative computation required, this feature is very important for online optimization of cooling tower performance. Although the model is simple, the results are very accurate. 84
Austin (1997) recommended regression methods to create the models of each component in air conditioning systems for predicting and optimizing the system performance. Flake (1997) utilized a different regression technique to determine parameters of the cooling tower model developed by Braun (1989) and to build a predictive model for optimal supervisory control strategies. Attempts to develop the cooling tower models have a relative long history, the first such work may trace back to 1925, when Merkel developed a practical model for cooling tower operation, which has been the basis for most modern cooling tower analyses. In his model, the water loss of evaporation is neglected and the Lewis number is assumed to be one in order to simplify the analysis. However, as evaporate water cannot be neglected in cooling tower operation, Merkel’s model is not accurate enough and not suitable for real applications. A more rigorous analysis of a cooling tower model that relaxed Merkel’s restriction was given by Sutherland (1983). In 1989, Braun developed “effectiveness models” for cooling towers, which utilized the assumption of a linearized air saturation enthalpy and the modified definition of number of transfer units. However, Braun’s model needs iterative computation to obtain the output results and is not suitable for online optimization. Bernier (1994) reviewed the heat and mass transfer process in cooling towers at water droplet level and analysed an idealized spray-type tower in one-dimension, which is useful for cooling tower designers, but not much information is provided to plant operators. Soylemez (1999) presented a quick method for estimating the size and performance of forced draft countercurrent cooling towers and experimental results were used to validate the prediction formulation. Unfortunately, this model also need iterative computation and not suitable for online optimization.In this part, a universal engineering model, which can be used to formulate both counter-flow and cross-flow cooling towers, is proposed. COOLING TOWER MODEL ANALYSIS The mechanism of heat and mass transfer between ambient air and condenser water inside a cooling tower is illustrated in Figure 1. Four governing equations can be used to express the mass and energy balance in the system: (1) Mass conservation of air: ma,i + me = ma,o (1) (2) Heat conservation of air: ma,i ha,I + Qrej – Qe = ma,o ha,o
(2)
(3) Mass conservation of condenser water: mw,I – me + mm = mw,o
(3)
(4) Heat conservation of condenser water: Mw,i Tw,i Cpw – Qrej + mm Tm Cpw = mw,o Two Cpw (4) In the governing equations, there are nine known parameters including: six input variables,ha,i, ,ma,i, mw,i, mm,Tm, Tw,i ; a constant Cpw; and two measurable output variablesmw,o,Tw,o, , and five unknowns: three output variables: ha,o, ma,o,and Qrej; and two immeasurable variables meand Qe. As the unknown variables are more than the number governing equations, it is insufficient to determine all outlet conditions by the four governing equations alone, additional equations that could depict the characteristics of the cooling tower should be added. In Braun’s model with effectiveness coefficient (1989), the derivative of 85
saturation air enthalpy with respect to temperature, Cs, is introduced and used to formulate the cooling load model. εais also added as a ratio of the actual heat transfer amount to the theoretical maximum amount.
Figure 7.6: Schematic representation of heat and mass transfer in the cooling tower
Analogous to a dry counter-flow heat exchanger, the effectiveness, εa, is evaluated by
Where NTU, m*, and Cs calculated respectively by
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Although Braun’s model is more accurate than Merkel’s one, it is having several problems. • The computations are very complicated, it needs iterative computation to get the final results, and the estimated outlet water temperature is needed before calculation; • It is hard to find the function derivatives, which are useful in real-time optimization analysis; • The model was derived based on mechanical principles, it only suitable for the counter-flow cooling towers. For the cross-flow cooling towers, a different model is needed. ENGINEERING MODEL DEVELOPMENT Since the main difficulties in real-time application of Braun’s model are the initial estimation of Csand highly nonlinearities of εa which resulted in a complicated and time consuming computation. To develop an effective engineering model, we will analysis both Csand εafrom fundamental laws of mass and energy balance. Analysis Cs In Braun’s model (1989), a straight line between water inlet temperature and water outlet temperature on the air saturation enthalpy with respect to temperature is used to approximate the curve between water inlet point and water outlet point (Figure 2), where Csis the ratio of length of line hs,w,I – hs,w,o (1) to line Tw,I –Tw,o (2).For control and optimization purpose, however, Tw,o, and hs,w,o, are output variables, which need to be controlled, therefore, these two variables should not be used as input variables to calculate the heat rejection ration. Instead, we may express Equation (9) with measurable variables as.
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Where, ΔT is the approach of the cooling tower and represents the difference between line (4) and line (2) in Figure 2; Δh is the saturated air enthalpy difference with respect to ΔT. By energy and mass conservation laws, the approach, ΔT, is a function of (ma/mw) and (Tw,i-Twb), as the approach is affected by the mass flow rate of both air and water and the temperature difference between inlet water and ambient air. Δh can be considered as a functionof ΔT, also the function of (ma/mw) and ( Tw,i-Twb), Therefore, Cs can be described as:
Analysis εa From the Equation (6), (7), and (8), it clear shows that the heat transfer effectiveness, εa, is the function of NTU and m*, where NTU is the function of (ma/mw)and m* is the function of (ma/mw) and Cs. By Equation (11), Cs is the function of (ma/mw) and ( Tw,i-Twb), Then, we can obtain a general expression for εaas:
Where x= (ma/mw) and y = (Tw,i-Twb),The heat transfer effectiveness is the function of two variables, which are the inlet conditions of the cooling tower. As finding the exact function for Equation (12) is neither practical nor necessary for real-time application, the following engineering solution is proposed.
Engineering model In order to solve the problem above, Taylor’s series expansion is used as an approximation of the unknown function in Equation (12). It is clear that εais a continuous variable under normal operating conditions, its derivative and high-order derivatives exist. Thus, we can apply Taylor’s series expansion for two variables into εa function. Because the characteristics of cooling towers are highly nonlinear, second-order Taylor’s series expansion is used to better reflect the nonlinearity.
Where, (x0, yo) is any reasonable operating point of cooling tower near (x, y). Once the point(x0, yo) is determined,
88
and
can be treated as constant. To express the equation in neat way, Equation (13) is rearranged and written as a function of two variables form.
Where, the coefficients, c0 - c5, are constants, and determined only by the cooling tower characteristics, which depend on the towers’ structure and design.
ALGORITHMS FOR DETERMINING ENGINEERING MODEL The real performance data of the cooling tower provided by manufacturers are used in our method. The objective function is given as:
where the function(.) is the right hand side of Equation (14) and the real performance data of cooling tower are represented by Fdatai. N is the number of the sampling points. Fdatai can be derived from manufacturers’ data bylookup-table or interpolation. In order to obtain accurate results, the number of sampling points must more thanthat of coefficients, i.e. N > 5. Furthermore, the sampling points should be distributed evenly among the wholerange of operation.Nonlinear least square method for curve fitting is used to solve Equation (15), both Gauss-Newton andLevenberg-Marquardt methods are implemented in the optimization algorithms (Coleman et al. 1999). In Gauss-Newton method, a search direction dk is obtained at each major iteration step. The search direction is expressed as:
Where (c1,c2,c3,c4,c5)Tand uk the u value of the kth iteration;
89
J (uk) is the Jacobian matrix with respect to uk. In the case of H (uk) (Hessian Matrix of F i(uk)) is significant, Levenberg-Marquardt method is adopted. It uses a search direction between the Gauss-Newton direction and the steepest descent. This makes it less effective but more robust than the Gauss-Newton method. The Levenberg- Marquardt method is given by
In this equation, λk controls both magnitude and direction of dk . When λk is zero, the direction dk is identical to that of the Gauss-Newton method. As λk tends to infinity, dk tends towards a vector of zeros and a steepest descent direction.
Remarks 1. In this method, the coefficients c0 - c5 are determined offline by curve fitting in the whole operating range. Therefore, the real-time output calculation is straightforward once the input variables are measured. 2. For more accurate results, it is possible to construct a look up table for coefficients c0 - c5 by dividing the whole operating range into sub-regions. One set of coefficients is selected at one time according to the cooling tower operation conditions. 3. In Bruan’s model, both NTU and εaare exponential functions which will require substantial computing effort. In the new model εais in a polynomial form which is much easier to calculate and suitable for on-line optimization. 4. For cross-flow cooling towers, the analysis is almost same except Equation (6), which takes the following form according to the heat exchange principle.
However, this change will not affect the model structure. The differences of the different cooling tower models are determined by coefficients of Equation (14). Therefore, both counter-flow and cross-flow cooling towers can be represented by the same model. 5. In practice, it is very hard to measure the inlet and outlet airflow rate (ma,i, and ma,o ) accurately. This problem could be solved as follows: • Using energy conservation principle, we can replace • Writing
Equation (19), according to the known variables: εa, Tw,i,Tw,b, andmw,o , Equation (14) is again used inversely to find the value of the mass airflow rate, a m& . The value will then be employed to determine the overall heat rejection rate at the next sample time. CONCLUSION ON UNIVERSAL MODEL The new engineering model for cooling towers, which can be used to formulate both counterflow and cross-flow cooling towers, has been presented in this paper. The methods of Merkel and Braun and fundamental laws of mass and energy balance are used to develop the effectiveness of heat exchange with polynomial form.Nonlinear least square curve-fitting methods are used to determine the coefficients of the model. Some engineering 90
considerations are also discussed. The comparison study of existing and the new model is given to show that the new model can predict the performance of both counter-flow and cross-flow cooling tower accurately with less computation. As the manufacture data are used to determine the coefficients for the model, it is predicted that it should have better performance compared with the existing ones. In practice, many unpredictable factors affect the performance of the cooling towers, such as outdoor airflow rate, interior problems of cooling tower, and measurement errors, etc. Therefore, the coefficients of cooling tower model may not be constant during the operational life span. Fault detection or adaptive scheme should be added to accommodate these changes; these aspects are also subject to future study.
CRITICAL INVESTIGATION OF LEWIS FACTOR IN HEAT AND MASS TRANSFER ANALYSIS The Lewis factor, Lef, appears in the governing equations of the heat and mass transfer processes (evaporative cooling) in a wet-cooling tower according to Merkel and Poppe and Rögener. Merkel assumed that the Lewis factor is equal to 1 to simplify the governing equations while Poppe and Rögener used the equation of Bosnjakovic to express the Lewis factor in their more rigorous approach. This approach is commonly known as the Poppe method and will be referred as such in this paper. The analysis of Poppe is employed in the current investigation as the value of the Lewis factor can be explicitly specified. The influence of the Lewis factor on wet-cooling tower performance can therefore be critically evaluated under a wide range of ambient conditions. There is a common misconception among researchers who refer to the Lewis number, Le, as the Lewis factor, Lef. The relation between the Lewis number and the Lewis factor is explained.
2. Lewis number The derivation and significance of the Lewis number, Le, is explained by its analogy to the derivation of the Prandtl, Pr, and Schmidt, Sc, numbers. The rate equation for momentum transfer is given by Newton’s law of viscosity
The rate equation for heat or energy transfer is given by Fourier’s law of heat conduction,
The rate equation for mass transfer is given by Fick’s law of diffusion,
The diffusivitiesν,αand D in Eqs. (1)–(3) have dimensions of [L2/T ], where L and T refer to the length and time scales respectively. Any ratio of two of these coefficients will result in a dimensionless number. In systems undergoing simultaneous convective heat and momentum transfer, the ratio of νto αwould be of importance and is defined as the Prandtl number, 91
In processes involving simultaneous momentum and mass transfer the Schmidt number is defined as the ratio of νto D,is defined as the Lewis number,
From Eq. (6) it can be seen that the Lewis number is equal to the ratio of the Schmidt to the Prandtl number and is relevant to simultaneous convective heat and mass transfer. The temperature and concentration profiles will coincide when Le = 1.
Lewis factor In addition to the Lewis number, Le, the Lewis factor, or Lewis relation, Lef, can be defined: it gives an indication of the relative rates of heat and mass transfer in an evaporative process. In some of the literature encountered there seems to be confusion about the definitions of these dimensionless numbers and the Lewis factor is often incorrectly referred to as the Lewis number. The Lewis factor, Lef, is equal to the ratio of the heat transfer Stanton number, St, to the mass transfer Stanton number, Stm where
Where, Nu= Nusselt number, or dimensionless heat flux, Sh= Sherwood number, or dimensionless mass flux. The Lewis factor can be obtained by dividing Eq. (7) by Eq. (8),
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Lewis tried to prove analytically that Lef = 1 for gas/liquid systems. In a later article he stated that the relation, Lef = 1, holds approximately for air/water mixtures but not for all liquid–gas mixtures. Although the proof given by Lewis was incorrect, the ratio h/cphm is today known as the Lewis factor. From the Chilton–Colburn analogy power law relations it follows that,
Bourillot states that the Lewis number is not constantand is tied to the nature of the vapour– gas mixture. It also dependson the nature of the boundary layer near the exchangesurfaces and the thermodynamic state of the mixture.Bosnjakovic pointed out that the mass transfer is not proportionalto the humidity potential, (wsw− w). A correctorterm, F(ξ), is applied to Eq. (12) and the expression for Lefin the Bosnjakovic form is obtained.
Where,d = Molecular weight of water/Molecular weight ofair = 0.622. Poppe and Rögener cited that the Lewis factor, Lef, is according to the Bosnjakovic form,
Where the Lewis number, Le, is assumed constant at 0.865. Bourillot and Grange state that the Lewis factor, Lef, for a wet-cooling tower, using Eq. (14), is approximately 0.92.In his classical work on evaporation Merkel assumed that Lef = 1. Häszler reports that the assumption of Merkel is not correct and that Lewis factors are in the range from 0.5 to 1.3. An analysis of both splash and film packings by Feltzin and Benton indicates that for counter flow towers a Lewis factor of 1.25 is more appropriate. Sutherland 93
used a Lewis factor of 0.9 in his “accurate” tower analysis. Häszler states that when the humidity potential (wsw−w) is large, Eq. (12) is not valid any more
Influence of Lewis factor on cooling tower performance evaluation Three different specifications of the Lewis factor are employed in this investigation to determine the effect of the Lewis factor on the performance prediction of natural draft wetcooling towers. The trends that are observed are applicable to mechanical draft towers as well. Eq. (13) is employed, which predicts Lewis factors of approximately 0.92, as well as the limiting values cited by Häszler of 0.5 and 1.3. The same definition of the Lewis factor is employed in the fill performance analysis and the subsequent cooling tower performance analysis. The method employed in this investigation to solve the governing heat and mass transfer equations for counter flow cooling towers, according to the Poppe method, is described in detail in Kloppers and Kröger. The application of these equations to cooling tower performance evaluation can be found in Kloppers and Kröger. As already mentioned, the Poppe method is employed to study the influence of the Lewis factor on cooling tower performance as the Lewis factor can be explicitly specified. Ambient air temperatures of 280, 290, 300 and 310 K are considered. The humidity of the air is varied from completely dry to saturated conditions at each of the four selected temperatures. The effect of the Lewis factor on cooling tower performance can therefore be determined over a wide range of atmospheric conditions.
Heat rejection rate The higher the Lewis factor, the more heat is rejected. In the natural draft cooling tower at an ambient temperature of 280 K the differences in heat rejection rates, between the analyses with Lewis factors of 0.5 and 1.3, are approximately 2.4%. The difference is 0.8% at 290 K andapproximately zero at 300 K. At 310 K in very dry conditions, the difference is almost 5% where the heat rejected, due to the smaller Lewis factor, is more than that predicted by the higher Lewis factor.When the inlet ambient air is relatively hot and humid there is virtually no difference between the results as can be seen in Fig. 1(b).
Water outlet temperature Because more heat is rejected at higher Lewis factors, the corresponding water outlet temperature is lower. The discrepancy between the water outlet temperatures, by applying Lewis factors of 0.5 and 1.3, respectively, is approximately 0.6 K at an ambient temperature of 280 K for all humidity and at 310 K for very low humidity. This discrepancy is practically zero at 300 K. When the inlet ambient air is relatively hot and humid there is again virtually no difference between the results of the different definitions of the Lewis factor for the water outlet temperatures. When the transfer coefficient, or Merkel number, is determined during a cooling tower fill performance test, the water outlet temperature is known. In the subsequent cooling tower performance test, the transfer coefficient is known and the water outlet temperature is generally unknown. The outlet water temperature can therefore be determined from theknown transfer coefficient. If the same method of analysis is employed, at the same ambient conditions, in the fill performance test and the subsequent cooling tower performance test, the water outlet temperature must be the same regardless of the method of analysis (i.e. Merkel or Poppe) that are employed. But it can be seen that the water outlet temperature is not the same, even though the same analysis and definitions are employed in the fill performance test and the subsequent cooling tower performance analysis. It is 94
therefore imperative that the cooling tower performance be evaluated at the same ambient conditions under which the performance of the fill was tested.
Water evaporation rate The water evaporation rate is higher when applying smaller Lewis factors than with higher ones. Thus, the air becomes saturated more quickly with lower Lewis factors. The discrepancy between the water evaporation rates in cooling towers with Lewis factors of 0.5 and 1.3 is approximately 15% at 280 K and reduces to 6% at 310 K. The Merkel method tends to underestimate the amount of water that evaporates when compared to the Poppe analysis, but that the discrepancy decreases with increasing ambient temperatures. The results of Grange were verified by the authors. It is therefore clear that the influence of the assumptions and definitions of the Merkel and Poppe analyses regarding the Lewis factor, on the results of cooling tower performance, diminishes when the inlet ambient air is relatively hot and humid.
Air outlet temperature The higher the Lewis factor, the higher the air outlet temperature. It can be seen from Fig. 4(a) that the trends for the predicted air outlet temperatures, according to the different Lewis factors, are not the same. This is because the outlet air is unsaturated for the Lef = 1.3 case below a humidity ratio of approximately 0.0035. For the other cases in Fig. 4(a) and all the cases in Fig. 4(b), the outlet air is supersaturated with water vapour. As already mentioned, the outlet air becomes saturated more quickly for lower Lewis factors. The Lewis factor is consistently applied when the same definition or equation of the Lewis factor is employed in the fill performance analysis and in the subsequent cooling tower performance analysis. The Lewis factor has little influence on the water outlet temperature and the heat rejected from the cooling tower in very humid ambient air. In dry conditions, at all ambient temperatures considered, the differences between the results of the different Lewis factors can be quite significant. The rate of water evaporation is strongly dependent on the Lewis factor for both the natural draft and mechanical draft towers. This is because the Lewis factor is an indication of the relative rates of heat and mass transfer in an evaporative process. The Lewis factor can therefore be tuned to represent the physically measured evaporation rates and outlet air temperatures more closely in fill performance analyses. It is therefore important to perform the fill performance tests in conditions that closely represent actual operational conditions, especially if the cooling tower is operated at a very low ambient humidity. If the fill performance test data is insufficient to accurately predicts theLewis factor of a particular fill, it is recommended that the equation of Bosnjakovic be used as the numerical value is approximately 0.92, which is approximately the mean between the limiting values of 0.5 and 1.3 given by Häszler. .
95
Fig 7.8. The difference between the water outlet temperatures predicted by three different values of Lewis factors for varying atmospheric humidity at 280 and 310 K.
96
Fig.7.9. The difference between the heat rejection rates predicted by three different values of Lewis factors for varying atmospheric humidity at 280 and 310 K.
The inconsistent application of the Lewis factor The analyses of the natural draft cooling towers are repeated with an inconsistent application of the Lewis factor specification, i.e. the equation of Bosnjakovic is used in the fill performance evaluation, while Lewis factors of 0.5 and 1.3 are used in the cooling tower performance evaluation. The inconsistent application of the Lewis factor results in larger discrepancies than is the case with the consistent application of the Lewis factor. The discrepancy between the heat rejection rate is approximately 8% at an ambient temperature of 280 K. The discrepancy is only 2.4% where the Lewis factors are applied consistently. The discrepancy reduces at higher ambient temperatures to approximately 2% at 310 K. This is consistent with the conclusion reached previously, that the influence of the Lewis factor diminishes at higher ambient temperatures. The discrepancy in the water outlet temperature
97
Fig.7.11. The difference between Fig.7.10. The difference between the air outlet temperatures the water evaporation rates predicted by three different predicted by three different values of Lewis factors for values of Lewis factors for varying atmospheric humidity at varying atmospheric humidity at 280 and 310 K. 280 and 310 K. and air outlet temperature for the natural draft cooling tower, for the inconsistent analysis of the Lewis factor, is larger than the consistent application.
