developing cost-effective and reliable drying heat pumps. Optimal ..... Heat can be recovered from exhaust air either by a heat exchanger or by recycling part of ...
In: Refrigeration: Theory, Technology and Applications ISBN: 978-1-61668-930-8 Editor: Mikkel E. Larsen, pp. 1-70 © 2011 Nova Science Publishers, Inc. The exclusive license for this PDF is limited to personal printing only. No part of this digital document may be reproduced, stored in a retrieval system or transmitted commercially in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services.
Chapter 1
INDUSTRIAL DRYING HEAT PUMPS Vasile Minea Hydro-Quebec Research Institute, Laboratoire des technologies de l’énergie - LTE, Shawnigan, QC, Canada
1. INTRODUCTION Most high-value agricultural, food and wood products need drying to minimize spoilage, preserve quality and reduce transportation costs [1]. Drying is a complex and energy intensive process where the water is evaporated from a product by supplying heat by convection, conduction, radiation, microwave, etc. About 85% of industrial driers are the convective type with hot air as drying medium, and 99% of them involve removal of water [2]. They consume up to 25% of the national industrial energy in the developed countries. Driers incorporating dehumidification cycles are called drying heat pumps. They recover energy by condensing moisture from the drying air. The recovered waste heat is recycled back to the dryer at higher temperature for reuse in the drying process. This is a method of improving the energy efficiency and reducing primary energy consumption of drying processes. Drying heat pumps based on the vapour compression cycle are largely the most used. Compared to heat pumps for space heating, they have several advantages, such as: High annual factors of utilisation; High energy efficiency due to low temperature lifts;Efficient control of product moisture content, and air dry and wet-bulb temperatures resulting in better final quality, especially for heat-sensitive food, agricultural and biomedical products; Waste heat recovery and heat demand occur simultaneously; Reduced global (electric and fossil) energy consumption; and Short pay-back periods. Among the limitations of drying heat pumps compared to basic hot air convective driers can be maintioned:
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Higher initial capital cost; Higher maintenance cost due to need to maintain compressor, refrigerant filters and charging of refrigerant; Possible leakage of refrigerant; More complex operation; Additional floor space required; Requirement for competent design engineers and operating technicians.
Although heat pump-assisted drying technology is generally considered as a mature technology being used during the last 2–3 decades, its industrial application has not been as widespread as it should have been, particularly in the field of drying agro-food and in wood industries [3]. Even where the requirement for very short payback periods has been met, heat pumps have not so far been installed. Among others, some reasons for their neglect were the following:
Uncertainty by potential users as to heat pump reliability, Lack of good hardware in some types of potential applications, Lack of experimental and demonstration installations in different types of industries, Lack of required knowledge of chemical engineering and heat pump technology in target industries, and Relative cost of electricity and fossil fuels affecting the commercial viability of drying heat pumps.
Another problem consists in frequent errors and/or mistakes spread by a large number of research studies involving drying heat pumps. These errors mainly concern the heat pump – dryer integration and control, as well as the correlation between the product moisture content and the heat pump dehumidification rate. Inadequate integration and wrong operation parameters of the drying heat pump may cause troubles such as high discharge pressures, low dehumidification efficiency, and even compressor mechanical damages. Most of these obstacles can be overcome by further R&DD activities aiming at developing cost-effective and reliable drying heat pumps. Optimal integration of driers and heat pumps remains a challenging R&D task. Wider dissemination of existing and coming knowledge may also contribute to avoid future failed applications. This contribution provides general information on drying heat pumps, and focuses on laboratory and industrial-scale wood drying applications. A number of design and control mistakes found in the drying literature are discussed and some solutions are proposed. The main scope is to avoid undesirable operation parameters, provide safe operation conditions to the heat pump components, improve the system‘s overall energy performances and accelerate future successful implementations of industrial drying heat pumps.
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2. DRYING PROCESS 2.1. Generalities Drying is a highly nonlinear coupled heat and mass transfer process. It occurs by vaporization of a liquid by supplying heat to a wet material. Drying with hot air implies humidification and cooling of the air in a well-insulated (adiabatic) drying chamber. Heat is transferred by convection from drying air to the product drying surface where heat is needed for evaporation of moisture. Figure 2.1 shows a schematic representation of energy balance of a drying system. The material enters the drying chamber at state a with an initial mass
M dry M wet
Ta . After drying, the dried material leaves the drying M dry T chamber at state b with the final mass and at temperature b . On the other hand, in the and temperature
kg /s drying chamber enters relatively dry air (at state 2) with the flow rate m air ( dry,air ). It absorbs water from the material during the isenthalpic process 2-3. If the drying air is heated at constant absolute humidity before entering the drier, the heating thermal power (kW) will be:
Q mdry, air (h2 h1 )
(2.2)
where h1 and h2 are the air mass enthalpy ( kJ / kg ) entering and leaving the air heater, respectively. The maximum rate of water extracted from the material will be:
M wet mdry, air ( 3 2 ) mdry, air ( 3 1 ) where
(2.3)
1 , 2 and 3 are the absolute humidity of the drying air during the process (
kgwater / kgdry, air
). If c is the specific heat of the material, the energy balance of this
simplified drying process will be: mdry, air h3 mdry, air h2 M wet cTa M dry cTa Tb
(2.4)
Heat supplied at the boundaries of the drying product diffuses into the material primarily by conduction. The heat and mass transfer coefficients which control the heat and moisture migration rate inside the product are strong functions of the moisture content as well as of the product temperature. Moisture content represents the weight of water present in the product
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expressed as a percentage of the weight of over-dry product. As the moisture content at the drying surface of the product drops, liquid moisture migrates from the product bulk to the drying surface before it is removed by the hot drying air. Local moisture diffusivity controls liquid moisture migration rate from bulk to the drying surface of the product. The transport of moisture can occur by one or a combination of several mechanisms as liquid (if the wet material is at a temperature below the boiling point of water) and vapour diffusion (if the water vaporizes inside the material), hydrostatic pressure differences (when internal vaporization rates exceed the rate of vapour transport through the solid to the ambient medium) or Knudsen diffusion (in the case of freeze drying) [1]. 3 Exhaust air
se
g
Ma s
in
mdry, air
ry
Material outet 2
3
1 T2
D
b Ta
Heating
2
Drying air
Absolute humidity, kg/kg
Mdry
a
alp
Tb
nth
Material inlet Mdry + Mwet
y, k
J/k g
T3
1 T1
Air heater
T1 T2 Dry-bulb temperature, °C
Q
Figure 2.1. Simplified schema of a drying system.