Conclusion Exactly the same definition of the Lewis factor must be employed in the fill performance analysis and in the subsequent cooling tower performance analysis. The fill performance test must be conducted at, or as close as possible, to conditions specified for operation of the cooling tower for which it is intended. The influence of the Lewis factor, on the performance evaluation of wet-cooling towers, diminishes when the inlet ambient air is relatively hot and humid. For increasing Lewis factors, the heat rejection rate increases,the water outlet temperature decreases and the water evaporation rate decreases.
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CHAPTER 8 ANALYSIS OF COOLING TOWER IN ZUARI AGRO CHEMICALS LIMITED – GOA Range and approach data over the year period Time of taking reading Morning: 06:00 a.m.
Afternoon: 03:00 p.m.
Night : 11.00 p.m.
Make up water: 70 litres/hour Blow down: 2 to 3 tonnes (3 to 4 times in a month) Month January Date: 18/01/2013 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4
00:00 hrs to 08:00 hrs 23oC 21 oC 83% Supply Return temperature temperature o 28.2 C 34.8 oC o 27.2 C 33.0 oC 32.0 oC 39.0 oC
Range
Approach Effectiveness%
6.6 oC 5.8 oC 7.0 oC
7.2 oC 6.2 oC 11 oC
08:00 hrs to 16:00 hrs 29.5oC 23.5 oC 59 % Supply Return temperature temperature o 29.9 C 36.4 oC o 28.4 C 34.3 oC 33.0 oC 39.0 oC
Range
Approach Effectiveness%
6.5 oC 5.9 oC 6.0 oC
6.4 oC 4.9 oC 9.5 oC
16:00 hrs to 00:00 hrs 25oC 21.5 oC 68 % Supply Return temperature temperature 28.6 oC 35.4 oC 28.1 oC 33.8 oC o 32 C 38.5 oC
Range
Approach Effectiveness%
6.8 oC 5.7 oC 6.5 oC
7.1 oC 6.6 oC 10.5 oC
99
47.8% 48.33% 38.89%
50.38% 54.63% 38.71%
48.92% 46.34% 38.23%
Month February Date: 15/02/2013 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4
00:00 hrs to 08:00 hrs 27.5oC 25 oC 81% Supply Return temperature temperature o 30.7 C 37.8 oC 30.5 oC 37.2 oC 33 oC 45 oC
Range
Approach Effectiveness%
7.1 oC 6.7 oC 12 oC
5.7oC 5.5 oC 8 oC
08:00 hrs to 16:00 hrs 30oC 27 oC 78 % Supply Return temperature temperature o 32.7 C 39.5oC o 31.9 C 38.7 oC 34 oC 47 oC
Range
Approach Effectiveness%
6.8oC 6.8 oC 13 oC
5.7oC 4.9 oC 7 oC
16:00 hrs to 00:00 hrs 29oC 26.5 oC 81 % Supply Return temperature temperature o 31.8 C 38.9 oC o 31.4 C 38.1 oC 36.3 oC 47 oC
Range
Approach Effectiveness%
7.1 oC 6.7 oC 10.7 oC
5.3 oC 4.9 oC 9.8 oC
Month March Shutdown due to annual maintenance. Month April Shutdown due to annual maintenance. Month May Shutdown due to annual maintenance. Month June Date: 15/06/2012 Period DBT WBT
00:00 hrs to 08:00 hrs 26oC 25 oC 100
55.46 % 55.47 % 60 %
54.4% 58.12% 65%
57.26% 57.76% 52.20%
RH Cooling tower
92% Supply temperature 29.3oC 30.9 oC 28.7 oC
Range
Approach Effectiveness%
7.5oC 6 oC 7.3 oC
4.3oC 5.9 oC 3.7 oC
08:00 hrs to 16:00 hrs 30oC 27.5 oC 82 % Supply Return temperature temperature 31.3oC 38.6oC 32.4 oC 38.1 oC o 31 C 39.2 oC
Range
Approach Effectiveness%
7.3oC 5.7 oC 8.2 oC
3.8 oC 4.9 oC 3.5 oC
16:00 hrs to 00:00 hrs 27oC 25.5 oC 88 % Supply Return temperature temperature 29.7oC 37.1 oC 30.9 oC 37.1 oC o 29 C 37 oC
Range
Approach Effectiveness%
7.4oC 6.2 oC 8 oC
4.2oC 5.4 oC 3.5 oC
Range
Approach Effectiveness%
CT 1 CT 2 CT 4
00:00 hrs to 08:00 hrs 25.5oC 25 oC 96% Supply Return temperature temperature 29.9oC 37.7oC 31.4 oC 37.5 oC o 31.2 C 40.7 oC
7.8oC 6.1 oC 9.5 oC
4.9oC 6.4 oC 6.2 oC
Period DBT WBT RH Cooling tower
08:00 hrs to 16:00 hrs 30oC 27.5 oC 82 % Supply Return
Range
Approach Effectiveness%
CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4
Return temperature 36.8oC 36.9 oC 36 oC
63.56 % 50.42 % 66.36 %
65.76% 53.77% 70.08%
63.79% 53.44% 69.56%
Month July Date: 17/07/2012 Period DBT WBT RH Cooling tower
101
61.42 % 48.8 % 60.51 %
CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4
temperature 31.3oC 32.4 oC 31 oC
temperature 38.6oC 38.1 oC 39.2 oC
7.3oC 5.7 oC 8.2 oC
3.8 oC 4.9 oC 3.5 oC
16:00 hrs to 00:00 hrs 24.5oC 24 oC 96 % Supply Return temperature temperature 28.9oC 36.7 oC o 30.8 C 35.9 oC 33.0 oC 42 oC
Range
Approach Effectiveness%
7.8oC 5.1 oC 9 oC
4.9oC 6.8 oC 9 oC
00:00 hrs to 08:00 hrs 26.5oC 25.5 oC 92% Supply Return temperature temperature 30.3oC 37.6oC 31.2 oC 38 oC o 32.5 C 41 oC
Range
Approach Effectiveness%
7.3oC 6.8 oC 8.5 oC
4.8oC 5.7 oC 7 oC
08:00 hrs to 16:00 hrs 27oC 26 oC 95 % Supply Return temperature temperature 31.2oC 38.3oC 31.7 oC 38.6 oC o 34.9 C 43.8 oC
Range
Approach Effectiveness%
7.1oC 6.9 oC 8.9 oC
5.2 oC 5.7 oC 8.9 oC
Range
Approach Effectiveness%
7.3oC
5.2oC
65.76% 53.77% 70.08%
61.42% 42.86% 50%
Month August Date: 16/08/2012 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1
16:00 hrs to 00:00 hrs 26oC 25 oC 92 % Supply Return temperature temperature 30.2oC 37.5 oC 102
60.33 % 54.4% 54.84 %
57.72% 54.76% 50 %
58.4%
CT 2 CT 4
31.2 oC 34.6 oC
37.9 oC 43.3 oC
6.7 oC 8.7 oC
6.2 oC 9.6 oC
00:00 hrs to 08:00 hrs 26.5oC 25 oC 88 % Supply Return temperature temperature o 27.7 C 36.6oC o 30.6 C 37.1 oC 30.8 oC 38.0 oC
Range
Approach Effectiveness%
7.3oC 6.8 oC 8.5 oC
4.8oC 5.7 oC 7 oC
08:00 hrs to 16:00 hrs 32oC 28 oC 73 % Supply Return temperature temperature o 32.3 C 37.2oC o 31.2 C 38oC 31.5 oC 39.4 oC
Range
Approach Effectiveness%
4.9oC 6.8 oC 7.9 oC
4.3 oC 3.2 oC 3.5 oC
16:00 hrs to 00:00 hrs 28oC 26 oC 85 % Supply Return temperature temperature 28.9oC 37 oC o 30.9 C 37.5 oC 30.5 oC 38.5 oC
Range
Approach Effectiveness%
8.1oC 6.6 oC 8.0 oC
2.9oC 4.9oC 4.5oC
Range
Approach Effectiveness%
51.94% 47.54%
Month September Date: 15/09/2012 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4
60.33 % 54.4% 54.84 %
53.26% 68% 69.30%
73.63% 57.39% 64%
Month October Date: 15/10/2012 Period DBT WBT RH Cooling tower
00:00 hrs to 08:00 hrs 24.5oC 23 oC 87 % Supply Return temperature temperature 103
CT 1 CT 2 CT 4
24.8oC 28.7 oC 30 oC
Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4
34.9oC 35.5 oC 40 oC
10.1oC 6.8 oC 10 oC
1.8oC 5.7 oC 7 oC
08:00 hrs to 16:00 hrs 33oC 27 oC 62 % Supply Return temperature temperature 34.1oC 36.3oC 30 oC 37oC o 31 C 41oC
Range
Approach Effectiveness%
2.2oC 7 oC 10 oC
7.1 oC 3 oC 4 oC
16:00 hrs to 00:00 hrs 26oC 25 oC 92 % Supply Return temperature temperature o 27 C 37 oC 30.5 oC 37.4 oC 31.6 oC 32 oC
Range
Approach Effectiveness%
10oC 6.9 oC 0.4 oC
2oC 5.5 oC 6.6 oC
84.87 % 54.4% 58.8 %
23.65% 70% 71.43%
83.33% 57.39% 64%
Month November : Shutdown due to technical problem. Month December Date: 29/12/2012 Period DBT WBT RH Cooling tower CT 1 CT 2 CT 4 Period DBT WBT RH Cooling tower CT 1
00:00 hrs to 08:00 hrs 23oC 19.5 oC 52 % Supply Return temperature temperature o 27 C 34oC 25.6 oC 31.8 oC 27 oC 35 oC 08:00 hrs to 16:00 hrs 31oC 23.5 oC 52 % Supply Return temperature temperature o 30.1 C 36.6oC 104
Range
Approach Effectiveness%
7oC 6.2 oC 8 oC
7.5oC 6.1 oC 7.5 oC
Range
Approach Effectiveness%
6.5oC
6.6oC
48.27 % 50.40% 51.61%
49.61%
CT 2 CT 4
27.8 oC 30 oC
Period DBT WBT RH Cooling tower
16:00 hrs to 00:00 hrs 26oC 22 oC 70 % Supply Return temperature temperature o 29.3 C 36.2 oC o 27.1 C 33.1 oC 29.1 oC 37.5 oC
CT 1 CT 2 CT 4
33.8oC 38oC
6 oC 8 oC
4.3 oC 6.5 oC
Range
Approach Effectiveness%
6.9oC 6 oC 8.4 oC
7.3oC 5.1 oC 7.1 oC
EFFICIENCY
EFFECIENCY VARIATION OVER A DAY OF CT 1 50.5 50 49.5 49 48.5 48 47.5 47 46.5 EFFECIENCY
SHIFT 1 47.8
SHIFT 2 50.38
SHIFT 3 48.92
Figure 8.1
EFFECIENCY VARIATION OVER A DAY OF CT4 39
EFFICIENCY
38.8 38.6 38.4 38.2 38 37.8 EFFECIENCY
SHIFT 1 38.89
SHIFT 2 38.71
FIGURE 8.2 105
SHIFT 3 38.23
58.25% 55.17%
48.59% 54.05% 54.19%
EFFICIENCY VARIATION OVER THE YEAR OF CT1 70 60
EFFICIENCY
50 40 30 20 10 0
JAN
FEB
MAR APRI MAY JUNE JULY AUG SEPT OCT NOV DEC CH L UST EFFECIENCY 49.03 55.7 0 0 0 64.37 62.87 58.82 62.4 63.94 0 48.82
FIGURE 8.3
EFFICIENCY VARIATION OVER THE YEAR OF CT4 70 60
EFFICIENCY
50 40 30 20 10 0
JAN
FEB
MAR APRIL MAY JUNE JULY AUG SEPT OCT NOV DEC CH UST EFFICIENCY 38.61 59.07 0 0 0 68.67 60.19 50.79 62.71 64.74 0 53.65
FIGURE 8.4
106
Water treatment data over a year period: Date
time
pH
Ammonica nitrogen AN
Orthophosphate OPO4
Inorganic phosphate IPO4
Poly phosphate PPO4
Free residual chlorine FrC
Oxidation reduction parameter ORP
Corrosio n rate ORC
15/01/13
0-8
6.8
6.4
18.1
19.1
1.6
-
483
2.4
January
8-16
-
-
-
-
-
-
-
-
16-24
6.5
23.2
14.3
15.5
1.2
-
494
2.9
15/02/13
0-8
6.9
45
20.2
20.8
0.6
-
409
3.2
February
8-16
-
-
-
-
-
-
395
-
16-24
6.9
56.8
18.2
22
3.8
0.1
386
4
MARCH : PLANT SHUTDOWN DUE TO ANNUAL MAINTENANCE APRIL : PLANT SHUTDOWN DUE TO ANNUAL MAINTENANCE MAY : PLANT SHUTDOWN DUE TO ANNUAL MAINTENANCE 15/06/12
0-8
7.2
50.9
18.1
19.6
1.5
-
310
2.3
June
8-16
7.4
49.6
16.7
17.4
0.7
-
300
2.9
16-24
7.1
49.8
16.2
17.2
1.0
-
456
4.56
15/07/12
0-8
6.8
10.5
17.0
18.5
1.5
-
251
1.2
July
8-16
-
-
-
-
-
-
258
1.3
16-24
6.9
10.8
15.2
16.6
1.4
-
261
1.3
15/08/12
0-8
6.7
14.2
18.5
20.3
1.8
-
398
1.7
August
8-16
6.8
10.9
17.3
17.6
0.3
-
294
1.8
16-24
6.5
12.3
15.9
16.7
0.8
-
13
2.4
15/09/12
0-8
7.1
39.3
17
18
1
0.6
473
3.2
Sep
8-16
-
-
-
-
-
-
449
3.2
16-24
6.6
34.9
12.2
14.1
1.9
0.8
450
40
15/10/12
0-8
7.2
24
24.7
0.7
0.2
-
387
0.93
October
8-16
-
-
-
-
-
0.2
351
2.6
16-24
7.0
9.6
25.5
25.9
0.4
0.1
433
2.5
NOVEMBER : PLANT SHUTDOWN DUE TO TECHNICAL PROBLEM 29/12/12
0-8
6.7
7.4
14.9
15.3
0.2
-
373
3.2
Dec
8-16
-
-
-
-
-
-
446
4.8
16-24
6.8
5
18.2
18.5
0.3
-
375
3.5
Calculation involved in cooling tower Cooling tower 1(CT1) 107
o
o
=34.8oC (Rated 44 C)
Inlet Cooling Water Temperature C o
o
=28.2oC(Rated 32 C)
Outlet Cooling Water Temperature C o
=21oC(Rated 28 C)
Air Dry Bulb Temperature near Cell C
o
=23oC(Rated 32 C)
Number of CT Cells on line with water flow
=10
o
Air Wet Bulb Temperature near Cell C
o
3
Total Measured Cooling Water Flow m /hr
=15000
3
Measured CT Fan Flow m /hr
=1625000
Analysis 3
CT water Flow/Cell, m /hr 3
CT Fan air Flow, m /hr (Avg.)
3
3
= 1500 m /hr (1500000 kg/hr) (rated 1585 m /hr) 3
3
= 1462500 m /hr (rated1625000 m /hr) 3
CT Fan air Flow kg/hr (Avg.) @ Density of 1.08 kg/m = 1579500 kg/hr L/G Ratio of C.T. kg/kg = .9496 CT Range =6.6 oC CT Approach = 7.2 oC % CT Effectiveness =47.8% Rated % CT Effectiveness = 75% Cooling Duty Handled/Cell in kCal =Flow * Temperature Difference in kCal/hr =1500 * 6.6 * 103 3
=9900 * 103 kCal/hr (Rated 19020 * 10 kCal/hr) 3
Evaporation Losses in m /hr
3
= 0.00085 x 1.8 x circulation rate (m /hr) x (T1-T2) =0.00085 x 1.8 x 1500 x (34.8-28.2) 3
=8.415m /hr per cell 3
=84.15 m /hr for 10 cells. Percentage Evaporation Loss
= [8.415/1500]*100 = 0.561% per cell = 0.561*10 =5.61% for 10 cells Blow down requirement for site COC of 2.7 =Evaporation losses / (COC–1) =8.415/(2.7-1) 3
=4.95m /hr per cell 3
=49.5 m /hr for 10 cells 3
Make up water requirement/cell in m /hr
=Evaporation Loss + Blow down Loss 108
=8.415+4.95 3
=13.365 m /hr per cell 3
= 133.65 m /hr for 10 cells.
Cooling tower 2(CT2) o
o
Inlet Cooling Water Temperature C
=33.0oC (Rated 40 C)
o
=27.2 oC(Rated 32 C)
o
Outlet Cooling Water Temperature C o
=21oC(Rated 28 C)
Air Dry Bulb Temperature near Cell C
o
=23oC(Rated 32 C)
Number of CT Cells on line with water flow
=3
o
Air Wet Bulb Temperature near Cell C
o
3
Total Measured Cooling Water Flow m /hr
=6529
3
Measured CT Fan Flow m /hr
=1422000
Analysis 3
CT water Flow/Cell, m /hr 3
CT Fan air Flow, m /hr (Avg.)
3
3
= 2176 m /hr (2176000 kg/hr) (rated 6900 m /hr) 3
3
= 1279800 m /hr (rated1422000 m /hr) 3
CT Fan air Flow kg/hr (Avg.) @ Density of 1.08 kg/m = 1382184 kg/hr L/G Ratio of C.T. kg/kg = 1.574 CT Range =5.8 oC CT Approach = 6.2 oC % CT Effectiveness =48.33% Rated % CT Effectiveness = 66.67% Cooling Duty Handled/Cell in kCal =Flow * Temperature Difference in kCal/hr =2176 * 5.8 * 103 3
=12620.8 * 103 kCal/hr (Rated 55200 * 10 kCal/hr) 3
Evaporation Losses in m /hr
3
= 0.00085 x 1.8 x circulation rate (m /hr) x (T -T ) 1
=0.00085 x 1.8 x 2176 x (33-27.2) 3
=19.31m /hr per cell 3
=57.93 m /hr for 3 cells. Percentage Evaporation Loss
= [19.31/2176]*100 = 0.887% per cell = 0.561*3 =2.6622% for 3 cells 109
2
Blow down requirement for site COC of 2.7 =Evaporation losses / (COC–1) =19.31/(2.7-1) 3
=11.35m /hr per cell 3
=34.07 m /hr for 3 cells 3
Make up water requirement/cell in m /hr
=Evaporation Loss + Blow down Loss =19.31+11.35 3
=30.66 m /hr per cell 3
= 91.98 m /hr for 3 cells.
Cooling tower 4(CT4) o
o
=39.0oC (Rated 55 C)
Inlet Cooling Water Temperature C o
o
Outlet Cooling Water Temperature C
=32oC(Rated 36 C)
o
=21oC(Rated 28 C)
Air Dry Bulb Temperature near Cell C
o
=23oC(Rated 32 C)
Number of CT Cells on line with water flow
=2
o
Air Wet Bulb Temperature near Cell C
o
3
Total Measured Cooling Water Flow m /hr
=146.5
3
Measured CT Fan Flow m /hr
=42718.5(approximated)
Analysis 3
CT water Flow/Cell, m /hr 3
CT Fan air Flow, m /hr (Avg.)
3
3
= 73.25 m /hr (73250 kg/hr) (rated 77.5 m /hr) 3
3
= 42718.5 m /hr (rated 47465 m /hr) 3
CT Fan air Flow kg/hr (Avg.) @ Density of 1.08 kg/m = 46135.98 kg/hr L/G Ratio of C.T. kg/kg = 1.587 CT Range =7 oC CT Approach = 11 oC % CT Effectiveness =38.89% Rated % CT Effectiveness = 70.37% Cooling Duty Handled/Cell in kCal =Flow * Temperature Difference in kCal/hr =73.25 * 7 * 103 3
=512.75 * 103 kCal/hr (Rated 1472.5 * 10 kCal/hr) 3
Evaporation Losses in m /hr
3
= 0.00085 x 1.8 x circulation rate (m /hr) x (T1-T2) =0.00085 x 1.8 x 73.25 x (39-32) 3
=0.7845m /hr per cell 3
=1.569 m /hr for 2 cells. 110
Percentage Evaporation Loss
= [.7845/73.25]*100 = 1.07% per cell = 1.07*2 =2.14% for 2 cells Blow down requirement for site COC of 2.7 =Evaporation losses / (COC–1) =0.7845/(2.7-1) 3
=0.4614m /hr per cell 3
=0.9229 m /hr for 2 cells 3
Make up water requirement/cell in m /hr
=Evaporation Loss + Blow down Loss =0.7845+..4614 3
=1.2459 m /hr per cell 3
= 2.4918 m /hr for 2 cells. Energy audit for cooling towers PERFORMANCE OF COOLING TOWER 1 Method of assessment at shop floor Step 1: note cooling tower sump water temperature Step 2: note ambient wet bulb temperature EVALUTION Formula: approach =sump water temperature – ambient wet bulb temperature Site measurements- efficiency measurements Values cooling tower sump water temperature 28.2 ambient wet bulb temperature 21 Approach 7.2 Recommended approach 4 Heat load on cooling tower Method of assessment at shop floor Step 1: note air flow at exhaust of cooling tower by anemometer Step 2: note dry bulb and wet bulb temperature of air at exhaust Step 3: note ambient dry bulb and wet bulb temperature Step 4: need to refer psycrometric calculator EVALUTION: heat load based on heat gained by air Formula: heat load : mass flow air X Sp enthalpy diff (exhaust air –ambient air) Site measurements- efficiency measurements Values Diameter at exhaust 7.3 Cross section 41.83 Avg air velocity at exhaust(by anemometer) 1462500 Volumetric air flow
1462500 111
Units o C o C o C o C
Units meter Meter2 3
m /hr Cum/sec
Mass flow of air Dry bulb temp of exhaust air Wet bulb temp of exhaust air Enthalpy of exhaust air(from psycrometric calculator) Dry bulb temp of ambient air Wet bulb temp of ambient air Enthalpy of ambient air(from psycrometric calculator) Total enthalpy difference Heat load in TR(refrigeration = 3024 kcal/h) Heat load in TR(cooling tower = 3782 kcal/h)
1500000 23 21 58 32 28 90 32 0.0105 0.00846
Kg/hr o C o C Kcal/kg o C o C Kcal/kg Kcal/kg TR TR
CALCULATING COOLING TOWER EFFECTIVENESS shop floor Method Step 1: Measure cooling water inlet temperature Step 2: Measure cooling water outlet temperature Step 3: Measure the ambient wet bulb temperature of air Step 4: calculate range and approach Step 5: calculate the ratio for effectiveness EVALUTION Formula: effectiveness = Range /(Range + Approach) Range = cooling water inlet temperature- cooling water outlet temperature Approach= cooling water outlet temperature - ambient wet bulb temperature Site measurements- capacityt measurements Present values Units o cooling water inlet temperature 34.8 C o cooling water outlet temperature 28.2 C o ambient wet bulb temperature of air 21 C o Range 6.6 C o approach 7.2 C Cooling tower effectiveness 47.8 % Standard values Approach Range as per design –min 2.5 o C cooling tower performance-L/G ratio, evaporation loss, heat load shop floorMethod of capacity assessment of CT Step 1: measure the quantity of water flow in kg/hr Step 2: measure the quantity of air flow in kg/hr ( measure average velocity/area of apparatus) Step 3: calculate L/G ratio – kg of liquid/kg of air Step 4: measure temperature of water at inlet Step 5: measure temperature of water at outlet Step 6: calculate temperature difference Step 7: calculate evaporation loss by formula( .00085*1.8*circulation rate *temp diff) Step 8: measure temp of air at inlet (wet/dry) Step 9: measure temp of air at inlet (wet/dry) Step 10: find out enthalpy difference from psyscrometric chart 112
EVALUTION: heat load based on heat gained by air Formula: .00085*1.8*circulation rate *temp diff Heat rejected/heat load =enthalpy diff *mass of air flow (kg/hr) L/G ratio = (mass of liquid flow in kg/hr)/ (mass of air flow in kg/hr) Site measurements- efficiency measurements Present values
Units M3/hr
3
Quantity of water flow
15000 m /hr
Quantity of air flow
1462500 m /hr 1.08 1579500 34.8 28.2 83 98 78 20
3
Density of air Air flow mass Temp at inlet Temp at outlet Relative humidity Enthalpy at inlet(from psychrometric chart) Enthalpy at outlet(from psychrometric chart) Difference in enthalpy heat rejected/heat load Measure water inlet temperature Measure water outlet temperature L/G ratio
9900 *103
34.8 28.2 0.9496 84.15 1.1-1.5/1.5-2.0/1.4-1.8
Evaporation loss Standard L/G ratio for splash/film/low clog/ (higher ratio means less air required) Standard value for evaporation rate
Kg/m3 Kg/hr o C o C % KJ/kg KJ/kg KJ/kg KJ o C o C M3/hr
1.8 m3 per 1000000 kcal heat rejected
Particulars of no.1 and no.2 cooling towers NO.1
NO.2
Cooling tower (item no)
C-EF101
C-EF102
Vendor’s model no
663-3-08
663-3-03
No of towers
01
01
No of cells
10
03
Type of cells
M3/hr
double flow induced draft cross flow
Hot water quantity
15850m3/h
6900m3/h
Hot water temperature(design)
44oC
40oC
Hot water temperature(operation)
32oC
32oC
113
cold water temperature(design)
32oC
32oC
cold water temperature(operation)
32oC
32oC
design W.B. temperature
28oC
28oC
evaporation loss
approx. 1% per 6oC range of cooling
drift losses
0.2% maximum of circulation rate
no of risers
2
2
Diameter of riser pipes(nominal)
42”
30”
42”-30”-24”
Diameter of distribution
30”
Pipes at top (nomimal) Static head of hot water
11.8m
11.9m
Above nominal water level
~11.95m
~12.02m
No &inlet size of flow control
16 nos X 14”
6 nos X 16”
Valves (nominal) dimensions Cell length
8540 mm
8540 mm
Total length(basin inside)
85800mm
25900 mm
Width at basin
15700 mm
Overall width at top
21050 mm
Fan diameter
7300 mm(24’)
Height of fan deck
12600 mm
(over basin curb) Overall height above basin curb
18100 mm
fans Item no
C-GB 101 A-J
No Type
C-GB 102 A-C
10
3
statically balanced multi-blade induced draft axial flow 7300mm(24”)
Fan diameter No of blades per fan
8
Blade angle of incidence
adjustable 114
approx 1440 mm(8”)
Fan hub diameter Fan speed
166 rpm
Tip speed
approx 63.6n/sec
Air delivery per fan
approx
approx
1625000m3/hr
1422000m3/hr
Total static pressure
5.78 mm
5.88 mm
Velocity pressure at fan outlet
8.42 mm
6.44 mm
Velocity recovery at fan cylinder
3.37 mm
2.58 mm
Fan static efficiency
41.5%
45.4%
Total fan efficiency
77.7%
75.1%
Fan BHP at motor shaft
86.5HP(64.5 KW)
71.6HP(52.9 KW)
Motor HP
100 HP(74 KW)
75 HP(55KW)
NO.1
NO.2
Drivers as follows: Reduction gear Type
spiral level cum double reduction
Gear ratio
8.72/1
No. of reductions
2
Efficiency
98%
motor
Type
NO.1
NO.2 TEFC- weather proof outdoor use squirrel cage induction
Speed
1450 rpm
Rated HP
100
75
No.of poles
4
4
Rated voltage
400 volt
Phase
three
Frequency
50 Hz
Insulation
class “E” tropicallized
pit for cooling tower
NO.1
NO.2
115
Item no
C-FB 101
C-FB 102
Basin width
15.7 m
15.7 m
Length
85.8 m
25.9 m
Depth
2..8 m
3.1 m
Gross volume
3770 m3
1260 m3
Net volume
3370 m3
1140 m3
Sump width
4.0 m
4.0 m
Length
85.8 m
25.9 m
Depth
5.2 m
5.2 m
Gross volume
1780 m3
540 m3
Net volume
1680 m3
500 m3
Total gross volume
5550 m3
1800 m3
Net volume
5050 m3
1650 m3
Dimensions
Nalco Chemical used for water treatment 1. 2. 3. 4. 5. 6. 7. 8. 9.
NALCO 8305 NALCO 8307 NALCO7348 NALCO 8301 D NALCO 73 NIN 42 NALCO73 NIN 20 NALCO 7330 NALCO 7356 NALCO 8088
corrosion inhibitor corrosion inhibitor bio-dispersant scale dispersant chlorine activator contingency biocide contingency biocide corrosion inhibitor non oxidizing biocide
Quality of make-up water(city water) Design basic quality of raw water (city water) pH
8-9
turbidity
mg/Ltr
5
total dissolved solids
mg/Ltr
158.0
calcium oxide
mg/Ltr
23.45
magnesium oxide
mg/Ltr
6.45
iron (Fe)
mg/Ltr
0.12
chlorine(Cl)
mg/Ltr
16.0
116
sulphate(SO4)
mg/Ltr
8.4
silica
mg/Ltr
10.0
P-alkalinity as CaCO3
mg/Ltr
4.0
M-alkalinity as CaCO3
mg/Ltr
50.0
Dissolved oxygen
mg/Ltr
0.2
Free Carbon dioxide
mg/Ltr
Nil
Oxygen consumed by organic matter
mg/Ltr
0.08
Biochemical oxygen demand (B.O.D)
mg/Ltr
chemical oxygen demand (C.O.D)
mg/Ltr
4.0 max
total hardness as CaCO3
mg/Ltr
63.7
carbonate hardness
mg/Ltr
50.0
permanent hardness
mg/Ltr
13.7
magnesium hardness
mg/Ltr
16.0
bicarbonate of calcium(CaCO3)
mg/Ltr
42.0
bicarbonate of magnesium(MgCO3)
mg/Ltr
6.74
magnesium sulphate (MgSO4)
mg/Ltr
12.6
magnesium chloride (MgCl2)
mg/Ltr
2.36
sodium chloride(NaCl)
mg/Ltr
23.49
0.5 at 20oC
probable composition
32OC max
temperature
0.7 kg/cm2g
pressure
Cost data 1. Cost of gear oil replacement in Gearbox Oil used is servoprime68 Total liters consumed-50 liters per year per gearbox 2. Cost of bearing replacements in motor or gearbox Bearings might be replaced once in 5 yrs Cost incurred- Rs 109050
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3.Cost of Dozing chemicals per year
Nalco-7308 Oil depressant Cost per kg CT1 (Amount CT2 (Amount Total name (Rs) consumed) (kg) consumed)(kg) (Rs) Nalco-7308 284 1400 635 577940
Cost
No water is blown down due to oil contamination.
Zuari fan data 1. Fan details: Make Encon (INDIA) – Dadar west Mumbai- 28 1) 2) 3) 4)
Energy saving FKP fan. 33 feet diameter. 25-30% saving in power consumption. Aerodynamically hollow construction FRP fan.
Fans Item no
C-GB 101 A-J
No Type
C-GB 102 A-C
10
3
statically balanced multi-blade induced draft axial flow 7300mm(24”)
Fan diameter No of blades per fan
8
Blade angle of incidence
adjustable approx 1440 mm(8”)
Fan hub diameter Fan speed
166 rpm
Tip speed Air delivery per fan
approx 63.6n/sec approx
approx
1625000m3/hr
1422000m3/hr
Total static pressure
5.78 mm
5.88 mm
Velocity pressure at fan outlet
8.42 mm
6.44 mm
Velocity recovery at fan cylinder
3.37 mm
2.58 mm
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Fan static efficiency
41.5%
45.4%
Total fan efficiency
77.7%
75.1%
Fan BHP at motor shaft
86.5HP(64.5 KW)
71.6HP(52.9 KW)
Motor HP
100 HP(74 KW)
75 HP(55KW)
1. Analysis of power consumption in axial flow fans: Minimum 80% efficiency. Features – Higher lift to drag ratio. Larger chord width along with blade twist. FRP material Better surface finish. Lower profile drag loss.
(A) KW at fan shaft =
Q X Tp 3.68 X 105 X ηf
Where, Q = air flow Tp= total pressure ηf= fan efficiency (B) KW at motor shaft =
KW at fan shaft ηg
Where, ηg = transmission efficiency (gear box)
(C) KW to motor =
KW at motor shaft ηm
Where, ηm = motor efficiency
Conclusion Higher efficiency require lower power consumption. 119
Installation data 1)On August 14, 1993 M/S Encon (INDIA) installed 35% energy saving aluminum fan blades. 2)Pan-century oleochemicals made modifications which lead to 27% energy saving and 4% increase in air flow. 3)Indofoil chemicals company on December 12, 1994 made modifications which lead to 26% energy saving and 20% increase in air flow.
Fan performance report Cell no: ‘A’ in cooling tower 1 Material of construction: Al Fan diameter: 24ft Hub diameter: 8 ft Traverse area: 440.6 Velometer readings: #Station readings Station I II III IV
R1 11 11 11 11
R2 12 11 11 11
R3 9 9 10 7
R4 4 4 4 4
R5 3 5 5 5
Voltage 415 415 415 415
I(amps) 100 100 100 100
Power factor .87 .87 .87 .87
Average reading (m/s) = 7.9 = 1554.72 ft/min Flow
= 1554.72 ft/min X traverse area =685009.63cfm
Pressure readings: (in mm Hg) Station 1 5
Station 2 6
Station 3 5
Air hp = K X flow X pressure drop = 0.0001573 X685009.63 X 0.1968 =21.20 hp 120
Station 4 4
Station 5 5
Current (I) amps 100
Voltage (v) 415
Power factor 0.87 0.8
Kilowatts 62.535 57.504
Motor hp 83.82 77.08
Cell no: ‘A’ in cooling tower 1 Fan diameter: 24ft No of blades: 8 Traverse area: 462.365 sq ft Material of construction: FRP Observations Velometer readings: (a) With blade angle 10o #Station readings Station I II III IV
R1 9.6 9.6 8.0 7.4
R2 10.0 10.0 9.6 9.6
R3 10.4 10.4 9.4 10.0
R4 6.8 10.0 10.0 11.0
R5 5.2 10.0 10.0 11.0
Voltage 415 415 415 415
I(amps) 90 90 90 90
Power factor .87 .87 .87 .87
Average reading (m/s) = 9.19 = 1808.592 ft/min Flow
= 1808.592 ft/min X traverse area = 836229.64 cfm
Pressure drop readings: (in mm Hg) Station 1 2.8
Station 2 2.6
Station 3 5.4
Station 4 3.6
Average 3.45
(a) With blade angle 8o #Station readings Station I II III IV
R1 9.6 8.4 3.0 6.0
R2 9.4 8.6 8.0 8.4
R3 10.8 9.6 8.8 8.8
R4 10.4 9.4 9.4 9.2
R5 8.8 9.0 9.6 8.0 121
Voltage 415 415 415 415
I(amps) 76 76 76 76
Power factor .87 .87 .87 .87
Average reading (m/s) = 8.66 m/s = 1704.288 ft/min Flow
= 1704.288 ft/min X traverse area = 788033.12 cfm
Pressure drop readings: (in mm Hg) Station 1 -
Station 2 -
Station 3 6.8
Station 4 2.4
Average 4.6
Air HP with blade angle 10o = 75.443 Power consumption blade angle 10o =56.28 KW Air HP with blade angle 8o = 63.709 Power consumption blade angle 8o =47.52 KW
Inferences Blade angle 10o 8o
Air flow 685009.63 836229.64 788003.12
Static pressure 5 3.45 4.6
Current 100 amps Al blades 90 amps FRP blades 76 amps FRP blades
Hub and fan blade assembly frp ct-1 & ct-2 hub and fan blade assembly should be dynamically balanced and confirm to the following specifications: 1) Fan diameter: 24 feet (7315 mm) 2) Fan speed: 170 rpm 3) Motor speed: 1480 rpm 4) Motor rating: 75kw 5) Flow required: 850000 cfm 6) No of blades: 8 (hollow blade type) 7) Moc of blades: frp (fibre glass reinforced with epoxy resin).material should be able to withstand the chlorine dioxide dosing carried out at the cooling tower. 8) Hub diameter: 1830 mm 9) Hub body construction from ms plates, 8 mm thick with two plates and clamps in between. 10) Hub spool in cast iron
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11) Split tapered bush in ss 304 to suit the output shaft of paharpur marley make gearbox series 32.2t 12) Clamps in ci with taper bore 13) Entire hub assembly should be hot dip galvanised as per is 3203 to achieve a thickness of 65-85 microns
Power cost i = 120a v = 440v 1 kw-hr = Rs 9 per unit total cost per month = Rs 3,07,929.6
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CHAPTER 9 CASE-STUDIES Case study on fibre filters In 2008, California Institute of Technology used 764,064,005 liters of water. Cooling towers at the central and satellite plants consumed 319,951,956 liters of water through evaporation and blow-down, contributing to over forty per cent of overall water usage on campus. In this Water efficiency Project, the amount of water loss due to evaporation and blow-down each month has been studied and has demonstrated that on average, 83 per cent of water loss is due to evaporation and the rest due to blow-down. This project aims to improve overall water efficiency of cooling towers by examining two applicable systems that can recover vapour and reduce blow-down. An ozone treatment reduces the use of chemicals and thus decreases the blow-down rate. A vapour recovery system consisting of a circular fibre filter on top of cooling towers absorbs and condenses water vapour coming out of the cooling towers. A feasibility experiment showed that approximately 10% of evaporated water could be recovered using this method. Further research should be carried out to study corrosion problems from the ozone treatment, and the type of the fibre filter that should be installed to optimize water vapour absorption without obstructing airflow.
Introduction Water is one of the most vital resources on the Earth, but is becoming increasingly scarce. Numerous attempts to conserve water and reduce water pollution have been made by all kinds of organizations, from the federal government of the United States to research universities such as California Institute of Technology. Caltech currently employs various water-efficient technologies in an attempt to reduce its water consumption level through its Water Efficiency Project. In 2008, Caltech used 764,064,004 liters of water. Over half of the water was consumed through Caltech’s own central and satellite utility plants. Each plant operates induced draft cooling towers with four cells. The cooling towers are used to lower the temperature of the water from condensers. A large fan pulls up cool and dry ambient air from below, and warm water sprayed near the top (Fig. 2) cools from evaporation and heat transfer to the cooler air. Warm, saturated air comes out of the cooling tower into the atmosphere (Fig. 1).
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Figure 9.1 | Satellite Plant Cooling Towers
Figure 9.2 | Warm water spray inside the central plant cooling tower Eighty per cent of water used by Caltech’s central and satellite plants is lost from the cooling towers. One factor of the water loss from cooling towers is evaporation. Concentration of chemicals (ions, acids, and so on) that are either originally present in water or added to increase cooling efficiency and reduce microbes increases after each cycle due to evaporation. As the concentration increases, ions might cluster together and form solids, the number of bacteria may increase significantly, thus decreasing overall cooling efficiency. Therefore, the concentration of solids should be maintained below a certain limit. Blow-down 125
is used to adjust the concentration by bleeding and refilling cooling water. This process results significant water loss in cooling towers and also results water pollution. Increasing water efficiency of Caltech’s cooling towers can lower overall water consumption significantly. Facilities employees currently attempt to decrease blow-down rate by controlling pH setup point – cooling water pH at which blow-down valve opens. The pH level of cooling water increases as water evaporates and the chemical concentration increases. Setting higher pH tolerance level – up to a point at which cooling efficiency of cooling towers does not decrease significantly – increases the number of cycles of cooling water per blow-down. Ozone treatment of cooling water is another possible solution to reduce blow-down. Ozone, a relatively unstable molecule composed of three oxygen atoms, acts as a strong biocide by oxidizing living organisms. Traditional chemical treatment to control microbes and prevent scaling requires frequent bleed-off of cooling water because of increase in concentration of chemicals due to evaporation. This further requires frequent refill of treatment chemicals. An alternative to the traditional chemical treatment is ozone treatment. As an effective microbial controller, ozone can replace chemicals to control microbes. Ozone treatment, an alternative to traditional chemical treatments, can improve water efficiency substantially according to R.J. Strittmatter, et al1. Ozone treatment for cooling water has several advantages over traditional controls: less or no use of chemicals (toxic and non-toxic), water conservation due to less blow-down (as less or no chemicals are used, the concentration level is much lower). However, there are several concerns about ozone treatment, including the impact on corrosion, effectiveness, and lack of general guidelines. The experiments by Strittmatter, et al show that ozone has a negligible impact on corrosion at typical use levels, does not increase mineral scales, controls fouling, and is excellent for microbial controls. Caltech had previously examined applicability of ozone treatment in its cooling towers, but did not introduce the treatment due to corrosion problems. Besides reducing blow, the amount of water used on campus can be reduced by reducing the amount of evaporative losses. Attempts to recover evaporation loss can be found in two Korean patents. Kyung Seok Kang uses hollow fibre membrane filter modules to recapture cooling water vapor2 and Hoogeun Lee uses honeycomb, pleated or web-like fibre filters to absorb water vapour coming out of cooling towers3. However, the amount of vapour that can be recovered from such systems is not indicated. The ultimate goal of the Water Efficiency Project is to find applicable mechanisms which improve overall water efficiency of cooling towers. For this summer, a cooling tower prototype – a humidifier – and two different types of filters were installed and tested how much vapour dismissed from the humidifier can be absorbed by each filter.