For materials of high diffusivity, moisture migrates rapidly to the drying surface even at low moisture content of the product. Therefore, the drying rate of such materials is controlled by external transport rates while the drying rate improves with improvement of the external conditions. Evaporation of liquid moisture takes place from exposed surface by absorbing the heat of vaporization Hot air is used both to supply the heat for evaporation and to carry away the evaporated moisture from the solid. At low moisture content of the product, the effect of low relative humidity becomes less significant. At low relative humidity of air, partial vapour pressure of drying air becomes low. This leads to a higher driving potential for mass transfer resulting in increased moisture evaporation rate. Liquid moisture evaporates from the drying surface of the product because of the difference of partial vapour pressures between the air and the surface of the product. Partial vapour pressure at the drying surface is function of the drying surface temperature and its water activity which is a function of surface moisture content and temperature. Water activity (
w ) depends on relative pressure and is defined as the ratio of the partial pressure (p) of
water over the wet solid to the equilibrium vapour pressure (
p w ) of water at the same
Industrial Drying Heat Pumps
temperature. Thus, is defined as:
5
w , which is also equal to the relative humidity of the ambient humid air, w
p pw
(2.1)
As the moisture content and temperature of the drying surface continuously change during drying process, partial vapour pressure at the drying surface becomes an uncontrollable parameter. The diffusivity at the exposed drying surface significantly drops during drying process particularly when the moisture content of the product becomes low. The moisture content of a wet material in equilibrium with the air of given relative humidity and temperature represents the equilibrium moisture content. The evaporation rate of moisture depends on the mass transfer coefficient of the drying air which depends mainly on the types of flow used such as stagnant, laminar or turbulent. Higher volume flow rate if drying air increases the mass transfer coefficient, but the size of blower becomes large and power consumption rate increases. Therefore, maintaining optimum flow of drying air is important for the economic operation of a dryer. The isotherm obtained by exposing the solid to air of decreasing humidity is known as the desorption isotherm. As the maximum allowable temperature of the drying air is limited particularly for heat sensitive materials, partial vapour pressure of the drying air is usually controlled by condensing moisture of the drying air by using heat pumps.
2.2. Drying Schedules Drying schedules are expressed in terms of dry-bulb temperature and either wet-bulb depression and equilibrium moisture content, or both. Because the relative humidity does not directly indicate the drying capacity of the kiln atmosphere, this parameter is determined by use of dry- and wet-bulb temperatures measured by psychrometers. Relative humidity is a measure of the amount of water vapour in the air, expressed as a percentage of the total amount contained in saturated air at a given temperature and pressure. Increasing the temperature of the air without adding more moisture will cause the relative humidity to decrease. To reduce the energy consumption per unit of dried product, it is necessary to reduce the drying time. To reduce the drying time and obtain the desired final product moisture content and quality, different time-dependent drying schemes, as intermittent, cycling and air reversal drying are used. With food products as corn, peanuts, maize, wheat, drying and rest periods of few seconds to few minutes provide shorter effective drying times, thermal energy savings, higher moisture removal rates and lower product surface temperature. The intermittent profiles are prescribed by raising the inlet ait temperature for a defined period and dropping the air temperature back to its original level until the periodic cycle interval. The intermittency i is defined as the fraction of time during which the inlet air temperature is raised (
on ), to the cycle time:
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i
on on cycle on off
(2.5)
In the case of drying heat pump it is not possible to use too short off periods during intermittent cycles because the heat pump‘s overall efficiency can be significantly affected. The transient regimes of heat pumps generally are of at least 10 minutes, before permanent regime is achieved.
2.3. Control of Driers Modern drying technologies make it imperative to found solutions allowing optimizing the processes with respect to complex quality factors of products. The drier control systems are important for energy efficiency since a poorly controlled process is likely to be wasteful, in terms of energy, throughput and quality of the product. It is also likely to have a reduced throughput and to produce inferior quality product. Theoretically, a large number of parameters can be measured and controlled in a dryer. However, most practical control systems limit themselves to one or two outputs and one or two inputs. The most important inputs to a dryer are the heating, air flow and material feed rates. The most common disturbance inputs are the ambient air and humidity and feedstock moisture content and composition. The most commonly used outputs are the moisture content of the dried material, temperature and humidity of exhaust or re-circulated air, and a measure of the product quality, as discoloration, etc. The moisture content of the dried material is often difficult to measure. With certain assumptions, it can be derived from the temperature and humidity of the exhaust air. Different types of control systems are available. Combinations of line-sensors and expert systems with feedback response allow immediate quality-related decision to be made. Sensors are placed in strategic locations to measure real-time quality parameters and the signals are fed to expert systems using software. Drier control systems are designed to change the moisture content of the product while minimizing the drying time and energy use, improving the product quality and reducing the maintenance costs. Since no control system can act directly on moisture content, drier operation is a function of parameters as product moisture content, air dry and wet-bulb temperatures, and velocity. In drier control systems, proportional integral derivative (PID) regulators control the heating, ventilation and humidification/dehumidification devices. The control loops provide automatic temperature maintenance at the product stack entry point. A comparator determines the difference between the actual and the target temperature set by the drying schedule. The most of product quality degradation is mainly due to the thermal effect of the drying air. It is thus possible to reduce these quality effects (surface cracking, nutrient degradation) through a proper feedback system to regulate the air and/or product temperature. To capture the product surface temperature can be used thermo-vision cameras or hypodermic thermocouples.
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2.4. Drying Efficiency Due to affinity of most materials to water and high latent vaporization heat of water, drying is highly intensive in energy. The energy consumption for thermal drying with conventional convective-type dryers is typically high. Contacting efficiency between the products and the drying medium determines the drying performance. Moreover, when heat losses exist through the air vented without recycle or energy recovery, overall efficiency decreases. A good insulation and reduced leakage of the drier minimize heat losses and increases the overall energy efficiency. The performance of a dryer also depends on its geographical location, ambient conditions, controls, maintenance, etc. Selection of energy efficient dryer depends on the physical properties of the product and the ratio of fossil fuel and electricity costs. Low energy performance of drying processes results in adverse environmental impact. Because the oil price has increased in recent years, the drying energy costs have escalated as well. The forecast rising energy costs may lead to a carbon tax which will add additional burden on industry. The solution is to develop highly energy-efficient technologies to reduce the energy consumption and mitigate the environmental impacts. The energy efficiency of a drying process can be expressed by the following equation [2]:
Eevap Etot
(2.6)
Where: Eevap is the energy used for water evaporation, and Etot - total energy supplied to the dryer. Because at low humidity and temperatures, energy is proportional to temperature difference, the energy efficiency may be approximated to thermal efficiency:
T
Tin Tout Tin Tamb
(2.7)
Where: Tin Tout is the difference between inlet and outlet temperatures, and Tin Tamb - difference between inlet and ambient temperatures. The maximum thermal efficiency is defined as the ratio of the highest temperature difference between inlet ( (
Tamb
):
Tin ) and outlet temperature to difference between inlet and ambient
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max
Tin Twb Tin Tamb
(2.8)
It is obtained when exhaust is saturated at wet-bulb temperature (Twb).
3. INDUSTRIAL DRIERS Drier selection approach has to include process specifications and economic evaluations. Are required quantitative information, as the mode of feedstock production, drier throughput and variability, and product quality parameters as physical, chemical and biochemical. At the dried inlet, the products can have different forms as solid (wood, ceramics), granular, particulate, suspension, solutions, sludge, crystalline, liquid, etc. Some materials involve special requirements as corrosion, toxicity, flammability, fire hazards, color, aroma, etc. Location of the moisture (near surface or distributed in the product), nature of moisture (free or strongly bound to solid), mechanisms of moisture transfer, physical size of product, conditions of drying medium (e.g., temperature, humidity, flow rate of hot air for convective dryers), pressure in dryer, as well as the operating conditions also influence the selection of the best drier. Location and average weather, drying kinetics, moist solid sorption isotherms, drying curves, effect of process variables, changes in operating conditions that may affect the quality of the product, as well as inlet and outlet moisture contents have to be determined. Final value of the product, need to automatic control, type and cost of fuel, cost of electricity, environmental regulations, safety aspects (fire and explosion hazards, toxicity) and available footprint of drying system and accessories in the facility have also to be known to avoid errors in selecting the driers [2]. Depending on heat input type, driers can be continuous or intermittent, adiabatic or nonadiabatic, and operate by convection, conduction, radiation, electromagnetic fields, or combinations. The convective dryers are the most common. About 85% of industrial driers are of this type despite their relatively low thermal