Results Water Usage Data Analysis Data analysis on cooling tower water usage and its relation with cooling degree days was performed. Central cooling tower and satellite cooling tower’s monthly make-up and blow-down levels are recorded in Utility Plant’s monthly reports. The reports show that the cooling water make-up and blow-down levels are low during winter and increase during the summer (Fig. 3). A quantitative measure of how much cooling is needed each month is obtained in a form of monthly cooling degree days (CDD) data from the weather station ID KCAPASAD5 (North Wilson Avenue, Pasadena, CA). Total cooling water make-up (central and satellite) correlates with cooling degree days each month (Fig. 4). Figures 3 and 4 show that cooling 126
water was lost more through evaporation than through blow-down. Numerical data of total make-up, blow-down and evaporative loss show that only 9 to24 per cent of make-up water is lost through blow-down (Table 1).
Vapour Recovery Experiment In order to estimate how much vapour can be absorbed by different filters and parameters, a simple experiment was designed using an ultrasonic humidifier (Fig. 5) as a cooling tower prototype. Supply rate of vapour from the humidifier was calculated by dividing the difference of water remaining in the tank of the humidifier by amount of time passed.
FIGURE 9.3 Figure9.3 | Central and satellite plants’ cooling tower water make-up and blow-down each month from January 2008 to May 2009. Note that the Satellite plant consumed noticeably more water before September 2008. Central plant’s cooling towers did not operate as much as Satellite plant’s as old chillers were being replaced by new ones. CT: Central Plant Cooling Tower. ST: Satellite Plant Cooling Tower
Time Passed 137 min 308 min 20 sec 739 min 1 sec
Water (ml) 1000 2000 2000
Initial Water After (ml) Difference (ml) 738 1302 355
262 698 1645
Rate (ml/min) 1.912 2.264 2.226
Table 9.1 | Amount of water emitted by the ultrasonic humidifier over different periods of time 127
Table 1 | Total water make-up (MU), blow-down (BD) and evaporation loss by central and satellite cooling towers from January 2008 to May 2009. The percentage of evaporation loss is quite significant – between 76 and 91%– each month.
128
Figure 9.4 | Total water make-up (MU), blow-down (BD) and evaporation loss by central and satellite Cooling towers and Cooling Degree Days (CDD). Water consumption by the cooling towers generally follows the trend of CDD as expected. However, it is noticeable that the water consumption during the summer break (June, July and August) does not increase as sharply as the CDD do. Table 2 shows that 1.9 to 2.2 g of vapour was emitted by the humidifier per minute. This variation, with changing temperature and humidity levels in the laboratory, the source of error is estimated to be around 15 per cent. Two different types of filters– fibre filter (Fig.6) and pleated air filter (Figs. 7,8,9) – are installed on top of a humidifier and the amount of time passed and according amount water (in grams) absorbed by each filter is measured. For fibre filter pad, variations in thickness and area were imposed. For pleated air filter, different orientations were used. Temperature of vapour coming out of the humidifier – 23.5°C on average – was measured in order to compare with the temperature of vapour from cooling towers – estimated to be between 24°C and 30°C from cooling water temperature going into and coming out of Caltech’s cooling towers. Attempts to measure air flow on top of the humidifier and after going through each filter was not successful as the flow was not constant without a filter on top, and filters in horizontal orientation obstruct most of the air flow.
Figure 9.5 | VICKS Ultrasonic Humidifier
129
Figure 9.6 | Clockwise from left upper corner: fibre filter 1, 2, 3,and 4. Filters 1, 2, 3 are of same size. Filters 1 and 2 are about 1cm thick. Filters 3 and 4 are of same thickness, about 2 cm thick
Figure 9.7 | Pleated air filter in horizontal orientation
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Figure 9.8 | Single walled pleated air filter in vertical orientation
Figure 9.9 | Multiple walled pleated air filter in vertical orientation
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Figure 9.10:Vapour absorption versus time of fibre filter 1. The relationship is linear (solid line) before saturation and concave down (dotted line) as the filter gets saturated.
Figure 9.11: Vapour absorption versus time of thin and thick fibre filters
Figure 11 shows that the vapour absorption by thin and thick filters is almost identical before saturation. The slope of both curves is approximately 0.2 g/min in the linear region. However, vapour absorption of thick filter concaves down more slowly than that of thin filter, indicating higher saturation level. The thin filter seems to begin deviating from the linear relation around 10 grams of water while the thick filter seems to deviate around 20 grams of 132
water. Hence, the filters may become saturation when they have absorbed 10 g of water per 1 cm of filter.
Figure 9.12: Vapour absorption versus time of large and small filters There is no significant difference in vapour absorption between small and large filters before saturation. Figure 12 shows that vapour absorption of large filters concaves down more slowly than that of small filters, indicating higher saturation level. There is no significant difference in vapour absorption between vertical and horizontal filters before saturation. Although there are not enough data points – no points between 4 hours and 14 hours – to draw a conclusion, vapour absorption of horizontal filter concaves down more slowly than that of thin filter, indicating higher saturation level. Since both horizontal and vertical filters have same filter size, differing saturation levels seem to be due to filters’ orientations. The data presented in figure 14 demonstrate that the results are similar for the single walled and multiple walled filters before saturation. Vapour absorption of multiple walled filter concaves down more slowly than that of single walled filter, indicating higher saturation level. Figures 10, 11, 12, 13, and 14 show that vapour absorption is linear with time until saturation, thickness and area do not scale proportionally with vapour absorption, vertical orientation results lower saturation than horizontal orientation, and multiple walled filter absorbs much faster than singled walled one. From the data points, around 10 per cent of vapor emitted from the humidifier was captured by the fibre filters and between 3 and 6 per cent by pleated air filter before getting saturated. Applying to Caltech’s cooling towers, such filters can save around 16 million litres per year.
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Figure 9.13: Vapour absorption versus time of horizontal and vertical pleated air filters
Figure 9.14: Vapour absorption versus time of single and multiple concentric vertical pleated air filters 134
Discussion Caltech cannot use seawater for cooling purposes due to its location over thirty miles from the ocean and fresh water is becoming scarcer every year. Cooling towers recycles their cooling water while keeping high cooling efficiency, yet takes up forty two per cent of Caltech’s total water consumption. Recovering a small amount of vapour dismissed from Caltech’s cooling towers by use of fibre filters can improve overall water efficiency significantly. Further research should be carried on to find optimal thickness, area, and orientation of filter – extensive parameter study. Efficient ways to extract condensed vapour from saturated filters and practical application to cooling towers should be studied thereafter.
Experimental Study of the Flue Gas Injection to improve the Natural Draft Cooling Tower Performance under Crosswind Natural draft dry cooling towers are the common towers that mostly used in waterless areas. Environmental conditions such as crosswind play an important role in the cooling tower performance. In this study, experimental model of a single cooling tower at the base in the KARAJ power station of IRAN is presented as a case study. Pressure coefficient around model at the entrance of the towers has been measured by using wind tunnel. Effects of crosswinds on the thermal performance of natural draft dry cooling towers have been investigated. Finally, the effect of flue gas injection on natural draft cooling tower performance through experimental simulation has been studied. Results indicate that flue gas injection will improve the performance of cooling tower under cross wind condition.
Introduction Natural draft cooling towers are utilized widely in large scale power plants. The performance of dry-cooling towers is highly sensitive to the environment conditions, particularly the crosswind which may cause reduce up to 40% of the total power generation capacity. Therefore, understanding the environmental conditions effect on cooling tower performance is of great importance. The conventional design of cooling towers does not sufficiently consider the impact of wind, which in fact exists most of time in reality. Hence, it is important to study the influence of wind on the performance of cooling towers and suggest suitable improving methods. Several investigations tried to understand the effect of wind on cooling tower performance. Wei et al experimentally investigated the mechanism ofunfavourable effects of wind on cooling efficiency of dry cooling towers. Du Preez and Kroeger carried out extensive experimental and numerical researches on the performance of wind-break walls. They studied dry cooling towers that have heat exchangers horizontally placed in the inlet cross-section of towers. Bender et al investigated the utilization of wind-break walls to balance the airflow rate into cooling tower intakes to prevent ice formations due to cold and windy weather. Both of them concluded that suitable arrangement of wind-break walls can lead to significant reduction in the adverse effects of crosswind. With the aim of finding optimal wind-break solutions, this study uses both wind tunnel experiment and CFD simulation approaches to investigate the performance of wind-break methods for cooling towers under windy conditions. The research interest has been consistently focused on the cooling towers with vertical heat exchangers around the bottom of towers, which generally confront the most significant impacts from crosswinds. 135
As a consequence, an idea to improve the performance of cooling towers is to utilize of flue gas injection to reduce wind effect on cooling tower performance. Eldredge et al numerically investigated the effects of flue gas injection on natural draft cooling tower performance. They considered five independent variables; flue gas flow rate, flue gas temperature, radial injection location, injection orientation, and liquid entrainment in the flue gas. They concluded that flue gas temperature have the most significant effect on tower performance. Su et al studied the thermal performance of a dry cooling tower under crosswind conditions using computational fluid dynamics (CFD) technologies. The tower had vertical heat exchangers placed around the bottom. They showed that the most important factors for the tower efficiency reduction is the wind-caused divergent airflow at external side of heat exchangers. Fu and Zhai numerically studied the effects of crosswind on two in-line dry cooling towers. The two-tower case demonstrated different heat transfer and airflow patterns from the single tower study, especially when the wind speed islarger than 10 m/s. However, the study verified that the wind induced around flow destroys the radial inflow into the cooling towers and thus extensively deteriorates the heattransfer performance at lateral sides. Cooling tower is normally designed for stagnant ambient air condition, but experimental observation showed that cooling efficiency was changed as a function of crosswind velocity. Experimental and numerical observations identically showed that heat transfer capacity of the cooling tower proportionally increased with wind velocity up to 3 m/s, and then decreased for higher wind velocity. Al-Waked and Behnia simulated the flow field in the presence of different wind breakers under the wind condition. They evaluated the air mass flux through the radiators. Also, they computed the heat flux from the radiators and concluded that in the presence of wind breakers both the mass and heat fluxes increased under the high wind velocities. Also, other researchers Parvizi reported the same beneficial concept of using the wind breakers. Although the wind breakers improve the cooling efficiency, designers have never used them practically. Al-Waked investigated crosswinds effect on the thermal performance of natural draft wet cooling towers numerically. Goodarzi proposed a stack configuration for dry cooling tower to improve cooling efficiency under crosswind. Numerical simulation of the proposed configuration showed improvement in the cooling efficiency up to 9 % compared to the present.
Wind tunnel experiments and accessories (i)
Reynolds number
Where, ρ = ambient air density V = mean velocity passing through the tower D=diameter of tower in base μ=dynamic viscosity
(ii)
Pressure coefficient
Where, 136
CP= pressure coefficient at a height of h and angle of θ P= local pressure at (h,θ), P∞=free stream static pressure V(h)=free stream mean velocity at a height of h h=the height of experiment. In general, the Reynolds number similarity cannot beachieved in wind tunnel testing; the Reynolds number of full scale tower order is 107, 108 and in wind tunnel order is 105 However outer pressure coefficient of model and prototype must be equal.
Case study All experiments hold in wind tunnel of ShahroodUniversity of Technology. This tunnel is open-circuit with at length of about 18m and 80cm ×80cm ×200 cm test section. The wind tunnel is equipped with a centrifugal fan and three phases motor. Maximum velocity of this is approximate 35m/s and turbulence intensity in maximum velocity is about 0.05% and 0.2% in minimum velocity as shows in Fig. 2. In these experiments, the inner and outer pressure distribution on the model was measured. Pressure measurement system consists of a Pressure transmitter, analog to digital converter, signals matching, Multimeter, Channel board, Computers and etc.
137
Fig. 9.15 Schematic of wind tunnel and test section
Figure 9.16
Model Hyperbolic model made of ceramic material is used to scalethe 1/400 of the prototype. The prototype was the KARAJ cooling tower which has the following dimensions: 92m height, 72m diameter of the base, 49m diameter of the tip, 48m diameter of throat and 15 degree circumferential spacing are located around the model inlet to measure the pressure distribution. The holes placed at a height of a / b = 0.5, where, a is the height of holes and b is radiator height. The test tubes connect the holes and the pressure transmitter. All 138
measurements were stored in computer data storage system. The rate of pressure signals were 200Sample/s. The model position in wind tunnel was at a height of 0.2m from test section floor and 1m from its beginning. Figs. 3 and 4 show the model and schematic of the used equipments, respectively.
Fig. 9.17 the model used in the experiments
Fig. 9.18 Schematic of equipments used in the test
139
Results and discussion In order to determine the pressure distribution, the dimensionless pressure coefficient was used, as in (2). Pressure distribution curves for different Reynolds numbers based on the angle at the outer surface and inner surface are plotted in Fig. 5, and Fig. 6. As is shown in Fig. 5, with increasing Reynolds number the external pressure distribution becomes more negative, in the side part of tower (outside the radiator), that means the flux of the air entering into the tower is reduced in that part of towerand so the cooling efficiency decrease because the air tangential velocity is lower similar to flow over a cylinder. According to Fig. 5, the flux of air entering the radiators increases in the front part of tower (facing the wind). From Fig. 6, it is clear that the internal pressure distribution increases and became worse with increasing Reynolds number. As a result the cooling efficiency decreases with increasing wind velocity.
Fig. 9.19 the outer pressure distribution
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Fig. 9.20 the inner pressure distribution
Fig. 9.21 Mean pressure distribution curve based on the Reynolds (a) internal surface (b) external surface
141
It is obvious that increasing the wind speed can cause the reduction of air entering into the tower and cooling tower efficiency. To reduce the adverse effects of wind on the performance of cooling towers, various proposals have been presented. One way was to use the flue gas injection into natural draft cooling tower. Although natural draft towers are not usually as tall as the stacks, benefit is taken of the upward momentum in the towers, along with the buoyant plume, to achieve the necessary atmospheric rise for the flue gas. Another advantage of gas injection into a cooling tower is that the discharged flue gas gets diluted by as much as a factor of ten before disposal to the atmosphere. As shown in Table I the pressure distribution become more negative due to the injection, and thus the tower performance can be improved. In Fig. 8 the influence of external pressure distribution is plotted for the flue gas injection. Notice that as a result the external pressure distribution does not change. So this method cannot be used to improve the distribution of external pressure.
TABLE 9.2 INTERNAL MEAN PRESSURE COEFFICIENT Reynolds x 105 0.66 0.76 1 1.2 1.9 2.8 3.2
Mean Pressure Coefficient Mean Pressure Coefficient Normal Flue Gas Injection -0.65 -0.85 -0.58 -0.73 -0.48 -0.56 -0.46 -0.50 -0.45 -0.48 -0.44 -0.47 -0.43 -0.46
Fig. 9.22 the outer pressure distribution
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Conclusion In this study, effects of crosswinds on the thermal performance of natural draft dry cooling towers have beeninvestigated. It was observed that with increasing the windspeed the circumferential pressure distribution becomes more unfavorable and the cooling efficiency decreases. To reduce the unfavorable effects of wind on the performance of cooling towers, effect of flue gas injection on natural draft cooling tower performance through experimental simulation has been studied. Using flue gas injection into the cooling tower the internal pressure distribution becomes more negative, which improves the performance of the cooling tower. It was observed that changes in external pressure distribution are not significantly influenced by gas injection either. As a conclusion, flue gas injection into natural draft dry cooling towers will improve their performance under cross wind condition.
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CHAPTER 10 MAINTENANCE COOLING TOWER MAINTENANCE When the cooling system’s installation has been completed, it is necessary to start the cooling tower and place it in service. Condenser water system commissioning can be broken down into four basic elements, with numerous requirements associated with each element, outlined as follows:
A-Condenser water pump 1. Check pump installation, including mountings, vibration isolators and connectors, and piping specialties (valves, strainer, pressure gauges, thermometers, etc.). 2. Check pump shaft and coupling alignment. 3. Lubricate pump shaft bearings as required by the manufacturer. 4. Lubricate motor shaft bearings as required by the manufacturer. 5. Turn shaft by hand to make sure the pump and motor turn freely. 6. “Bump” the motor on and check for proper rotation direction.
B- Cooling tower 1. Clean all tower surfaces flush and clean the wet deck and basin. 2. Clean the basin strainer. 3. Lubricate fan shaft bearings as required by the manufacturer. 4. Lubricate motor shaft bearings as required by the manufacturer. 5. Test and adjust belt drive (if installed). a) Adjust belt(s) tension. b) Check and adjust belt(s) alignment. 6. Test and adjust gear drive (if installed). a) Fill oil reservoir. b). Check shaft alignment. c). Check couplings for bolt tightness, excess play, etc. 7. Turn shaft by hand to make sure fan and drive turn freely. 8. “Bump” motor(s) on and check fan(s) for correct rotation. 9. Run fan(s) for a short period and check for unusual noises and/or vibration. Verify that motor amps are in accordance with manufacturer’s data. 10. Confirm that the condenser water piping has been flushed and chemically cleaned. 11. Fill the basin and piping to the manufacturer’s recommended basin operating level. 12. Run condenser water pump for a short period. a). Verify that pump motor amps are within motor nameplate rating. b). Test the wet deck distribution, fill, and basin for proper water flow. c). Check basin for vortexing. 13. Test basin freeze protection thermostat and heater. 14. for multi-cell installations: a). Ensure that automatic isolation valves function properly. b). Balance condenser water flow to and from each cell. c). Balance equalizer line to maintain proper basin operating level in each cell. 15. Place tower fan and condenser water motor starters in “automatic “position”. 144
C. Controls 1. Check that all temperature sensors are properly installed. 2. Test operation of bypass control valve and adjust for 708 rotation. 3. Check controller set point. 4. Test fan control relays for proper functioning. 5. Test VFD (if installed) for proper operation. 6. Test operation of vibration cutout switch. 7. Test operation of basin level controls, including high and lowlevel alarms. 8. Place controls in operation.
D. Water treatment 1. Check that all pH and/or conductivity probes are properly installed. 2. Test operation of blow-down solenoid valve. 3. Place water treatment equipment in operation. Once the cooling tower system is placed into fulltime service, routine inspection and maintenance must be done to ensure proper tower operation and to obtain the expected service life of the equipment. The required maintenance can be broken into two areas: water treatment and mechanical maintenance
A. Water treatment management 1. Require and evaluate regular and frequent reports by the water treatment contractor, first to ensure that regular water treatment is being done and, second, to “track” the various treatment parameters such as pH, TDS, chemical types, quantities used, etc. 2. At least twice each year, send a water sample to an independent laboratory for analysis and compare the results with the most recent monthly report from the water service contractor. 3. During shutdown periods, the maintenance staff should inspect the tower and as much piping as possible for scaling or fouling that is being inadequately addressed by the water treatment program. 4. Track chiller and tower performance on a routine basis to determine if the system is remaining free of deposition or fouling. The relationships between load, temperature difference, and power input tracked. If the relationships between these values change appreciably, this could indicate chiller fouling, tower fouling, or other performance problems.
B. Mechanical maintenance When the tower is to be started after being shut down for a lengthy period of time, it must be thoroughly inspected and repaired, as follows: 1. Check drift eliminators for proper position, being clean, etc. 2. Check fans, bearings, motors, and drives for proper lubrication. 145
3. Rotate fan shaft(s) by hand to make sure they turn freely. 4. Check fan motors for proper rotation and adjust belt tension for beltdrives. 5. Fill basin with fresh water and check operation of level controller. 6. Start condenser pump and check wet deck for proper distribution. 7. Check fill for fouling and/or clogging and clean or replace if necessary. 8. Check access door gaskets and replace as necessary. 9. Thoroughly inspect all metal surfaces for corrosion, scale or fouling, or sludge. Clean as required and any damaged metal should be cleaned down to bare metal and refinished with a cold zinc coating. 10. Operate tower and look for and repair any water or air leaks from the basin, casing, or piping. During the cooling season, regular inspection and maintenance of the tower is required to ensure proper operation, as follows:
A. Weekly 1) Clean basin strainer. 2) Check blow down valve and make-up water valves to make sure they are working properly. 3) Test water and adjust chemical treatment as necessary. 4) Check/fill gear drive oil reservoir.
B. Monthly 1. Clean and flush basin. (This may be required more often for towers located adjacent to highways,industrial sites, etc. with high particulate emissions.) 2. Check operating level in basin and adjust as necessary. 3. Check water distribution system and sprays. 4. Check drift eliminators for proper position. 5. Check belts or gearbox and adjust as necessary. 6. Check fans, inlet screens, and louvers for dirt and debris. Clean as necessary. 7. Check keys and set screws.
C. Regularly 1. Lubricate motor in accordance with MFGR’s instructions. 2. Lubricate fan shaft bearings every 1000 hr or 3 months. 3. Check gear drive in accordance with MFGR’s instructions. 4. Blow-down condenser water pump strainer.
D. Yearly 1. Clean and touch-up paint or other protective finish as necessary (including grillage) 2. Dismantle and clean condenser water pump starter.
Condition monitoring of cooling tower fan gearboxes Abstract To reduce visual and environmental impact, modern power stations are built with induced draft cooling towers replacing the large natural draft cooling towers previously used. The ability of the cooling towers to provide adequate cooling for the main circulating water of the power station is affected by the availability of the cooling tower fans. 146
A key component of the cooling tower fan is the gearbox. Maintenance of gearboxes is difficult and expensive, given the location and therefore condition monitoring techniques have been applied to detect common failure mechanisms. Future condition monitoring may be possible using low cost on-line techniques.
Introduction Industrial main circulating water (MCW) provides the primary cooling for the industry.Before being returned to the industry by the MCW pumps, the water is cooled.As it flows over the baffles in the cooling tower, some water evaporates to providethe cooling
Objectives of condition monitoring The objectives of condition monitoring are as follows: 1. To collect the minimum information needed to plan predictive maintenance and to avoid high cost breakdown maintenance. 2. To assist in identifying appropriate actions when alarm levels are exceeded or significant changes are detected. 3. To maximize proactive maintenance by the detection of faults in plant installation or reinstallation which, if left uncorrected, would lead to plant damage. 4. In many cases, a combination of techniques is required to fulfill these objectives.
Condition monitoring techniques in use Hand held vibration monitoring The majority of auxiliary plant on a industry is periodically monitored using hand held data collectors, collecting vibration data using a portable accelerometer. The data is then loaded into a software package on a PC in order to analyze the data. Access to the cooling tower fan gearbox is prohibited during operation, so the only data available to the operator is from the driving motor. Experience has shown that the timely detection of gearbox faults from analysis of motor vibration readings alone is difficult, but readings at the drive end of the motor can be analyzed to provide corroborating evidence.
Location of permanently installed accelerometers on gearboxes The attenuation of gearbox vibration signals (e.g. from a damaged rolling element bearing) through the gearbox, input shaft and motor housing to an accelerometer placed on the motor means that an accelerometer must be placed on the gearbox to obtain adequate information. In most cases, accelerometers have been mounted on the gearboxes radially in line with the top output shaft (i.e. lowest speed) bearing. This location was initially selected due to concerns over failures of the top output shaft bearing, but experience has shown that this location is also suitable for monitoring of the higher speed input and intermediate shafts. The vibration signals from these shafts are higher in magnitude that from the low speed output shaft and, even with attenuation through the gearbox, are detectable at the output shaft accelerometer. Conversely, accelerometers mounted at the bearing locations of the input shaft are not able to effectively monitor output shaft vibration signals.
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Velocity vibration measurements Vibration of auxiliary plant has traditionally been measured in units of velocity (mm/s RMS), which gives an indication of the condition of the plant relatively insensitive to speed (1). Initially, velocity readings on gearboxes were set up to detect 2 failure mechanisms: 1. Fan and installation defects (e.g. unbalance misalignment). 2. Defects appearing in the vibration spectrum at gear mesh frequencies.
Fan and installation defects Fan and installation defects (e.g. unbalance, misalignment) appear at low frequencies, usually less at less than 10 orders (multiples) of running speed. For the case of the output shaft, the 1st order frequency can be less then 1Hz. Detection of the low frequency defects is difficult, firstly due to the poor response at low frequencies of standard accelerometers and secondly because there is significant low frequency vibration of the cooling tower structure, saturating the accelerometer input to the data collector.
Defects appearing at gear mesh frequencies The maximum frequency of data collected is above the 3rd harmonic of the gear mesh frequency (the product of the shaft speed and the number of teeth on the gear). In all cases of cooling tower fan gearboxes, the highest frequency of interest is less than 2000Hz, so high frequency accelerometers are not needed and data is collected in units of velocity mm/s RMS. Enveloped signal measurements Enveloped Signal Processing (2) is a well proven technique for the detection of defects in rolling element bearings. Despite the location of a single accelerometer at the output shaft top bearing location, it was hoped that rolling element bearing defects on other shafts within the gearbox could be detected. The envelope most commonly used is 5kHz - 10kHz and the maximum frequency of interest is 20 orders of input shaft speed. The data is collected in units of acceleration g’s Peak.
Motor current analysis Motor stator current analysis (3) has traditionally been used to monitor the condition of induction motor rotors and the sensitivity of the technique to load fluctuations is known. A variation on this technique has been used, analyzing the time waveform of the stator current over a short period (e.g. 10s). A normal cooling tower fan gearbox motor current has little variation of current over time but a gearbox or fan with a serious defect (e.g. sheared gearbox holding down bolts, unbalanced fan blades) shows modulation of motor current at output shaft frequency.
Oil analysis Oil analysis has been a prime condition monitoring technique for gearboxes, often able to detect gearbox wear before vibration analysis (4). The difficulty in applying this technique to cooling tower fan gearboxes has been the collection of a representative, repetitive sample. One station installed dedicated sample collection and return piping from the gearbox to a location external to the cooling tower cell. When a sample is needed, a sample pump is connected to the collection and return piping and, after allowing time to flush the sample 148
piping, a representative sample can be taken. The disadvantage of this method is that the small bore piping connected to the gearbox is vulnerable to damage, which presents an environmental risk from 80l of oil leaking from the gearbox. The other method in use is to use the oil filler pipe as the connection to the gearbox. The pipe is flushed into a container until warm oil is detected, so that the sample is reasonably representative of the oil inside the gearbox. The flushed oil is then put back. Standard industrial tests are performed on the oil (kinematic viscosity, water content, total acid number, elemental analysis and particle quantifier). The results of oil analysis have generally been good but an equally important use of oil analysis has been the ability to prolong oil life in gearboxes, based on the oil condition. In one application, the increased cost of using synthetic oil was justified because the frequency of oil changes was reduced (saving the associated maintenance costs of the oil change as well as the cost of the oil). Quadrupling of oil life was achieved.
Common failure mechanisms It is not the intention of this article to provide a list of the all potential failure mechanisms of cooling tower fan gearboxes. Some examples of the common failure mechanisms are included, together with the suggested condition monitoring techniques.
Gear teeth failures Some gearboxes experienced single teeth failures on the intermediate shaft bevel gear. Metallurgical analysis of the damaged teeth suggested a manufacturing defect r a transient overload on the gear, possibly when the motor was started. The defect was detected using Enveloped Signal Processing of the signal from the accelerometer mounted at the output shaft. The output shaft accelerometer signal is integrated to give a velocity reading. Figure shows a typical gear with the damaged tooth clearly visible.
Input and intermediate shaft bearing failure Possible causes of bearing failure are solid or water ingress via a shaft seal, poor lubrication or contamination of oil. A gearbox was returned to service after repairs to gear teeth but
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failed soon after being installed. Using Enveloped Signal Processing an input shaft bearing defect was detected using the accelerometer mounted at the output shaft.
Output shaft bearing failure A number of different failure mechanisms are responsible for output shaft bearing failures. Some designs of gearbox are prone to oil starvation of the top output shaft bearing when the fan rotates in reverse. This has been addressed by the fitting of anti-reverse rotation devices. If the fans are left stationary false brinnelling damage can take place. Over long periods (>3 weeks) the thickness of the oil film reduces to the point where fretting corrosion can occur if water ingress occurs via the shaft seal. Such damage can occur during commissioning, when some fans run but others are left stationary for long periods. Enveloped Signal Processing is recommended for detection of these bearing failures.
Gearbox overheating Atonestation,over half thegearboxesexperiencedfailuresoftheoutputshaftbottombearing. One of the root causes of failure was oxidation of the oil at the elevated temperatures at which the gearbox was running. The gearbox did not have a cooling fan mounted on the input shaft and the result was a “dead space” in the airflow around the gearbox. Oil analysis detected the oxidation and the grade of oil was changed from ISO VG220 to ISO VG320.
Fan unbalance Water logging of fan blades can cause fan unbalance. Porous GRP materialdelaminates and small pieces break off inside the hollow fan blade, lodging at theend of the blades and blocking the water drain holes. With no accelerometer installed on the gearbox, vibration on the driving motorwas monitored using a hand held accelerometer. A harmonic of the fan speedtriggered an alarm on the motor drive end bearing velocity spectrum reading.
Unusual failure mechanisms in temperature extremes At one station in the US, oil analysis detected the production of wear metals fromgearbox teeth. Subsequently, high levels of vibration were detected on the driveend bearing of the driving motor. The gearbox was stripped down and severeadhesive wear was found on the back of the output shaft gear teeth. During normal operation, the gearbox temperatures were over 90 ˚ C and soISO VG680 grade oil was used. A detailed investigation revealed that, while the cooling tower fans run forwards in normal operation, in extremely cold weather(-10˚C), the fans are run backwards at half speed in order to de-ice the coolingtower after a short period off-load. At this temperature, the kinematic viscosity ofthe oil is over 20000 centistokes (cSt) i.e. near its pour point (the temperature atwhich it stops flowing). The splash lubrication regime for the gear teeth did notwork resulting in severe adhesive wear.
Future condition monitoring Online vibration analysis
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The advantage of using hand held data collectors for vibration monitoring is thatit provides cost effective condition monitoring for the majority of auxiliary plant.On power stations, the periodic (e.g. weekly or monthly) vibration survey is backedup by the station operators during routine or ad hoc plant visits. A “noisy” bearingor change in vibration is logged and follow up instigated. With cooling tower fangearboxes, as with other inaccessible plant, operator plant visits are less likely tobe able to detect impending failures. With the reduction in cost of Vibration Isolated Measurement Pod (VIMP)type technologies, on-line monitoring of cooling tower fan gearboxes is becomingcost effective. A 16-channel VIMP type unit wired up to accelerometers ongearboxes can monitor all the gearboxes in industry. The VIMPtype unit is connected to an on-line version of the normal off-line vibration analysissoftware and the data displayed in the plant control room.The correct setting of alarm levels is important. While an experienced vibrationanalyst looking at monthly collected data can disregard readings causing “ false ” alarms, the control room operator may be overloaded with alarms in theon-line system. The development of multivariable alarms in software packagesmay help solve this problem. Laying communications cabling on an existing installation represents thebiggest problem (and cost) of an on-line system. A number of condition monitoringcompanies are developing radio link technology to solve this problem.
Online oil condition monitoring As previously discussed, oil analysis is a potentially powerful diagnostic tool ingearbox condition monitoring, but can be limited by the ability to collect a sample.A recently developed product is on trial (5), which provides a continuousindication of oil condition. The principle of operation is that the loss tangent (Tanδ) of the dielectricconstant (ε) of oil is affected by oxidation degradation, liquid or solid contaminationand wears metal production. The loss tangent is related to the dielectric constant(or permittivity) as follows: ε= ε’(1-jTanδ) Where ε’ is the real component of the dielectric constant. A low voltage dc powered sensor is located in the oil flow and gives a voltageoutput and an alarm output when a threshold is exceeded. If successful, theunits could be wired into the station distributed control system to prompt actiononce an alarm is triggered. Recent experience with the online oil condition monitoring has been on coalmill gearboxes. The online sensor has produced a “fault” indication when ingressof pulverized fuel occurred, which would not have been picked up by periodic oilanalysis until damage had been done to the gearbox.
Conclusions With a combination of techniques, correct setting of alarm levels and interpretationof data, gearbox failures can be detected, even if it is not possible to distinguishthe individual components that are failing. A combination of enveloped signal processing and high resolution velocityspectra are suitable techniques for detecting gear tooth failures, using vibrationmonitoring.
Improving cooling tower fan system efficiencies
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After a look at the problem for air cooled heat exchangers and cooling towers using axial fans,ways to improve system efficiencies in three areas are discussed: before the fan system design isfinalized, improvements in the physical equipment as installed, and recognition of performanceproblems caused by adjacent equipment.
Fan Systems This discussion is limited to the scope of two types of fan systems: those used in Dry Cooling Towers (air-cooled heat exchangers) and Wet Cooling Towers. Each of these devices are used to transfer heat and both have several things in common: 1. Both have an axial fan to move the air. 2. Both have a shroud to contain the fan and funnel air into the fan. 3. Both have plenums into which the air is directed so that heat can be transferred by direct or indirect contact. In the case of air coolers or radiators the contact is indirect while in cooling towers the air comes in direct contact with the heated water.
Fan System Efficiency When we design an air-moving device one of the most important tools we use is the fan performance curve. Using this curve of fan performance we plot a system resistance line to establish an operating point at which the fan performance exactly matches the system requirements. From the operating point we can define the fan pitch and power requirements. With almost any fan the pitch can be changed from the original estimate, if airflow is too low, to a higher pitch and greater flow. However, if our system efficiency or losses are not as assumed and we need more air, horsepower increases by the cube of the flow. A ten percent increase in flow requires a thirty-three percent increase in horsepower. Fan performance curves generally are obtained under ideal, reproducible, conditions. To refresh memory as to terminology, the head or total pressure that an axial fan works against is made up of two components. These are static pressure which is the sum of the system resistances and velocity pressure which is a loss associated with accelerating the surrounding air from zero to the design velocity. The only useful work done is by the static pressure component. This is measured in inches of water and an axial fan normally works in the regime of 0 to 2 inches total pressure. Air Horsepower is calculated by: HPAir = Total Pressure X CFM Efficiency An induced draft air cooler with the fan above the bundle, is also in wide use. It has more advantages but problems of a different nature. Induced draft fans are in the hot exit air which may create problems with maintenance although there are several other offsetting advantages over forced type units. Each type unit would be subject to the fan tip losses and fan hub seal losses. However, the major problems of inlet conditions to the fan ring and hot air recirculation are magnified by the high inlet velocity to the fan and the low exit velocity from the bundle of a forced draft unit.
System Losses Potential losses in system efficiency occur in several areas: 152
a)Losses caused by the fixed system design rather than by variable physical properties. Once the operating point of the fan is fixed these losses are built-in and cannot be easily detected or corrected. They are losses because they rob the system of potential efficiency. Examples of this type of system "loss" would be: choice of fan design, fan diameter selection, fan design operating point. b) Losses caused by "variable environmental properties" would be: lack of fan hub seals, excessive fan tip clearance, poor inlet conditions of the fan ring or stack, excessively high approach velocity to the fan, or random air leaks in the fan plenum. Often allowances for losses in louvers, bug screens, re-circulating ducts, and steam coils are simply omitted in design. c) Other performance losses could occur because of hot air recirculation. Of the above losses, the only easily corrected problems would be in category b) which we call "variable environmental properties. In the following discussion category a) will be covered in The Fan Itself. Category b) will be discussed in The Fan Housing and c) will be covered under Unwanted Air Movements.
(a) The Fan Itself.The ways a fan system could be inefficient are sometimes obvious but most of the time they are not. For instance, the blade design itself is a major factor. Modern axial fans are usually made by extruding aluminum or molding fiberglass. Extruded aluminum blades are by nature always of uniform chord width while molded fiberglass blades can have an irregular shape. See Figure 3, Fan Blade Shapes. One of the basic design criteria for blade design is to produce uniform air flow over the entire plane of the fan. One of the aerodynamic principles involved is that the work done at any radius along the blade is a function of blade width, angle of attack and tangential velocity squared. The "angle of attack" in airfoil design dictates the amount of blade twist required at any particular radius along the blade. It follows that as a point on the blade decreases from tip toward the hub the tangential velocity sharply decreases and in order to produce uniform airflow, the blade width and twist must increase. If the blade chord cannot increase in width, the twist must be increased to compensate. With an extruded blade the twist is created by mechanically yielding the blade to a prescribed degree. Due to limits in elasticity only limited twist can be created. In a molded blade there is no such limitation to chord width or twist so the "ideal" blade can be more closely approached. The point is that the blade design itself can create problems of non-uniform air flow and inefficiency. Another inherent property of an axial fan is the problem of "swirl." Swirl is the tangential deflection of the exit air direction caused by the effect of torque. The air vectors at the extreme inboard sections of the blade actually reverse direction and subtract from the net airflow. This is a very measurable quantity. An inexpensive hub component, the Hub Seal Disc prevents this and should be standard equipment on any axial fan. The point here is that, within limits, the fan speed can be varied so that a pitch angle can be selected which will optimize fan blade efficiency and will satisfy the required system resistance. Often it would be desirable to slow the fan down to attain a higher, more efficient operating pitch angle as an operating point. This also has a side benefit of reducing noise and vibration because normally the lower pitch angles which appear obvious choices to handle the duty have lower efficiencies. 153
Still another aspect of system efficiency is the proper selection of the Fan Diameter for any given conditions, operating and economic. There are several things which influence the choice of fan diameter. 1. Air Flow Range 2. "Fan Coverage" 3. Optimum Cell Size 4. Evaluated Horsepower 5. Standard Sizes Available Of these, the most logical influence is that the fan must provide the amount of Air Flow required for any duty in a sensible operating range. A quick look at any vendor's fan curve will yield several sizes of fans to do any particular job. A poorly sized fan will waste horsepower at the least and fail to do the required duty at the worst. For wet cooling towers, and recently for dry towers as well, the optimum cell size and evaluated horsepower comes into play. Both are purely economic considerations. Optimum cell size is obviously matching fan size to minimized construction cost per cell. The Evaluated Horsepower is increasingly becoming the major factor in deciding fan diameters. It is the variation of the Velocity Pressure Loss at each fan's operating point which greatly effects the required horsepower. The Velocity Pressure Loss is a fixed loss in every fan which reflects the energy used to collect the air into the throat of the fan. It is dependent on the Net Free Area of the fan and not on the exact entrance conditions. In reviewing the potential losses in efficiency in the fan itself we have discussed two inherent losses that were built-in to the system by design: 1. Poor fan blade design 2. Poor selection of operating point The two factors which could be physically modified to reduce fan system losses would be the addition of the Hub Seal Disc and the revision of the fan operating point to a more efficient condition, although a change in the number of blades or gear ratio might be required for the latter.
(b) The Fan Housing.The components that make up the fan housing would be considered a Fan Ring for air coolers or a straight or Velocity Recovery Stack for cooling towers.The most important system loss for both types would be the air leakage around the tips of the fan blades. This loss is a direct function of the Tip Clearance with the ring or stack and the Velocity Pressure at the operating point. This leakage is caused by the tendency of the high pressure exit air to re-circulate around the tips into the low pressure air in the inlet. The loss takes the form of reducing the Total Efficiency and Total Pressure capability of the fan.
Inlet conditions There are several areas where inlet conditions can seriously affect the fan system. For instance, the inlet condition to the Fan Ring for a forced draft air cooler, the inlet condition into the Velocity Recovery Stack of a wet cooling tower or the approach area itself to the whole air cooler or cooling tower. 154
The most obvious case is the inlet condition to the fan ring.These losses are not constant but vary with Velocity Pressure.
Velocity Recovery Stacks. In the case of Wet Cooling Towers, a relatively common means of improving inlet conditions and conserving horsepower is known as a Velocity Recovery Stack. These stacks incorporate a slightly tapered exit cone and a well-rounded inlet bell. In theory, there is a significantly reduced Velocity Pressure at the exit compared to the plane of the fan. Since the quantity of air is the same as both planes, the recovery of Velocity Pressure is converted into "static regain" which lowers the Total Pressure requirements of the fan, thus saving horsepower.
Approach Velocity Consideration. Sometimes the economics of structural costs may unintentionally create very serious effects upon the system performance of heat exchangers. As with inlet losses to the fan, the magnitude of the loss is a function of the Velocity Pressure which itself is a function of air velocity. It is considered good practice to insure that the air velocity at the entrance to the fan is no more than approximately one-half of the velocity through the fan throat.
(c) Unwanted Air Movements. There are often cases where in order to increase performance, you need to reduce air flow. These are cases where the warm exit air flow re-circulates to the inlet side of the fan and decreases the mean temperature difference between the cold entering air and the hot fluid temperature inside the tubes or in the fill thus lowering efficiency of the air cooler or cooling tower. The main factors which influence the tendency to re-circulate are primarily inlet or approach velocity, exit velocity and velocity of prevailing winds. A simple analytical method to predict recirculation in an air cooler utilizing the above parameters. The primary causes of recirculation could be summarized as follows: 1. Excessively high approach velocities. 2. Units placed in line with the prevailing wind direction. 3. Units placed at elevations so that the exit of one is upstream of the inlet of the adjacent unit. 4. Low exit velocities, such as those encountered in forced draft air coolers. Severe performance problems can result if recirculation is encountered. Recirculation can be confirmed by smoke testing and by temperature surveys of the exit and inlet air streams to a unit. To eliminate recirculation it is usually necessary to increase the exit air flow or change the elevation of the exit flow by adding straight sided fan stacks. In some cases baffles may have to be constructed.