efficiency caused by the difficulty in recovering the latent heat of vaporization contained in the dryer exhaust air in a cost-effective manner. Hot air is generated by indirect heating or direct firing. Superheated steam can also be used. As primary energy sources, oil, natural gas or electricity are used. Combustion gases can be used when the product is not heat-sensitive or affected by the presence of combustion products. In direct dryers, the drying medium contacts the material to be dried directly and supplies the heat required for drying by convection. The evaporated moisture is carried out by the same drying medium. Drying hot air or gas temperatures may range from 50°C to 400°C depending on the material. Dehumidified air can be needed when drying highly heat-sensitive materials. Industrial driers can operate at atmospheric pressure or below it (vacuum). Drying temperatures can be below or above the water boiling temperature, or below the water freezing temperature. According to the relative flowing between the drying air and the drying products, driers can be in co- or counter-current, or mixed. Several drying stages can also be used in more complex industrial processes. Finally, the product residence time inside the drier can be short (less than, for example, one minute), medium (i.e., between one minute and one hour) and long (more than one hour). Each type of dryer has specific characteristics
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which make it suitable or unsuitable for specific applications. Certain types are inherently expensive (e.g. freeze dryers) while others are inherently more efficient (e.g., indirect or conductive dryers). Thus, it is necessary to be aware of the wide variety of dryers available, as well as their special advantages and limitations. Solar-assisted heat pump dryers can be used in regions where solar energy is abundant in order to further improve the energy efficiency of overall drying systems. Instead of using conventional heating systems to provide auxiliary heating, the solar energy can be stored in phase-change materials for discharging sensible energy to the drying air. The solar-assisted heat pump dryer may operate at higher drying temperatures, is environmental-friendly process and is relatively easy to implement and control. Such a system offers the flexibility of operating with the heat pump, solar system or with both, and consists of solar collectors, blowers, phase-change storage tanks, air ducting and valve. However, higher capital costs are incurred for additional solar panels, blowers, storage tanks, ducting, valves and controls. The amount of stored solar energy is greatly subjected to the weather conditions. The implementation of such a system is cost-effective if the average annual sunshine is greater than 2600 hours per year and if the annual total quantity of radiation is more than 6x106kJ/m2. The products can be dried in batch or continuous modes, and they can be in stationary, moving, agitated or dispersed state inside the dryer. In batch driers, the material to be dried is introduced on trays inside the drying enclosure. Batch dryers can utilise cross- or through circulated convective and contact heating. In continuous driers, a series of trays is moved slowly through a heated tunnel. Drying takes place in a current of warm air. Conveyor driers consist of perforated metal conveyor belts passing through an oven. Most solid is fed onto the conveyor from an extruder, and heated air is forced through the bed of solid on the conveyor as it passes through the oven. Conveyor dryers operate continuously and are particularly suitable for solids requiring gentle physical handling. Heat loss sources of these dryers are the dryer air leaks and exhausts, and the typical specific energy consumption rages from 0.945 – 4.32 kWh/kgwater. Spray dryers consist of a vertical cylindrical or conical chamber into which a liquid or slurry is spayed using a rotating wheel or nozzle atomiser. Droplets from the atomiser come into contact with hot air. The dried solid is separated from the exit air in a cyclone. Spray dryers operate in continuous mode and are suitable for heat-sensitive materials. Rotary dryers consist of a rotating cylinder inclined a few degrees to the horizontal. Rotation speed is generally in the range of 5-20 RPM. The cylinder is rotated by small electrically driven rollers on which it rests. There are various types of indirectly heated rotary dryers. The most common is the steam tube dryer having a bundle of steam-heated tubes inside the dryer cylinder itself. Another type of indirectly heated rotary dryer has a heated jacket around the dryer cylinder. Rotary dryers usually operate in continuous mode. The material to be dried is fed in at the higher end of the cylinder while air is drawn in by an induced-draught fan and moves either co-current or counter-current to the solid. The air is heated either by a heat exchanger or directly by a flame. The specific energy consumption of spray and rotary dryers varies between 0.8 and 3.24 kWh/kgwater. Fluidised bed dryers consist of a vessel in which air is blown through a perforated plate above which lie a bed of solid particles. The air-flow rate is such that the solids become suspended in the upward flow of air. Fluidised bed dryers can operate in batch or continuous mode, and are mainly suitable for drying granular solids. In batch mode, the dryer is operated until a desired level of moisture has been achieved. In continuous mode, fresh material can be
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fed from above and dried material removed from a suitable point on the side. The main heat loss source is the exhaust air, and the specific energy consumption is of 0.945 – 2.16 kWh/ kgwater. Fluidized bed drying is applied for drying products as granular solids in the food industry, ceramic, pharmaceutical and agricultural industries. This method has high drying rates due to excellent gas-particle contact leading to high heat and mass transfer rates. Smaller flow area is required compared to other drying technologies as rotary, conveyor or tunnel. The thermal efficiency is higher, and the capital and maintenance costs are lower compared to rotary dryers. However, the power demand is high due to the need to suspend the entire bed in gas phase leading to high pressure drops. There is high potential of attrition, and in some cases, of granulation and agglomeration. The flexibility is low and there is a potential of defluidization if the feed is too wet. The dyer chamber receives wet material and discharge dried product through the product inlet and outlet ducts.