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In cooling towers the effect of the Velocity Recovery Stack is to reduce the exit air velocity which could promote recirculation. It may be necessary to utilize straight stacks to jet the hot exit air further away from the approach or inlet areas.
Air Leakage This is another category of unwanted air flow. Air leakage could occur in an air cooler at several places which lower system efficiency: 1. Ineffective or missing tube bundle air seals alongside frames, between tube bundles or at the ends of tube bundles. 2. In older unit’s plenums could be rusted out causing holes and loss of effective air flow over the bundles. In a cooling tower you could have: 1. Missing panels in the casing 2. Holes in the fan stacks 3. Missing boards or holes in the fan deck The net result of these problems is that the air movement intended to go through the tube bundle or fill takes the path of least resistance and consumes power but does not work.
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CHAPTER 11 BUYING AND REPLACING OF COOLING TOWER Defining tower performance requirements The first step in purchasing a new or replacement HVAC cooling tower is to define the performance or capacity for it. To do this, the following fourparameters must be specified: 1. Condenser water flow rate, gpm(circulating rate) 2. Entering (return) condenser water temperature, oC 3. Leaving (supply) condenser water temperature, oC 4. Entering (ambient) wet bulb temperature, oC The difference between the entering and leaving condenser watertemperatures defines the required range, while the difference between theleaving condenser water temperature and the entering wet bulb temperaturedefines the required approach. With range and flow rate, the total Btu/h ofheat rejection capacity is defined. With this data, any manufacturer can selecta cooling tower. The next step is to define one or more tower configurations that would beacceptable, as follows: A- Forced/propeller fan B- Forced/centrifugal fan C- Induced/propeller fan Flow arrangement 1—Crossflow 2—Counter flow D- Assembly a) Factory assembled b) Field erected E- Construction a) Wood b)Galvanized steel c)Galvanized steel with stainless steel basin (and wet deck, for across flow tower) d)All stainless steel e)Fiberglass reinforced plastic (or equivalent) f)Concrete/masonry When one of the acceptable alternatives is a galvanized tower, it isrecommended that the basin and, for cross flow towers, the wet deck beconstructed of stainless steel. This option is offered by all of the towermanufacturers and will increase tower costs by 20–40%. But,the typicalcooling tower life is extended by at least 33%, making itan excellent investment. 157
Estimated economic life Equipment Pump, base mounted Pump, line mounted Cooling tower, galvanized steel Cooling tower, FRP Cooling tower galvanized steel, basin and wet deck Cooling tower, wood Cooling tower, all stainless steel Table 11.1
Life(years) 20 10 15 20 22 25 35
Factory-assembled towers are much easier and faster to install and tend tobe less expensive. Thus, field-erected towers are rarely used for HVACapplications except, again, for very large loads (2,000 tons).
CTI ratings and performance Guarantees Except in very special circumstances or for large field-erected towers, HVACcooling towers should be specified to have their performance certified by the CoolingTechnology Institute (CTI) in accordance with CTI Standard 201. This certificationamounts to a third party guarantee that the tower will perform as advertised providedthe tower is installed correctly and the condenser water is “clean.” CTI certification eliminates the need and expense of field-testing a coolingtower to confirm that it performs as specified.
First costs The initial capital costs associated with each potential cooling tower are all of thecosts that would be incurred in the design and construction of that tower and theassociated condenser water pump. Equipment costs can be obtained directly fromthe prospective equipment vendors. Installation cost estimates can be obtainedfrom local contractors. The construction cost estimate must include the following, in addition tothe cost of the tower itself: 1. Tower dunnage and grillage 2. Rigging 3. Demolition 4. Electrical power 5. Controls 6. Contractor Overhead (Insurance, bonds, taxes, and general officeoperations, special conditions), typically 15–20% 7. Contractor Profit, typically 5–20% Other costs that may be included in the capital requirement are design fees,which may increase or decrease as a function of the selected alternative; specialconsultants’ fees; special testing, etc. Also, condenser water, make-up water, and/or drainage piping may change configuration and cost between alternativetowers. Unless the cost of the tower is being met from operating revenues, at least aportion of the capital expense will be met with borrowed funds. The use of thismoney has a cost in the form 158
of the applied interest rate and information aboutthe amount of borrowed funds, the applied interest rate, and the period of the loanmust be determined for the analysis.
Annual recurring costs Once the tower system is placed into operation, two annual recurring costs mustbe met each year of its economic life: energy costs and maintenance costs. The economic life for an alternative is the time frame within which itprovides a positive benefit to the owner. Thus, when it costs more to operate andmaintain a piece of equipment than it would to replace it, the economic life hasended. Typically, the economic life (sometimes called “service life”) is the periodover which the equipment is expected to last physically
Buying CT The computation of energy cost requires that two quantities be known: (1)the amount of electrical energy consumed by the tower and the associatedcondenser water pump and (2) the unit cost or rate schedule for that energy. Thesecond quantity is relatively easy to determine by contacting the utilities servingthe site or, for some campus facilities, obtaining the cost for steam, power, chilledwater, etc., that may be furnished from a central source. To accurately evaluate and compare alternative cooling towers, it isnecessary to also consider the condenser water pump. Since different towers mayimpose different pressure losses on the pump, this energy and cost componentmust be incorporated into the analysis. To determine the total energy use by the condenser water pump and coolingtower, the first step is develop a “load profile” for the condenser water system interms of system load (tons) as a function of outdoor temperature, usually for eachfive degree “bin” of outdoor temperature. At each bin, the power input to thecondenser water pump and cooling tower must be determined in terms of kW andthen multiplied by the hours of occurrence of that bin to yield energyconsumption in terms of kWh. Adding the energy consumption in all of the binsyields the total annual energy consumption. For single speed motors, the “percent load” is 100% anytime the fan is on. For 2-speed or pony motor arrangements, the full speed load is100%, while the half speed load is approximately 30%. When a VFD is used, thepercent load can be estimated. Cooling system load profiles fall into three basic forms: 1. Where airside economizer systems are used or the system is located in acold climate and is not operated in the winter, the load can be proratedbetween the design summer temperature and about 558F. In each ofthese bins, since some cooling is required, the condenser water pumpmust run and, therefore, its load is 100% in each bin. The tower fan, essentially equal to the percent load imposed on it. 2. When the cooling system operates on a year around basis, the coolingload will be prorated to all bins of temperature. 3. When a waterside economizer is in use, the winter use of the condenserwater pump and cooling tower must be estimated. Again, if there is anycooling load imposed in a bin, the condenser water pump powerrequirement is 100% in that bin. The tower fan use can be prorated indirect proportion to the imposed cooling load in the winter.
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The energy cost, then, is computed by multiplying the electrical energyconsumption by the unit cost for electricity.Annual recurring maintenance cost is a very difficult element to estimate. Lacking other information, the annual routine maintenance cost associated withpumps and cooling towers can be estimated as a percentage of the initialequipment cost, as follows: 1) Galvanized steel or wood cooling tower 3% 2) Galvanized steel tower with stainless wet deck and basin or Fiberglass reinforced plastic (FRP) tower 2% 3) Stainless steel or masonry tower 1% These costs do not include the cost of make-up water or the water treatmentprogram. Since the amount of make-up water and water treatment chemicalconsumption is a function of (a) cooling tower loads, (b) the water treatmentprogram required, and (c) the water flow rate, it is essentially independent of the power itself. Therefore, this element can be ignored when comparing alternativecooling towers unless there is a significant difference in drift losses betweenalternative towers. (The one exception to this is the additional water treatmentrequired for wooden towers and this additional cost should be included in theanalysis.
Nonrecurring repair and replacement costs Nonrecurring costs represent repair and/or replacement costs that occur atintervals longer than 1 year. For example, a galvanized wet deck and basin in across flow induced draft tower may require significant repair (or evenreplacement) after 10 years of service. These costs must be determined and theyear of their occurrence estimated. Total owning and operating cost comparison The total owning and operating cost for a cooling tower and its associatedcondenser water pump, over the system economic life, can be computed Life Cycle Cost = C + ( Sum of Repair/Replacement Costs) +[(Economic Life) *Annual Energy Cost + Annual Maintenance Cost] -----------------------------------------(11.1) C is the total capital cost computed
Procurement specifications Recommended specifications for both induced draft and forced draft coolingtowers are provided in Appendix. These specifications are based on factory assembledgalvanized steel towers with stainless steel basins and, for cross flowtowers, stainless steel wet decks. Each specification includes both cross flow andcounter flow configurations and, therefore, must be edited carefully if only one ofthese configurations is acceptable.
Water treatment program contracting The following are the recommended minimum standards that should be appliedto the selection of a water treatment service company: 160
1. To be considered, a water treatment service company should be anestablished company with full service capabilities. The companyshould have been in business at least five years, have corporate staffwith sufficient expertise and experience to competently address allaspects of water related issues, and be capable of providing areasonable list of references for whom they have provided service for aminimum of two years. Call and verify the listed references. 2. The local service personnel should have sufficient expertise andexperience to competently address all aspects of water related issues.After all, it is the local service technician who will actually provide theday-to-day service, not a “water expert” at the corporate office. Ask fordetailed resume for the service technician. Further, check into thecompany’s service technician turnover rate. If the local technician isconstantly changing, the quality of service provided by this companywill be uneven. 3. The proposed approach to water treatment should incorporate proven technology. This does not mean totally excluding “new” technologies,products, or methods, but it does mean that the water treatment servicecompany must demonstrate that their proposed technology has been successfully applied at other locations with similar water treatmentconditions and needs. Ask for a list of these locations and call them. 4. Ensure the program performance and its cost. Even with the mostreputable water service companies, there are many instances where thewater treatment program performance and/or cost did not meetpromises. Therefore, develop detailed and rigid performance and coststandards and include them in the contract with the service company,including penalties if the standards are not met. As a basis of comparing alternative water treatment programs by different servicecompanies, prepare a detailed “request for proposals (RFP)” that definesrequirements and standards and send it to several service companies whopotentially meet the standards outlined above. Thus, there will be severaldifferent proposals that will have a common basis for comparison. For largersystems, it may be necessary to retain an independent consultant to develop theRFP and/or evaluate the vendor proposals. Costs incurred for the consultant willbe more than offset by the proper selection of a cost-effective water treatmentprogram. Initial water service treatment contracts should be for a two-year period.This gives the service company ample opportunity to address all of the problemsthat may be in the system and meet the cost and performance goals that have beenestablished. After this initial period, contracts should be for one year.
In situ tower performance testing For large towers or for towers with special requirements that are not CTI certified, in-situ testing is the only way to guarantee that the tower will perform as required. This is an expensive and time-consuming process that should be undertaken only after due consideration.
Why in-situ testing? Thermal performance testing of an operating cooling tower is a complicated and expensive undertaking. Costs to test even a medium-sized tower can run well into five figures. These tests are normally conducted for one of two purposes: 1) Acceptance tests may be required to demonstrate that the installed cooling tower meets the performance standards were specified for the majority of HVAC applications, tower performance for many brands and types of cooling towers are certified by the CTI, and it is more cost-effective to simply a certified tower and, thus, avoid the cost of acceptance testing 161
entirely. In some cases, such as for very large towers or field-erected towers, an acceptance test may be necessary. 2) Performance testing to evaluate a cooling tower’s performance relative to correcting problems and/or changing the tower’s thermal requirements. In-situ performance testing yields actual tower performance data that can be used to indicate required tower modifications or improvements.
Testing criteria and methods The field testing of cooling tower performance must be done in accordance with CTI Standard ATC-105 or Standard PTC-23 issued by The American Society of Mechanical Engineers (ASME). Testing is typically done by third party agencies and it is recommended that the selected agency be an agency that is licensed by CTI Commonly. Thermal performance tests are referred to as either Class A or B tests. The Class A test is one conducted by using mercury-in-glass thermometers and grade level psychrometers, while the Class B test uses a data acquisition system with psychrometers arranged in an array over the entire air inlet face of the tower. Most testing today is the Class B test, using data acquisition systems to measure temperatures. With this test method, the first step is to inspect the tower to ensure it is ready for the test and to identify points of measurement. In the case of an acceptance test, the installer and/or manufacturer will normally be much more thorough in this area to ensure the tower’s full potential is measured. Once all parties are satisfied that the tower is ready for testing, instruments are deployed and the testing begins. For typical large HVAC cooling towers, the testing process requires one to two days. However, weather and operating conditions can sometimes increase this time. To begin the testing process, the test technicians begin taking data. Usually, the thermal data are started and monitored for a brief period. If any problems with instrumentation or conditions are noted, efforts will be made to correct them. Once any corrections are completed, the test begins and must last at least 1 hr after steady-state conditions have been achieved. During the test, the technicians will monitor the system temperatures and measure the water flow rate and fan power. The two test standards offer recommendations on deviation from design conditions for the test parameters. While it is preferable to comply with all these limitations, it is not always possible. CTI licensed testing agencies report deviations from recommended parameters in 70–75% of all tests. Recognizing this, the standards allow for deviation provided all parties agree. If at any time during the process, it is determined a parameter is outside the recommended limitations, all parties must review the situation and reach a unanimous solution. This can result in data being discarded and restarts required. Two parameters, the limits on oil, tar, or fatty substances or the total dissolved solids in the condenser water are not routinely checked during a tower test. However, if the tower fails the test, and any party thinks that these agents are present and could have contributed to the failure, these parameters are measured. To measure the water flow rate, a pitot tube traverse of the piping carrying water to the tower is the preferred method. A Wattmeter is used to measure fan input power on mechanical draft tower systems up to 600 V. Temperatures are measured with thermometers, RTD’s, or thermistors. Any other factor affecting the tower’s operation or the data taken must be recorded. These other factors may include pump discharge pressure, make-up flow and temperature, blow down flow and temperature, auxiliary streams entering the collection basin, etc. The hot water temperature is normally taken in the wet deck or in a tap in the piping carrying water to the tower. The cold water temperature is normally taken at taps on the discharge side
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of the pressure gauges at this location and these gauges can be replaced with flowing wells for temperature measurement. If this is not possible, separate taps are required. The CTI and ASME standards have defined the instrumentation and procedures very clearly. Unfortunately, the many installation variations and test circumstances provide multiple obstacles. This can create serious problems for the technicians and increased uncertainty of results. The straightforward process alone does not protect against completely meaningless results. For this reason, pumps in most cases, installations have the CTI carefully tests those individuals licensed by CTI to lead tests and inspects and approves their test equipment. Manufacturers also have highly skilled and trained staff to participate in the testing process, particularly for an acceptance test, to help ensure the products are properly evaluated. Evaluation of the data collected during the test must follow the requirements outlined by the test standard that is being used, either CTI or ASME. Tower installation requirements for testing for a thermal performance test, especially an acceptance test, there are certain site requirements that need to be met by the customer or his representative. Parameters Water flow temperature Hot water temperature Cold water temperature Wet bulb temperature Fan power Wind speed
Test frequency(readings/hr) 3 12 12 36 per station 4 or monitor continuously 12 Table 11.2
The following list includes the common considerations for a standard tower installation operating on a closed-loop system: 1) Pitot tube taps must be installed in the pipe(s) delivering water to the cooling tower. 2) Sensor taps must be installed for the measurement of water temperatures. Hot water temperature can normally be measured in the distribution basin of cross flow towers. Often, the pitot taps can also be a measurement point for hot water temperature. If site-specific circumstances make neither of these options acceptable, special taps will be required. The cold water temperature is normally accomplished at the discharge of the circulating water pumps. The most common location is at the pressure gauge tap present on most systems. If this is not available or applicable on a system, special taps or another solution must be identified. Measurement in a tower basin is not acceptable. Measurement of cold water temperature in a flume or channel can sometimes be accomplished with acceptable accuracy, but specifics should be reviewed. 3) At the time of the test, safe access to any elevated points of measurement must be provided. All access must conform to safe work practices, OSHA requirements, and any local requirements. 4) Power for test instruments (typically 120/1/60) must be available adjacent to the tower. On very large towers, multiple sources around the tower may be necessary. The tower must be prepared for testing before the test technicians arrive: 1) The tower must be clean. The wet deck must not have damaged, missing, or plugged nozzles or orifices and be balanced as well as the design allows. The air inlet should be cleared of any blockage. If the tower has louvers, they should be in the normal design position, if adjustable. The eliminators should be free of foreign matter. Fan discharge should be clear and unobstructed. 163
2) Water flow and heat load to the tower, or representative cells, should be as close to design as the system will permit. If the test standard recommended limitations cannot be met, all parties should review the situation to agree on the deviation or delay/cancel the test. 3) Any water bypass should be closed and inspected to ensure there is no leakage. 4) Any source of air leakage such as access doors, mechanical equipment supports, or holes in the casing or fan cylinders must be closed /blocked. 5) All fans must be operating at full speed and cannot cycle during the test period. In the case of tower fans operating with VFD’s, they should be placed in by-pass mode. 6) The owner or his representative should designate a coordinator qualified to integrate the testing activity and the normal process operation being served by the cooling tower. 7) The owner or his representative should have an electrician or qualified operator available to assist in the measurement of fan power. All parties to the test must be advised in advance of any special safety issues required at the site.
Fan Tests To illustrate the negative effects on fan system efficiency a series of full scale fan tests can be performed. The basic scheme is to test a forced draft air cooler at three different air flow rates in four conditions each: (a) Standard (with Inlet Bell, Seal Disc, and Close Tip Clearance) (b) Remove Inlet Bells only. Test unit and replace Inlet Bells. (c) Remove Seal Disc only. Test unit and replace Seal Disc. (d) Increase blade tip clearance. A total of twelve tests is to be performed. The testing equipment will include the following: 1. Anemometer 2. Draft Gauge 3. Tachometer 4. Power Analyzer
Procedure For each test, air flow (CFM), static pressure, pressure, temperature, and electrical power consumed is measured. Electrical measurements included volts, amperes, watts, and power factor. Electrical power input is calculated by the relation: HPin = V X A X P.F.X 1.732 746 Velocity Pressure was calculated by: VP=(CFM/Net Free Area )2
System Efficiency was calculated by: 164
E = Total Pressure (act.) X CFM HPin
Discussion of Results In reviewing the results, it is seen that the negative effects that rob system efficiency are a function of the Velocity Pressure.
Conclusions From the previous discussion the most important points worth emphasizing could be summed up as follows: 1. In any real life fan system there are inevitable losses that degrade system performance below that of the idealized curve performance. These should be taken into consideration. 2. Some losses are built-in by poorly designed fans or system designs that are not optimized. 3. Some losses are correctable by inexpensive standard components. 4. It is very important that an analysis is made of the complete fan system so that fan system efficiency can be computed. To do this complete information must be furnished from the supplier of the equipment for static and velocity pressure losses for each component in the system.
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CHAPTER 12 RECOMMENDATIONS LG SONIC Sound Sound can be described as mechanical energy transmitted by pressure waves in a material medium. Thus, sound can be described as a form of energy or a sound is said to be mechanical. This distinguishes sound energy form other forms of energy, such as electromagnetic energy. This general definition encompasses all types of sound, including audible sound, low frequency seismic waves (infrasound), and ultrasound.
Ultrasound Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults and thus, 20 kHz serves as a useful lower limit in describing ultrasound.
Ultrasound applications Current applications of ultrasound includes, for Example:-Sonochemistry (emulsification, acceleration of chemical reactions, extraction etc) dispersion, and disruption of biological cells (ultrasonic disintegration), removal of trapped gases, cleaning of microscopic contamination, ultrasonic humidifier, ultrasound identification (USID), and typically to penetrate a medium and measure the reflection signature or supply focused energy. The reflection signature can reveal details about the inner structure of the medium. Most well- known application of this technique is its use in sonography to produce pictures of fetuses in the human womb. Other application is using ultrasound in cancer diagnose. The numbers of ultrasound application is numerous. Combining the right frequencies, the right amplitude and using the right transducer numerous types of ultrasound application can be achieved. The sky is the limit.