3.4. Heat Recovery Methods Dryers are usually one stage of complex manufacturing processes. Energy efficiency measures, including drying controls, for a drying operation firstly consist in identifying the parameters which minimize the heating requirement. Because energy efficiency requires capital investment, it can be considered attractive if the resulting cost savings are sufficiently high. The most common form of heat recovery option on a dryer involves the use of waste heat to pre-heat the inlet air to the dryer. This is used in industries as food, ceramics, chemicals, wood, paper, laundries and textiles. There are cases where heat is recovered elsewhere in the process and transferred to the drying. In drying using electric heating, the key energy savings measures may include infra-red, microwave and dielectric technologies. These technologies can be highly efficient and costeffective in certain situations because of their ability to deliver the heat directly to the moisture to be evaporated, thereby avoiding the losses incurred through heating large volumes of air. Direct heating involves the use of hot combustion gases directly from a gas or oil burner, usually diluted with fresh air, as the drying medium. It improves efficiency by avoiding the losses associated with boilers, steam distribution systems ani heat transfer equipment. Heat recovery is one of the most important energy saving measure for any drier. It refers to any operation which heat from the exhaust air or the product is transferred to the input air or the dried material. Heat can be recovered from exhaust air either by a heat exchanger or by recycling part of the exhaust and mixing it with fresh input air. Heat recovery is applied to a wide range of dryers in a wide range of industries. These include batch, fluidised bed, rotary and spray dryers in food and drink, and chemical industries, tumble dryers in textiles industry and paper machine drying in paper industry. Heat recovery can be applied by several methods: exhaust air recycle, plate heat exchangers, run-around coils, heat wheels, heat pipes, phase changing materials, special membranes and heat pumps.
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Exhaust air recycle, used on both batch and continuous, recovers a part of the enthalpy of the hot and humid exhaust air. It is important for drying processes operating at high air velocity and short contact time between the drying medium and the material. It involves taking a proportion of the exhaust air and mixing it with fresh heated drying air before it enters the dryer itself. Such a system has the advantage that no heat exchangers are required and so the capital cost is less. The problem is that in most industrial operations, the humidity if the exhaust air is well below its equilibrium value in relation to the moisture content of the material being dried. This means that it has removed less water from that material than it could have done and that more energy has been used to heat the air than was necessary. It is not possible to achieve 100% of equilibrium humidity because the rate of drying is proportional to the difference between the equilibrium and actual humidity: the smaller the difference, the slower the rate of drying. There are many ways in which this problem could be handled. The most obvious is to increase the efficiency of mass transfer between the material to be dried and the drying air so that the air picks up more moisture from the material. It may be possible to do this by maximizing the area of contact, or by increasing the length of the dryer. However, increasing the length of the dryer substantially increases the cost. Another approach could be to reduce the flow rate of air through the dryer. Because this technology reduces the throughput, it is not usually an option. Plate heat exchangers consist of an array of parallel thin metal plates which separate the two streams. They are relatively low cost although not particularly compact. Run-around coil is used when it is not possible to transfer heat directly between the exhaust and inlet air streams on a dryer, usually because they are too far apart and ducting would be too expensive. In such cases, heat can be recovered using a secondary heat transfer medium. The heat transfer medium is usually a mixture of water and antifreeze. The heat exchangers are usually tubular recuperators with liquid the liquid on the tube side. The advantage of the run-around coil is that expensive air ducting is replaced by lower cost small diameter pipe. Space constraints are therefore not a problem, and the cost can be less. However, the heat transfer efficiency is less because two heat transfer steps occur in series. Heat wheels consist of rotating matrices through which hot and cold streams pass alternately. They are most commonly used in high temperature applications. The main disadvantage for drying is that they do not cope well with entrained particles. If the exhaust air of a dryer is cooled below its dew point, the vapour will condense, releasing its latent heat of vaporisation. Since this latent heat represents the majority of the heat input to the dryer, its reuse is essential of major improvements in energy efficiency are to be achieved. However, this heat is usually released at too low temperature for much of it to be cost-effectively transferred to the input air. Other energy savings measures are mechanical dewatering, insulation and use of superheated steam as a drying medium. An alternative method of dehumidification drying is to use dessicant wheels. It is similar to a heat wheel heat exchanger, but which consists of a powerful dessicant material. One half of the wheel passes through the moist air, removing water from it, and the other half is heated by a gas burner, driving off its absorbed moisture. The dehumidified air remains as hot as it was when it emerged from the dryer and can be recycled directly to the dryer inlet. A particularly simple and cost-effective form of heat pump is mechanical vapour recompression. Although not applicable to most dryers, it can be used on those where superheated steam is used as drying medium.
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The following section discusses some aspects of heat pumps utilization as efficient heat recovery and energy saving devices in industrial drying processes.
3.5. Heat Pumps In the common thermodynamic sense, a heat pump is a system in which refrigeration components (compressors, condensers, evaporators and expansion valves) are used in such a manner as to take heat from a source (air, water, ground, etc.) and give it up to a heat sink (air, water, ground, etc.) that is at a higher temperature than the source. Closed compression cycle heat pumps are theoretically based on the ideal Carnot cycle. This type of heat pump is mostly used to recover waste heat at relatively low temperatures, and to upgrade it for process temperatures between 50°C and 75°C. The most common type of compressor for closed-cycle is the electric compressor which achieves an efficiency of above 90%. Rotary and screw compressors are most suitable for large systems.
3.3.1. Principlup to state 1 before entering the compressor. At this point, the cycle is repeated. Heat sink (Tsink)
4
2'
Expansion valve
Evaporator 5
2
Compressor
4
Tcond
3
2' 2s
2 pcd Tsink
T, K
3
Condenser
1 5
Tevap
1
pev Tsource
s, kJ/kgK
Heat source (Tsource)
Figure 3.1. Principle of heat pumps and thermodynamic cycle in T-s diagram.