Ultrasound forces Exposing liquids to high mechanical pressure waves (or sound waves), forces as acoustical streaming, stable cavitation and transient (unstable or inertial) cavitation can be induced. For Eg:- ultrasonic disintegration, sonochemistry and sonoluminescence arises from acoustic cavitation: the formation, growth, and implosive collapse of bubbles in a liquid. 166
Cavitational collapse produces intense local heating (~5000 K), high pressures (~1000 atm), and enormous heating and cooling rates (>10 9 K/sec). Acoustic cavitation provides a unique interaction of energy and matter, and ultrasonic irradiation of liquids causes high energy chemical reactions to occur, often accompanied by the emission of light. This can only be achieved in specific situation involving specific frequencies of high ultrasound power (high W/h & dB) exposed to relatively low liquid volumes of relatively low temperatures
Ultrasound and water treatment At present, ultrasound is also being used in the field of water treatment. In this scenario, forces other than cavitation forces are being used to achieve a certain goal. An example of such ultrasound systems which can be found on the market are the LG Sonic® systems which are manufactured to suppress algal growth and biofilm formation. The ultrasounds produced by these kind of systems does not produce any stable (non- inertial) nor unstable cavitations. They do not even come close to reaching cavitation levels. Other mechanical forces induced by the produced mechanical pressure waves are use to suppress algal growth and reduce biofilm growth, such as resonance forces, longitudinal and transversal sound wave forces. To reach this goal, the LG Sonic systems for example use a “blend’’ of very specific ultrasound frequencies of certain power which are being send into the water by very special transducers. This will enhance the specificity and selectivity of the ultrasonic treatment. The algae are treated with ultrasonic sound waves set in precise frequencies, which directly target the cellular structure of the algae. The amount of algae in the water are reduced and controlled in an efficient, cost effective manner and further growth is inhibited. Green layers disappear, biofilm formation is prevented, and the appearance and clarity of the water is visibly improved. The continuous use of such a device prevents the water from becoming polluted again. These kinds of ultrasound algae control systems can be used in all situations where water is stored, from large industrial water applications to small private pools or ornamental ponds. These systems range from large capacity units to small ones, enabling a “tailor-made” solution to all purposes. The amount of time needed to see improvements depends on certain Physic-chemical parameters of the water such as the type of the algae present in the algal population, water temperature, the amount of light, the amount of nutrition (especially phosphate and nitrate), size and depth of the water body, TSS levels, TDS levels, turbidity, retention time, etc. To achieve a successful treatment of the water, one should first know that no water is the same, every water is unique and should be treated uniquely. Such a ultrasound system does not use chemicals, needs a low supply of electrical energy, and does not harm water plants, fishes, zooplankton, and other types of life present in the water. Thus, the environment is spared. At the other hand many of the traditional methods to fight algae or biofilm growth are either insufficient cumbersome, environmentally unfriendly, or all of these.
Algal growth control mechanism by means of ultrasound The ultrasound produced these devices are target to different types of algae such as unicellular algae, colonial forming algae, filamentous algae and cyanobacteria.
Eukaryotic algal cells and ultrasound Each Eukaryotic (unicellular and filamentous) algal cell has one or more relatively big cell compartment(s), the vacuole(s). This compartment can occupy about 70-90% of the cell 167
volume and can have different functions. Lipids, water, starch, pigments, other nutrients and some biochemical components can be stored in this vacuole. Some of these cellular compartments also function to maintain the fluid balance (turgor).These specific ultrasound frequencies can negatively affect the membrane (tonoplast) of the vacuoles and cause the detachment of the cell membrane from the cell wall. Other cell components can also be affected by these ultrasounds forces. All these are lethal to the algae cells.
Blue-green algal cells and ultrasound Blue-green algae (cyanobacteria) are bacteria (prokaryotic organisms) capable of photosynthesis and nitrogen fixation. Most of them have small cell compartments (gas vesicles). These gas vesicles are small and hollow, air filled structures of a cylindrical shape that provide buoyancy to these cyanobacteria. Each cyanobacteria cell can contain up to 5000 gas vesicles. The gas vesicles enable the bacteria, after periods of water mixing, to float up from the deeper water layer back into the eutrophic zone, where light for photosynthesis is provided, or to reach deeper nutrient-rich layers by sinking when the loose the air form there gas vesicles. Therefore, these organisms have means to overcome spatial separation of nutrition and light. The ability to regulate their buoyancy is discussed as a major advantage over other phytoplankton species and may partly explain the enormous success of the toxinproducing species in the field. The produced ultrasound forces will fracture these gas vesicles, thus causing the blue-green algae to sink and (eventually) die. Furthermore, some cyanobacterial types (strains) are able to produce toxins. Older and senescent blooms tend to release toxins into the water as the cells break open (or via treatment with copper sulphate). Cyanobacteria can produce a wide array of neurotoxins, hepatoxins, cytotoxins and skin irritants. In addition, many genera, such as Anabaena, can produce multiple toxins. By reducing the amount of the cyanobacteria, reduction of the produced toxins will be achieved. Thus, used for anti-algae and growth inhibition purposes, the ultrasonic water treatment system can have an outstanding effect on the reduction of toxins by the control of toxic cyanobacterial growth. In several scientific publications, scientists showed that degradation of these toxins can be achieved at lab scale by certain ultrasound forces.
Biofilm control by means of ultrasound Biofilm: Many industrial and professional applications use water. Whether streaming or stagnant, algal growth and biofilm formation may occur, which can damage the installations and reduce the efficiency. Many methods to control biofilm formation involve chemical treatment which is expensive, damages the circuit or lowers the water quality. Ultrasound treatment can inhibit the formation of biofilm on an environmentally friendly, cost effective manner without inducing damage to the installation in which the treatment is being applied. A biofilm can grow on different types of substrates which can be found in water. When temperatures are high, for eg: in cooling towers, a matrix of different microorganisms such as bacteria, fungi, protozoa and algae can grow very rapidly.
The formation of biofilm Biofilm consists of communities of microorganisms, which develop on surfaces in natural and artificial environments. Under certain conditions, many bacteria can be induced to produce Extracellular Polymeric Substances (EPS) which include polysaccharides, proteins, and nucleic acids. EPS are the “cement” of biofilm. Subsequently, other microbial aggregates 168
settle in the pore spaces of the EPS, thus helping in the further formation the biofilm. Some of the bacterial species that can produce EPS are Pseudomonas, Burkholderia, Aeromonas, Pasteurella, Pantoea, Alcaligenes and Sphingomonas. Microalgae can also contribute to biofilm formation by the production of exopolysaccharides (EPS) under certain stressed conditions. In particular, surface adherent biofilm and bacteria living within protozoa pose potential health problems that are unrecognised by conventional laboratory culture methods. A host is required for Legionella pneumophila multiplication, but in the absence of a host, L. pneumophila can survive within a biofilm layer (sessile Legionella) and yet others will be suspended in the water (planktonic Legionella). Mostly, protozoa serve as host cells for the intracellular replication of certain Legionella species in a variety of environmental settings.
The disadvantages of biofilm formation Even a small layer of biofilm within a pipe reduces the diameter of the pipeline. This means that less water can be pumped around the circuit but also the hydraulic pressure needs to be increased to still cope with the system/industrial demand. This can result in higher energy costs and lower performance efficiency of the cooling tower. A biofilm consists for 85-95% of stagnant water this can function as a insulating layer around the grids and pipes, thereby reducing the cooling efficiency of the tower. A biofilm can contain several bacteria which can produce corrosive chemicals. For Eg: anaerobic sulphate reducing bacteria. These bacteria produce sulphuric acid which can cause corrosion of metal pipes. Also the so called iron-oxidising bacteria can cause corrosion of metal, resulting in expensive repairs of leaking pipes. Biofilm can be a host for the pathogenic Legionella bacteria. These bacteria can become aerosol and infect humans when they inhale them causing severe pneumonia.
Ultrasound treatment The produced ultrasound attacks most of the unicellular and blue-green algae as well as certain bacteria responsible for the formation of the biofilm. Further algae (and other micro -organisms) growth will be inhibited, the biofilm will slowly deteriorate, thus enabling easy cleaning and removal/maintenance operations. Legionella control strategies should also include the control of cyanobacteria, which enhance growth and improve the survival of Legionella in an aquatic reservoir which subsequently enhance the chance that Legionella bacteria will be present in formed aerosols. The ultrasound makes the environment in the water less favourable for the Legionella bacteria to multiply and/or attached to surfaces including post formed biofilm surfaces to help in the formation (or further formation) of biofilm.
Reduction of other micro organisms In the irrigation, ultrasound treatment seems to suppress and control the growth of funguses such as Pithium, Fusarium and Phytophthora. Pithium (ed. P. insidiosum) are plant pathogens that produce motile oospores. Organisms of this genus are sometimes called aquatic fungi, but they really are not considered to be true fungi. These organisms may now actually be placed into a new Kingdom, Kingdom Stramenopila. They are often studied as part of medical mycology due to their ability to produce a chronic granulomatous process in which one sees hyphal structures. The disease is sometimes called “swamp cancer” due to its association with water exposure. Fusarium is a parasitic type of fungus which can affect plants and animals. Furthermore, they can produce mycotoxines (trichothecenen and fumonisines) which can cause food contamination. The growth of another plant pathogen Phytophthora (ed. Phytophthora infestans) can be suppressed and control. Phytophthora 169
belongs to the water-funguses, Oömycetes. The usually infect dycotile plants. In other applications (inlcluding irrigation), ultrasound treatment can also suppress and control the amount of E.coli, Enteroccocus and total coliforms.
BIOCIDES USED IN ZUARI INDUSTRIES
Biocide Name
Cost per kg
Amount consumed(kg) (CT 1) per year Nalco-N10WB 310 1300 Nalco-N5705 367 3200 Nalco-N7320 310 1130 Nalco-N7330 324 3300 Table 12.1 Total cost for biocides in Zuari industries= Rs 40,94,400 C-EF 101 - No. of cells 10 C-EF 102 - No. of cells 3 C-EF 103 - No. of cells 4 C-EF 104 - No. of cells 2
Amount consumed(kg) (CT 2) per year 630 1200 340 1100
Total (Rs)
Cost
598300 1614800 455700 1425600
Total No. of cells 19 Considering that in each cell there should be a unit I recommend 19 e-XL units for each cooling towers cell and the costs per unit is EUR 2150,- or there is also an option to install up to 4 e-XL unit on a single e-box. Total cost for LGSONIC equipment= Rs24,64,699.575 The payback analysis depends on the type and dosage of biocides you are applying. From our previous cases we have results of 69% reduction in biocide use, but again this depends on the case and related details of your cooling tower. However, you should not stop using the biocides right away but you can start reducing them gradually in accordance with the results of the treatment with our units.
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BALDOR ADJUSTABLE SPEED DIRECT DRIVE COOLING TOWER MOTOR AND DRIVE SYSTEM Why Baldor? 1) 2) 3) 4) 5) 6)
Baldor offers the industry’s broadest line of stock products Energy-efficiency leader Baldor products are available at more locations than any other brand Continuous innovation to improve reliability Industry’s shortest lead times/Flexible manufacturing Industry’s best information
NEW DIRECT DRIVE TECHNOLOGY IMPROVES RELIABILITY, REDUCES MAINTENANCE, RUNS QUIETER & SAVES ENERGY By combining the technologies of the field proven and power dense AC laminated frame RPM AC motor with high performance permanent magnet (PM) salient pole rotor designs and the matched performance of an adjustable speed drive. Baldor Electric can offer high torque direct drive motors for cooling tower applications with all the benefits of variable speed control and eliminating the cost and maintenance required for traditional gearbox or belted solutions. The fan couples directly to the motor and is controlled by a unique AC drive to provide optimal speed and cooling tower performance that runs quieter with reduced energy consumption. The drive is designed to accommodate the most common HVAC communication protocols.
DIRECT DRIVE RPM AC SYNCHRONOUS PM MOTOR REDUCES MAINTENANCE COST The RPM AC™ synchronous PM motor uses laminated finned frame construction to provide a highly efficient power dense package with flange mounting dimensions that can replace the right angle gearbox and jack shaft installation in many conventional cooling towers. This same technology is offered in conventional, yet power dense, foot mounted designs that can replace the belt and sheave application where more vertical mounting space is available. Derived from one of the toughest motor platforms used in the most demanding industrial applications, the RPM AC motor is the right solution for operation inside the tower’s hot and humid environment. The TEAO (totally enclosed air over) RPM AC cooling tower motor is designed for minimal maintenance. Bearings require lubrication only once per year. Water ingress along the shaft is prevented with the use of an Inpro/ Seal® bearing isolator and a slinger. The electrical insulation system is manufactured using a VPI (vacuum pressure impregnation) process that ensures long motor life even in the most extreme environmental conditions. Condensation drains relieve any moisture that may collect inside the motor. No more changing gear oil, lubricating pillow block bearings or changing out belts.
BALDOR VS1CTD PM COOLING TOWER DRIVE
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The Baldor VS1CTD integrates custom features that have been designed exclusively for the cooling tower industry. The VS1CTD utilizes our Matched Performance philosophy to pair each RPM AC PM motor with a specific control. Critical motor operational parameters are integrated right into the VS1CTD firmware to provide simplified Cooling tower startups, eliminating the need for users to enter complex information into the control or the need to tune the control to the motor prior to operation. Since the VS1CTD is targeted for use in the cooling tower market, much of the complexity that typically resides in general purpose controls has been eliminated further adding to the simplicity of our Adjustable Speed Direct Drive Cooling Tower Motor and Drive System. When system automation and control is a requirement for your cooling tower operation, the Baldor drive readily communicates with multiple communication networks including MODBUS-RTU, MODBUS/TCP, LonWorks, Metasys-N2, BACnet, EtherNet/IP, DeviceNet, PROFIBUS and Siemens P1 Apogee.
Field Tested Reliability After extensive Lab testing at Baldor’s facility, motor and drive systems have been installed and field tested for as long as three years. One system is running under a controlled environment on one of two identical cooling towers at Clemson University. Both towers were instrumented and the traditional geared system was compared to the one converted to use Baldor’s Adjustable Speed Direct Drive Motor and Drive System. Each tower had the same 5 blade 18 foot diameter fan. The conversion was made in less than a day. Performance results, which were verified by a third party, measured an input kW power savings of 11.8% compared to a traditional geared system, with high speed noise reduction from 82.3 dBA to 74.4 dBA and reduced vibration.
RPM AC Direct Drive Cooling Tower Features & Benefits Direct Drive Motor 1) Eliminates the need for a gearbox, jack shaft, pillow block bearings and couplings 2) Reduces maintenance and provides improved reliability 3) Eliminates cooling water contamination by eliminating gearbox oil and leakage 4) Reduces power consumption 5) Results in increased safety because there are fewer mechanical components 6) Water-tight motor design operates in the air stream 7) Eliminates the alignment of mechanical components for quicker installation and reduced installation costs Bearings and Seals 1) Oversized to maintain longer bearing life exceeding L-10 100,000 hours 2) Grease lubricated for long life 3) Handles fan loads with improved reliability 4) Proven Inpro/Seal® bearing isolator with slinger umbrella over seal 5) Only one ingress point 6) Insulated opposite drive end bearing on FL440 and FL5800\ Adjustable Speed Control 1) Designed specifically for the Cooling Tower Industry and can be set at the optimum speed point 2) Sensorless Permanent Magnet motor control operates without an encoder or resolver 3) Trickle heating eliminates need for motor space heaters 4) No tuning is required due to the Matched Performance of the motor and control 173
5) 6) 7) 8) 9)
Allows for a soft start (controlled ramp) Saves energy and reduces mechanical stress on the system Improves system reliability and extends life Reduces noise Allows for optimized return water cooling temperature for optimized compressor operation 10) Trickle current for braking prevents fan windmilling when not in operation 11) System resonance speeds can be bypassed Communication Protocols 1) Utilizes MODBUS-RTU, MODBUS/TCP, EtherNet/IP, LonWorks, Metasys-N2, BACnet, DeviceNet, PROFIBUS or Siemens P1 Apogee protocols 2) Interfaces with existing building automation systems MATCHED
PERFORMANCE Retrofit or New Tower Designs RPM AC cooling tower motors are available in either flange mount or foot mountdesigns for mounting in the air stream. The flange mount units are designed to be interchangeable with many popular gearbox bolt hole mounting configurations. Shaft height, diameter and flange mounting dimensions can be directly interchangeable with some existing cooling tower gearbox designs. Higher motor torque ratings are available using taller motors when space is available. In addition, traditional foot mount construction is available. Both flange and foot mount designs are available in a wide torque range in frame sizes FL250, FL280, FL440 and FL5800.
Fig 12.1: Baldor’s Direct Drive motor eliminates many components of a right angle geared system.
Motor Features 1) 2) 3) 4) 5)
Mounting pad for vibration sensor Thermostats one per phase normally closed Heavy build external coatings Proven Inpro/Seal® bearing isolator with slinger umbrella over seal Proven insulation system technology used in off-shore drilling applications
V*S Control Provides Optimized Cooling Tower Performance and 174
Energy Savings Even Under Low Load Conditions By optimizing motor speed considerable energy can be saved. The entire cooling tower system must be designed for the “Worst Case” (or highest air flow) scenario. For optimum system performance the fan may need to operate at reduced speed. As the speed of the motor is decreased, the air flow drops in a corresponding linear fashion. So, for example, if the motor runs at only 50% speed, the air flow is correspondingly reduced to 50% of maximum air flow. However, the input power to the motor varies with the cube of the motor speed. For example, if a motor is run at half-speed, the power consumed by the motor is 12.5% or 1/8 [i.e. (½)3] of the power consumed at full speed. So, if the needed airflow can be achieved by running at half-speed, it is possible to save a large amount of energy.
Figure 12.2
Figure 12.3 Another important aspect of the PM motor design versus a traditional induction motor is its ability to maintain high efficiency performance when operating under low load conditions which are typical for variable speed fan applications.
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Case Study This case study involves retrofitting an existing cooling tower constructed in 1986 at a south eastern university. This tower is comprised of two identical cells. For this study, one cell was retrofitted with a slow-speed, direct drive PM motor and ASD, while the other was left as originally configured, allowing for a direct comparison of the two fan-drive solutions. Prior to the installation, the current being drawn by the two original induction motors was measured with the fans running at full speed. An ammeter was used, and the current was measured at 47 A (rms) on both induction motors. As the induction motors were identical, this is a good indication that both cells were operating under the same load conditions. After the installation was complete on the PM motor with the ASD, the current was again rechecked on that motor and found to be only 41 A. The induction motor on the original, identical tower was still drawing 47 A. A power meter was used to measure the input power to both solutions. The fans were both running at 208 r/min. Data were taken at both the input and output of the drive toallow for a direct comparison of the induction motor/ gearbox combination to the PM motor. The results of the measurements are shown in Table 3.
Figure 12.4: Cell 1 in the original configuration
Figure 12.5: PM motor installed in place of the gearbox in Cell 2 Table 12.2Power consumption comparison: original 9o blade pitch, Manufacturer’s data 176
Location
Volts (Mean) 477
Amps (rms) 46.7
Input (kW) 31.5
Input to induction (Cell 1) Input to ASD , PM 477 44.5 28.5 (Cell 2) Input to PM (Cell 2) 459 40.9 28.0 From this data, it was determined that both cells were running at less than full load and that the load could be increased on each cell. To this end, the pitch of the blades on each fan was increased to 12°. This change of pitch caused the fans to draw more air, thus increasing the load on each motor. This increased air flow improved the effectiveness of the overall tower performance. Again, power measurements were made, and a third-party testing service was engaged to verify the manufacturer’s results. The data are shown in Tables 4 and 5. Table 12.3Power consumption comparison: original 12o blade pitch, Manufacturer’s data Location Input to induction (Cell 1) Input to ASD, PM (Cell 2)
Volts (Mean) 477
Amps (rms) 54.8
Input (kW) 38.1
477
49.8
33.6
Table 12.4.Power consumption comparison: original 12o blade pitch, testing service data Location Input to induction (Cell 1) Input to ASD, PM (Cell 2)
Volts (Mean) 477
Amps (rms) 54.8
Input (kW) 38.1
477
49.8
33.6
For the final blade pitch of 12°, a lower power consumption of 4.5 kW was measured on Cell 2 with the PM motor installed. To document the savings realized at various speeds on this application, the input power was recorded at intermediate speeds for the PM motor cell. Figure 9 shows the actual measured input power for theinduction motor/gearbox solution and the PM motor solution at various speeds. As shown in Tables 2 and 4, the PM motor solution requires less input power for the given blade pitch setting. Figure 9 shows the total input power in kilowatts for each solution over a range of operating speeds from 50% to 100%.