3.5.2. Advanced cycles Refrigerant circuits of heat pumps can be modified in order to minimize the irreversibility and improving the cycle overall efficiency. It is well known that the vapour generated during the expansion process (4-5) does not contribute to the cooling capacity of the evaporator. However, the compressor has to compress this vapour. The role of the flash tank (Figure 3.2a) is to extract a portion of this vapour at an intermediate pressure level, sends it to the compressor and reducing the compressor power input. So, this vapour is compressed from that intermediate pressure to the condensing pressure avoiding the expansion portion to the evaporator pressure level. The loss of compression work is thus avoided. On the other hand, the isenthalpic expansion process that reduces the pressure of the refrigerant from the condenser level to the evaporator level is highly irreversible. Instead the
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13
expansion valve, a work-producing expansion device can be employed (Figure 3.2b). The work output of this device (expander) can be supplied to the compressor to reduce the compressor power input. The expander reduces the power requirement of the compressor and, at the same time, increases the evaporator cooling capacity. This last benefit is available even when the work produced by the expander is not utilized. By using both benefits, the overall efficiency gain is of about 5%. CD
CD
EX1
Flash tank
C
Expander Power
Vapour
EV
EV
EX2
C
(b)
(a)
Figure 3.2. Heat pump vapour compression cycles with flash tank (a) and expander (b). C2
EV
CD
EX1
CD
(a)
EV Evaporator/ condenser
EX2 Inter-cooler
C1
C2
C1
EX1
EX2 (b)
Figure 3.3. Two-stage (a) and cascade (b) heat pump closed compression cycles.
The two-stage closed compression cycle with one evaporating temperature (Figure 3.3a) aims also at increasing the cycle overall performance. A flash intercooler allows increasing the proportion of vapour to be compressed by using the superheat of the vapour coming from the first stage compressor (C1) to evaporate some of the liquid working fluid from the first expansion stage. The disadvantages of this cycle are the pressure drop in the intercooler and a risk of entrainment of liquid drops in the second-stage compressor. When large temperature lifts are needed cascade cycles are used (Figure 3.3b). They allow using different working fluids at each stage and reasonable pressure ratios to be achieved in each compressor.
3.5.3. Working fluids Selection of refrigerants as working fluids for heat pumps concerns their thermodynamic properties and environmental impacts. The refrigerants are generally categorised in short-
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(HCFC) and long-term (HFC) alternatives, as well as mixtures and pure fluids. Mixtures can be azeotropic, near-azeotropic or non-azeotropic. Azeotropic fluids evaporate and condense at a constant temperature, while non-azeotropic mixtures undergo the phase changes at varying temperatures. The temperature change at the phase change is known as the temperature glide of the mixture. If the glide fits with the temperature change of the source/sink, the glide will contribute to a higher COP. Ozone depletion is due to a complex reaction with chlorine compounds, such as hydro-chlorofluorocarbons (HCFCs) that are used as working fluids in heat pumps. A useful parameter for estimating the impact of substances on ozone destruction is the ozone depletion potential (ODP). It is a measure of the destructive potential of a particular substance, relative to depletion caused by an equal amount of a reference substance. CFC-11 is typically defined as the reference compound, and is assigned an ODP of 1.0. On the other hand, the global warming potential (GWP) is an index that quantifies the capability of chemical substances to absorb infrared radiation relative to carbon dioxide. The direct and indirect capacities to absorb different wavelengths of infrared radiation, the residence time in the atmosphere and the time period over which the effect on radiation are factors contributing to this index . Since certain molecules used as working fluids in heat pumps absorb radiation emitted by the earth, the greenhouse effect accelerates. As low-temperature refrigerants for heat pumps currently are used, among others, HFC-134a and HFC-410A (Table 3.1), and as high-temperature refrigerants, R-236fa and R-245fa (Figure 3.4). The type of electricity production has also an indirect impact on greenhouse effect. Production of electricity for powering electrical plants is associated with emissions of CO2 and has to be considered for a total picture of the global warming impact. Local conditions for power generation vary considerably from one country to another (Table 3.2). The power demand for a specified heat output depends on the efficiency of the heat pump. This implies that indirect emissions can be reduced by selecting more efficient units. Table 3.1. ODP and GWP values for some refrigerants. Refrigerant HFC-134a HFC-410A R-236fa R-245fa
ODP 0 0 0 0
R-236fa - saturated temperatures
140
GWP (100) 1 300 1 900 6 300 950 R-245fa - saturated temperatures
180 160
120 Temperature, °C
Temperature, °C
140 100 80 60
120 100 80 60
40 40 20
20 0
0 0
500
1000
1500 Pressure, kPa,a
a
2000
2500
3000
0
1000
2000
3000
Pressure, kPa,a
b
Figure 3.4. Saturated temperatures of two high-temperature refrigerants; a) R-236fa; b) R-245fa.
4000
Industrial Drying Heat Pumps
15
Table 3.2. CO2 emissions associated with electricity production (indirect emissions). Type of production Coal-fired plant Oil-fired plant Natural gas-fired plant Hydraulic and nuclear plants
CO2 emission kgCO2/kWhel 1 000 700 450 0
3.5.4. Design of heat pumps The heat pump evaporator and condenser generally are finned refrigerant-to-air heat exchangers. Their heat transfer surfaces are calculated using the following equation:
Q S U * LMTD
(3.1)
where Q is the thermal capacity of the designed heat exchanger. The thermal capacities of evaporator and condenser (kW) are respectively calculated from energy conservation equations with refrigerant-side enthalpy changes and flow rates, or with the air-side temperature changes, mass flow rates and specific heat:
Q EV EV mEV , air c p, air Ti Ta ha hb mR h1 h5 UAEV LMTDEV
(3.2)
Q CD CD m R h2 h4 mCD,air c p ,air Td Tm mCD,air hd hm UACD LMTDCD
(3.3)
where τ is the operating time. The overall heat transfer coefficients of the evaporator and condenser are expressed as:
U
1 A f 1 A f ln r f / rint 1 Aint hR 2kL f hair
(3.3)
where f is the temperature effectiveness of the finned surface Af [4]. The evaporator refrigerant-side convective boiling heat transfer coefficient can be calculated with the well known Liu and Winterton asymptotic correlation, and the refrigerant-side condensation heat transfer coefficient, with the Shah‘s enhancement model [5]. For both evaporator and condenser, the logarithmic mean temperature difference is defined as:
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LMTD
Tin Tout ln Tin / Tout
(3.4)
Tin is temperature difference between the hot fluid inlet and cold fluid outlet ( Thf ,in Tcf ,out Tout - temperature difference between the hot fluid outlet and cold fluid ), and
where
T
T
h
cf ,in inlet ( hf ,out ). The air-side heat transfer coefficients ( air ) of flat fin evaporators and condensers are calculated based on Webb and Gupte correlation (1992) depending on Graetz number values. The mass flow rate of reciprocating compressor is related to the volumetric displacement rate according to the following equation:
VC mR a v1
(3.5)
Where a is the actual compressor volumetric efficiency, V C - compressor volumetric displacement rate and v1 - specific volume of refrigerant the vapor entering the compressor. The actual volumetric efficiency is less than the theoretical volumetric efficiency due to pressure losses in the compressor valves and the heating of suction gaz. The isentropic efficiency of a compressor is defined as follows (Figure 3.1):
s
h2 h1 h2 s h1
(3.6)
Where the compressor discharge point 2s is defined by
s1 s 2 s and p2 p2 s . The
isentropic efficiency varies with the pressure ratio ( p2 / p1 ) as follows:
s a0 a1 a2 2
(3.7)
The compressor electrical power input (kW) is determined by the conservation of energy:
WC m R (h2 h1 ) Q CD Q EV
(3.8)
The thermal power rejected by the compressor is a function of the electrical efficiency of the compressor motor (
cm ) and is determined by:
Industrial Drying Heat Pumps
Q C WC (
1
cm
17
1) (3.9)
The expansion device EX can be a thermostatic, electric or electronic valve. It controls the amount of superheat of the refrigerant within the evaporator. The liquid receiver LR is an inactive component for the thermodynamic cycle.
3.5.5. Performance of heat pumps The energy performance of electrically-driven heat pumps is dependent upon the degree of exhaust or re-circulated air cooling/dehumidification required and on the temperature lift needed. The maximum amount of heat which can be recovered depends on the ambient temperature (-30 to 30°C), required dryer air inlet and exit temperature and humidity. The reported energy saving levels through heat recovery from heat pumps vary from 10 to 50%. For heat recovery, the energy saving is equal to heat recovered, minus any additional fan power to overcome increased pressure drop. The fan power as a function of pressure drop is given by the formula:
Required power V p / f
(3.13)
Where:
V is the volumetric flow tare, Δp – the pressure drop, f - efficiency factor (typically 80-85%). The majority of the electrically driven heat pump heat recovery systems are dehumidifiers where the latent heat within the exhaust is removed by a refrigerant which is then compressed and the heat released to the inlet air. The energy performance of electrically driven heat pumps is dependent upon the degree of exhaust or re-circulated air cooling required and on the temperature lift needed to preheat the inlet or re-circulated air. A closed compression cycle based on the ideal Carnot cycle operates between the temperatures T1 (heat source) and T2 (heat sink). The coefficient of performance for the ideal Carnot cycle is defined as maximum theoretical efficiency:
COPCarnot
T1 Tcond T1 T2 Tcond Tevap
Where:
Tcond is the condensing temperature (K), and Tevap - evaporating temperature (K).
(3.14)
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Q thermal power output at the condenser ( CD ) and the electrical power input at the
compressor and blower ( W C B ):
COP
Q CD
W CB
Q CD
(3.15)
Q CD Q EV
The actual COP of heat pumps is usually 40 to 50% of the COP of the theoretical Carnot cycle. Consequently, the COP of the actual cycles is expressed as:
COP CarnotCOPCarnot where
(3.16)
Carnot is the Carnot efficiency.