177
Figure 12.6 Again, the PM motor has an advantage over the induction motor/gearbox combination. Using an average price of US$0.08/kWh, the annual cost savings for various applications and duty cycles are shown in Table 6. This table does not account for the additional savings achieved by using an ASD, which offers the ability to run at speeds between 50% and 100% of the rated speed. The largest savings will be related to reduce downtime and maintenance costs and will be discussed in the “Plant Implications” section. Table 12.5 Annual energy savings based on various duty cycles
Application Petrochemical plant Hospital University
Annual Savings(% High Speed/% Low Speed) Daily 100/0 75/25 Use (h) (US $) (US $) 24 3,154 2,488
50/50 (US $) 1,822
18 12
1,367 911
2,365 1,577
1,866 1,244
Many cooling towers are in locations where airborne noise can be an issue. To this end, a third-party testing company was engaged to conduct comparative sound tests between the two cells. Data were taken at both high and low speeds for both cells. The induction motor cell was designated as Cell 1 while the PM motor cell was designated as Cell 2. Sound level measurements were taken on Cell 1 while Cell 2 was turned off. There were 30-s readings taken at high speed and 30-s readings taken at low speed around the perimeter of the tower and the fan motor. As there was no motor outside of the fan stack on Cell 2, only nine readings were taken on Cell 2 with Cell 1 turned off. A single-point measurement was taken where the old induction motor was mounted on Cell 2 to have some reference to Cell 1. It was not possible to turn off the water flow for either cell at any time, so there was a significant amount of background noise, but, as this condition was the same for both cells, it should not affect the comparative data. Average A-weighted sound pressure results are shown in Table 7 for both high- and low-speed operations. Table 12.6: Sound pressure data A-Weighted Average Cell Induction (Cell 1)
High Speed (dBA) 82.3
Low Speed (dBA) 74.4 178
PM (Cell 2)
77.7
69.0
At high speed, the PM motor cell was 4.6dBA lowerthan the induction motor cell. For lowspeed operation,the PM motor cell was 5.4 dBA lower. Although there may be some slight differences in the background noise for each cell, these likely do not account for all of the noise level reduction realized with the PM motor solution. The removal of the higher speed induction motor from the outside of the fan stack appears to have the biggest influence on the overall noise level of the tower itself.
Electrical Considerations Permanent Magnet Control Algorithm In addition to the PM motor design features already detailed, another challenge of this application was that the PM motor had to be run sensorless. There was no room to install a speed feedback device, such as an encoder or resolver, and still meet the height restriction of the existing gearbox. In the harsh environment, a feedback device would be a reliability concern. Therefore a sensorless PM control scheme was developed to satisfy the requirements of this application. Several things had to be considered when forming this algorithm. One challenge was the inertia of the fan. This was taken into account to prevent the motor from pulling out of synchronization when starting and changing speeds. Figure 10 shows a portion of the motor voltage, speed, and current for a typical start from rest. Note the smooth acceleration and low starting current required. A typical 50-hp, 480-V induction motor started across the line would draw 347 A, compared to 12 A for the PM design started on the ASD.
Improved Process Control As mentioned earlier, the addition of the ASD allows the user to more accurately and efficiently control the process. Figure 11 shows how the motor speed is changed automatically with control logic as the heat demand on the system changes with time.
Fig 12.7: Motor starting performance
179
Fig 12.8: Motor speed variation with changing heat load
Braking and Condensation Control The use of an ASD also provides the opportunity to offer some additional features that across-the-line systems do not. The drive may be configured to apply a trickle current to the motor windings to act as a brake during down time. This prevents the fan from wind milling due to nominal winds or adjacent cooling tower turbulence. However, a mechanical locking mechanism should be employed during any maintenance procedures, as is commonly used with the existing technology. This trickle current may also be used as an internal space heater by raising the winding temperature and preventing condensation when the motor is not running.
Insulation System Inside the fan stack is an extremely humid environment. Therefore, the insulation system on the stator windings must be robust and highly moisture resistant. It is recommended that an insulation system utilizing an epoxy compound applied via a vacuum pressure impregnation(VPI) system should be employed. The VPI system is widely recognized as a superior insulation system for harsh applications such as this.
Insulated Bearings The magnetic fields present inside the running PM motor are no greater in magnitude than the fields present in a comparable induction motor. As with all inverter-fed motors, shaft currents may be present depending upon a number of factors, including cable type, lead length, and switching frequency of the ASD. Insulated bearings may be applied when the particular installation warrants. However, it should be noted that PM motors are no more or less susceptible to shaft currents than similar induction motor designs .
Generating Capability A PM motor will act as a generator when the shaft is driven by a mechanical means, such as wind milling of the fan. The voltage generated at the terminals on an open circuit is typically in the range of 1–2 Vrms line to- line per revolution per minute (1–2 Vrms/r/min). 180
This is not a particularly high voltage at a low r/min, but it is necessary for maintenance and other personnel to be made aware of the potential of the generated terminal voltage even on a disconnected motor.
Mechanical Considerations Mounting The motor is mounted using a flange located on the opposite drive end. In many cases, the motor-mounting holes can match the bolt-hole pattern of the existing gearbox. In other instances, an adapter plate can be used to mount the motor to an existing base design. On new towers, this would not be an issue; however, it should be considered for retrofit installations.
Shaft Seal Due to the harsh environment inherent with a cooling tower’s application, the motor’s drive end is protected by a metallic, non-contacting, non-wearing, permanent compound: a multi-labyrinth shaft seal that incorporates a vapour blocking ring to prevent ingress of moisture. This seal has been proven to exclude all types of bearing contamination and meets the requirements of the IEEE-841 motor specification for severe duty applications. This type of seal has been successfully used in cooling tower gearboxes for many years. In addition, a slinger is used on the drive end to further protect the seal from the direct spray of moisture. There are only two ingress points for water into the motor: the shaft and the conduit box. By choosing the right seal and conduit box configuration, an IP56 protection rating can be achieved.
Corrosion and Paint The environment inside the cooling tower is well known as being highly corrosive and is a concern for any equipment installed within. High levels of chlorides and sulphides are not uncommon in cooling tower water, and any piece of equipment operating inside the fan stack is subject to a constant mist of water containing these chemicals. For this reason, it is important to address corrosion resistance as it applies to a motor used in this application. As with any severe duty application, the user must specify the type of environment in which the motor willoperate so the manufacturer can provide an appropriatepaint system. Many different products are available for coating the motor to withstand the harsh chemicals present in the water of a petrochemical cooling tower. In fact, many petrochemical users have their own paint system specifications. The authors acknowledge the existence of multiple solutions to this problem but offer the following as guidance in the selection process.One option would be to coat the steel portions of the motor, such as the stator laminations, with an epoxy resin or other suitable corrosion-preventing material. The cast iron parts may be ecoated (an electrically applied paint coating), which is a process that provides superior adhesion and corrosion resistance. Finally, a multilayer epoxy top coat may be applied. If corrosion of the motor shaft is a concern, a stainless steel shaft material can be used.
181
BALDOR COOLING TOWER DIRECT DRIVE PROPOSAL WITH PM MOTOR, VFD AND NO GEARBOX NAME OF ZUARI INDUSTRIES FAN SPEED, RPM 170 CUSTOMER LOCATION GOA FAN DIAMETER, FT 24 TOTAL ANNUAL 8400 FAN DELIVERY 850000 HOURS M^3/HR 9 No Of Units 4 Nos ENERGY RATE
RS/UNIT LOW RPM HOURS 75% EXISTING MOTOR KW BALDOR MOTOR CATALOGE NO
6300
LOADING FACTOR %
75.0
75
FAN BHP
Assumed as 56 kw
FL4440
BALDOR MOTOR FRAME
above
BALDOR DRIVE REFERENCE LOSSE IN VFD %
VS1CTD475-1B, 116 amps 5
MAX.AMBIENT 45 AROUND MOTOR ENERGY SAVINGS DUE TO 510300 GEARBOX ELIMINATION ENERGY SAVINGS DUE TO 1210176 RPM REDUCTION SAVINGS DUE TO NO OIL 12000 CHANGE BEARING REPLACEMENT 30000 COST SAVINGS SHAFT BREAKAGE COST 11667 SAVINGS SAVINGS IN COST OF 233333 GEARBOX REPLACEMENT POWER FACTOR 1875 IMPROVEMENT CAPACITOR COST SAVING BENEFITS OF OIL 56700 CONTAMINATION ELIMINATION BENEFITS OF NOISE 0 REDUCTION TOTAL ENERGY SAVINGS/YEAR TOTAL MAINTENANCE COST SAVINGS/YEAR
AIR VELOCITY @MOTOR M/S
5
OTHER SAVINGS
56700 2075843
TOTAL SAVINGS/ YEAR SALVAGE VALUE OF OLD MOTOR+GEARBOX+FRAME BALDOR DRIVE UNIT PRICE INR
DUE TO VFD 20% RPM REDUCTION QUARTERLY OIL CHANGE ONCE /YEAR MOTOR OR GEARBOX ONCE IN 3 YEARS ONCE IN 3 YEARS ONCE IN 3 YEARS
1% REDUCTION IN PUMP POWER BY VALVE CONTROL NOISE LEVEL GOES DOWN TO 70DB 1720476 298667
15000
MOTOR AT 50RS/KG AND STELL AT RS 15/KG ONLY SUPPLY . TAXES , PNF,FREIGHT EXTRA ESTIMATE OF COSTS AT YOUR END
3198182
UNIT PRICE OF PANEL WITH CONTROLLER
300000
COST OF UNINSTALLATION
100000
COST OF INSTALLATION
150000
COST OF COMMISSIONING
50000
TOTAL COST INR
12 % ASSURED SAVINGS
ESTIMATE OF COSTS AT YOUR END ESTIMATE OF COSTS AT YOUR END ESTIMATE OF COSTS AT YOUR END 3798182
Maintenance 182
Another consideration is overall system maintenance. For motor/gearbox combination drives, the lubrication interval is determined by the high-speed gear set. The recommended lubrication interval for this type of gear is typically 2,500 h or six months, whichever comes first. Inaddition, gear manufacturers recommend a daily visual inspection for oil leaks, unusual noises, or vibrations. As these units are installed in areas that are not readily accessible or frequented, this is an unreasonable expectation and burden on maintenance personnel. When a gear is idle for more than a week, it should be run periodically to keep the internal components lubricated as they are highly susceptible to rust and corrosion. When being stored for an extended period, it is recommended that the gearboxes be completely filled with oil and then drained to the proper level prior to resuming operation. Because the high-speed input has been eliminated with the slow-speed PM motor design, the lubrication cycle can now be extended to one year. Once more field data are collected, it is believed that this interval can be increased, but a cautious approach is being taken until more history is developed. The PM motor need not be inspected daily for oil leaks, as the motor contains no oil. As mentioned previously, the ASD can provide a trickle current to heat the stator windings to a temperature slightly above ambient to prevent moisture from forming inside the motor.
Vibration With the elimination of the high-speed input to the gearbox, the system dynamics from a vibration standpoint have been simplified. There are no longer any resonance issues with the driveshaft. The maximum rotational excitation is now limited to the rotational speed of the fan. The number of bearings in the drive system has been reduced from six to two for a single reduction gearbox and from eight to two for a double reduction gearbox. This reduces the number of forcing frequencies present in the system. For added protection, a vibration switch can be used as with typical gearbox installations. To this end, a flat pad may be incorporated on the side of the motor, which can be drilled and tapped to accept commonly used vibration sensors.
Plant Implications Reduced Maintenance Costs To quantify the potential benefits of using the direct drive motor solution, two petrochemical facilities with multiple cooling towers were audited to determine maintenance and repair costs over a six-year time period. Therewere 16 towers with a total of 102 cells. The total six year maintenance cost associated with these towers was US$3,189,957. This equates to an average maintenance cost of US$531,660 per year. Unplanned maintenance accounted for 79% of this total. The data indicated that approximately 81% of the unplanned maintenance was caused by problems inherent with the traditional fan drive solution. Additionally, some users manually adjust the fan blade pitch depending on the season. The use of an ASD would eliminate the need for this planned maintenance as well because the speed of the fan could be increased or decreased to optimize air flow rather than using a manual adjustment of the fan blades. A break- 18 down of the maintenance costs is shown in Table 8. Table 12.7 Failure data
183
Facility
Number of Cells
A B Total
82 20 102
Electrical Repairs (US $) 626,845 462,899 1,089,744
Mechanical Repairs (US $) 994,800 1,105,413 2,100,213
Total (US $) 1,621,645 1,568,312 3,189,957
Serviceability With the installation of PM motors becoming more commonplace, the question of serviceability often arises. Rewinding the stator, replacing the bearings, and other strategies are possible for a PM motor just as with a typical induction motor, although some precautions must be taken due to the magnetized rotor assembly. The rotor assembly is actually the only real difference between this type of PM motor and induction motors historically used in most industrial applications. The stator windings are generally lap wound, as with a common induction motor. Therefore, rewinding the stator core is no more complex than for an induction motor. No special tools or processes are needed. There is a magnetic force holding the rotor in the stator. The magnitude of this force will vary depending upon the rotor diameter and core length of the motor. When removing or installing the rotor, it is important to guard against the rotor, pulling itself against the coil head and damaging the stator windings. Simple fixtures may be used as guides during the removal or insertion of a magnetized rotor. Another often-asked question is, “What happens if the rotor becomes demagnetized?” The rotor can be demagnetized if the temperature capability of the magnets is exceeded. However, there are many different grades of magnets available, and the motor manufacturer can avoid this problem by choosing the right one. Generally speaking, the rotor temperature will be lower than that of the stator windings due to the low rotor losses achieved by the use of PMs. Also, the allowable operating temperature of the magnets can be selected to be greater than that of the winding insulation. Thus, by using winding temperature detectors or thermostats, both the windings and magnets will be protected.
Conclusion Cooling tower fan drives have changed very little over the past two decades. Failures of the gearbox, driveshaft, or disc couplings have been the biggest reliability issue facing tower manufacturers and end users. Increasing energy costs have placed a premium on power consumption for all motors and applications. Many of the problems associated with cooling tower maintenance and reliability are solved with a direct drive, low-speed PM motor. The relatively high-speed (typically 1,750 r/min) induction motor has been eliminated. The motor itself has not historically been a problem, but the associated resonances and potential
ZUARI COOLING TOWER FAN DETAILS HUB AND FAN BLADE ASSEMBLY FRP CT-1 & CT-2 Hub and fan blade assembly should be dynamically balanced and confirm to the following specifications: 1) Fan diameter: 24 feet (7315 mm) 2) Fan speed: 170 rpm 184
3) Motor speed: 1480 rpm 4) Motor rating: 75 kW 5) Flow required: 850000 cfm 6) No of blades: 8 (hollow blade type) 7) Moc of blades: frp (fibre glass reinforced with epoxy resin).material should be able to withstand the chlorine dioxide dosing carried out at the cooling tower. 8) Hub diameter: 1830 mm 9) Hub body construction from ms plates, 8 mm thick with two plates and clamps in between. 10) Hub spool in cast iron 11) Split tapered bush in ss 304 to suit the output shaft of Paharpur Marley make gearbox series 32.2t 12) Clamps in CI with taper bore 13) Entire hub assembly should be hot dip galvanised as per is 3203 to achieve a thickness of 65 85 microns Power cost: i = 120a v = 440v 1 kw-hr = Rs.9 per unit Total cost per month = Rs.3,07,929.6
MIOX ABOUT MIOX MIOX designs and manufacture s a full range of on-site disinfectant generators to cost-effectively produce on-demand chemistry for a variety of applications. MIO’s patented on-site generation technology reduces carbon emissions by up to 80% and replaces the need to transport dangerous chemicals while typically realizing returns on capital equipment in 1 to 2 years. With more than 2000 MIOX installations in sites ranging from industrial cooling towers and municipal water utilities to resorts and clean-in-place providers, MIOX systems safely provide on-demand chemistry to millions of people worldwide.
Why Consider MIOX?
185
Consider these three benefits: (a) Economic savings (b) Safer operations, and (c) The most sustainable disinfection technique available. Compared to delivered chemical, MIOX can typically generate chemical on-site for 1/3rd to 1/5th the cost. MIOX is 10 times more powerful than chlorine, offering enhanced inactivation efficacy for pathogens such as Giardia. MIOX eliminates biofilm, driving greater operational efficiencies and offering greater protection against Legionella-related outbreaks. MIOX is generated at < 1 percent concentrations, the OSHA threshold for consideration as a hazardous chemical, and is therefore environmentally benign. And at dilute concentrations, degradation is far less of a concern. Operators exchange the management of hazardous disinfection chemicals with the handling of salt only. Salt is stored and converted on site to achlorine-based sanitizer, replacing the transport, handling, and storage of delivered chemicals. To generate the same amount of free available chlorine, there is a 3-to-1 carbon emission reduction between the transport of salt and 12.5% liquid bleach.
What is a Mixed Oxidant? A mixed oxidant is a blend of advanced chlor-oxygenated species. Average concentrations are 30-40% mixed oxidants (MOS) and 60-70% sodium hypochlorite. Compared to the generation of standard Sodium Hypochlorite alone, more energy is used to strip electrons off of chloride and oxygen. The MIOX on-site generation (OSG) systems have been developed with a proprietary geometrical design, electrolytic scheme, and solution flow characteristics. Extensive 3rd party research in both the lab and, more importantly, in the field with and by customers has confirmed the competitive advantages of MOS. It has also been confirmed that MOS does not contain chlorine dioxide or ozone.
How does MIOX’s Conversion Efficiencies Compare to Competitors? We rarely encounter alternative on-site generation systems in institutional and heavy industrial environments, much less for cooling tower applications. Nonetheless, MIOX’s saltand electrical-conversion efficiencies lead the industry. MIOX’s standard sodium hypochlorite OSG systems have salt conversion efficiencies (SCEs) of 3 and electrical conversion efficiencies (ECEs) of 2. This means it takes 3 pounds of salt to generate one pound of 100% Free Available Chlorine (FAC) liquid solution, and 2 Kilowatt Hours (KwH) of electricity. Further, MIOX is the only company that offers mixed oxidant OSGs. MIOX’s mixed oxidant OSGs have SCEs of 2.5 and ECEs of 3.5. Depending on the competitor in question, MIOX is typically 15-30% superior.
As a Stronger Oxidant, is MOS Going to Damage my Cooling Tower? There have been numerous studies conducted on the corrosivity of the MOS solution and how it reacts with various cooling tower materials. MOS is no more corrosive than any other type of biocide, non-oxidizing or otherwise, when combined with the proper corrosion and scale control programs. The industry standard for corrosion is < 2 mils per year (MPY) for steel, copper, and other relevant materials. A mil is one thousandth of an inch.
Will the Use of MIOX Damage the Performance of Other Chemicals Used for Cooling Tower Water Treatment? 186
An important distinction is organic versus inorganic-based chemicals. As an oxidizing disinfectant chemical, MOS can damage the performance of organic based scale inhibitors such as certain azoles and phosphonates. Chemicals that serve the exact same purpose, that are inorganic, are readily available that are compatible with MOS. As necessary, inquire with the end user or consultant water treatment and chemical supplier and MIOX can comment and recommend an alternative. Can MOS Perform in High pH Conditions? Similar to bromine, MOS performs effectively in high pH conditions. Chlorine notoriously is not the disinfectant of choice when source water conditions include a high pH, based on its relationship with chlorine and the production of hypochlorite ions versus hypochlorous acid. In high pH conditions, for example at 8 and above, there are a higher percentage of hypochlorite ions which are known as a weaker disinfectant as compared to hypochlorous acid. While MOS is a chlorine-based disinfectant, MOS also includes the unique chlor-oxygenated species (i.e., oxygen component of the oxidizer). These chloroxygenated species perform well in a high pH condition.
How do you size MOS in cases where ozone or chlorine dioxide is being replaced? MOS should be sized based on equivalent oxidative strength or “stochiometry.” Oxidative strength can beequating on a basis of milligrams per litre (mg/L) or parts per million (PPM). There is an oxidation strength measurement, as shown in the below graph. As a rule of thumb, MIOX historically has sized MOS as compared to chlorine dioxide or ozone on a pound for equivalent pound basis. But the greater oxidative strength, plus residual strength, of the 30-40% active MOS chemistry must be considered. When replacing either chlorine dioxide or ozone, MOS has been able to accommodate the disinfection needs of a given cooling tower on a pound for pound basis.
Does the use of MIOX’s OSG Systems require FIFRA Registration? MIOX has been told by EPA’s FIFRA that, since MOS is not a “regulated” insecticide, neither MIOX nor itscustomers require FIFRA registration, for two (2) reasons: (a) mixed oxidants are generated using salt, water, andelectricity (i.e., no harmful constituents), and (b) it is generated on-site at < 1% concentrations. At