4. DRYING HEAT PUMPS In the 1970s and 1980s, the drying industry promoted dehumidifier concepts, but the performance of such systems was often disappointing due to air flow and control problems, inadequate dehumidifying capacity and inappropriate kiln structures [3]. The reliability of the systems was often low and equipment suppliers did not provide enough information on the actual performance of their systems. However, the best solution that would allow heat pumps to be effective as a means of drying would be to use them in combination with a traditional energy source at high temperatures to obtain similar drying rates and energy savings. The majority of drying heat pump systems is dehumidifiers and the closed compression cycle, electrical motor-driven is the most used. The electrically-driven heat pumps benefit from high fuel prices, due to their larger absolute savings at high fuel prices. High COPs are also beneficial. Any convection-type dryer can be fitted with a suitable designed heat pump. Batch shelf, kilns (for wood), fluid bed, tray and rotary dryers can be used. They recover heat from the dryer hot and humid air by condensing out the water vapor, which is then removed as liquid. Additional heat input may be supplied by convection or by other sources (microwave, radio-frequency or infrared). The sensible and latent heat recovered is used to reheat the dehumidified air. Heat pump dryers improve the quality of dried high-value products by controlling the air humidity and temperature, substantially reduce electrical and/or fossil energy consumptions, and may reduce the drying time. Since heat pump drying is carried out in closed systems, smell from the drying products (food, etc.) are reduced. Heat pumps are used in industrial dehumidification and drying processes at low, moderate and high temperatures, but not higher than 110°C. They Heat pump dryers operate with high coefficients of performance, normally higher than 4.
Industrial Drying Heat Pumps
19
For selecting a drying heat pump and integrating it to a drier, the first step is to identify technical and economical feasibility of installation. Factors of importance are the characteristics if heat source (temperature, load, phase and location), as well as maximum sink temperature and lift temperature that are of about 75°C and 50°C, respectively. The ratio of heat source and heat sink amounts is also important. To make full use of the heat pump, the heat source should be little more than the same size of the sink by a factor of 10% to 50%. Drying heat pumps remove moisture from the air circulated through the drying chamber. They are efficient systems, impose fewer restrictions on materials and construction of the drying chamber, and can be adapted to any enclosure which is reasonably air-tight, moistureresistant and insulated. Initial capital investment, maintenance costs and peak power requirements are relatively low. However, with low-temperature heat pumps, drying times can be longer and drying rates lower as compared to conventional driers. If located outside in cold climates, driers must be well insulated to prevent heat loss during the winter months. Also, it is necessary to use a supplementary source of heat to maintain effective drying temperatures. A back-up energy source (electrical or fossil) is used to bring the drier charge up to operating temperature at the beginning of a drying cycle, after which the heat pump supplies the majority of required heat to continue the drying process.
4.1. Principle Figure 4.1 represents a heat pump integrated with a drying enclosure, as well as the refrigerant and air thermodynamic cycles in p-h and h-T diagrams [6]. The refrigerant-side of the heat pump consists of an evaporator EV, a compressor C, a condenser CD, sub-cooling devices as a heat rejector HR and/or a sub-cooler SC, a liquid receiver LR and an expansion valve EX. Air is drawn from the drying chamber and forced over the cold evaporator where the moisture condenses and the water is drained out. The sensible heat removed from the air and the latent heat of condensation, are both transferred to the refrigerant that vaporizes. The refrigerant absorbs heat from the air and undergoes a two-phase change from a vapour-liquid mixture (state 6) to superheated vapour (state 1a). The refrigerant superheated vapour at state 1a is further superheated inside the internal sub-cooler/super-heater SC up to the state 1, prior entering the compressor suction line. The refrigerant vapour is then compressed by the compressor and absorbs additional energy equivalent to the electrical input. The compressor electrical energy input is converted into mechanical work to raise the refrigerant vapour pressure. This work is transferred to the condenser for heating the drying air. By raising the vapour pressure, the condensing temperature increases to a level higher than that of the dryer temperature. As a high-pressure superheated vapour, the refrigerant passes through the condenser where it condenses and transfers heat back to the drying air. Within the CD, the refrigerant vapour first undergoes a change from superheated (2) to saturated vapour (2‖), and then condenses (2‖-2‘) and sub-cools (2‘-3). The heat rejection process 3-4 occurring in the HR coil aims at controlling the heat pump dehumidification capacity. Sometimes, the SC is required to further sub-cool the liquid (process 4-5). The expansion valve EX expands the liquid refrigerant (isenthalpic process 5-6) in order to reduce its pressure to the vaporization
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value, below the heat source level. The two-phase refrigerant enters the EV at state 6, and the thermodynamic cycle starts all over again. On the air-side, the air cooling (i-a) and dehumidification (a-b) processes occur at the evaporator EV. The air at state b leaving the evaporator is mixed with moisture-laden drying air (state i). The mixed air (state m) is directed through the condenser and heated at constant absolute humidity (process m-d) up to a temperature higher than the dry-bulb temperature of the drying air. When required, the back-up heater supplies additional heat to keep the air drybulb temperature close to the setting point. The drying air passes through the material stack and picks up moisture according to the process d-i at constant enthalpy. The described process recycles the heat to maintain the drying conditions in the drier, whereas in conventional driers considerable quantity of heat is exhausted to the atmosphere through venting of excess humidity.
4.2. Energy Performances Both design and economic parameters influence the overall economic viability of heat pump dryers. The heat pump energy efficiency is expressed by the coefficient of performance (COP) defined as the total heat supplied by the condenser (expressed in kWh) divided by the compressor and blower electrical energy consumption (also expressed in kWh):
COP
QCD Q CD EC B W * CB
(4.1)
where W C B is the electrical power input of the heat pump compressor and blower, and the heat pump total running time (s). Coefficients of performance below 4 are normally not acceptable, but COPs of 6 have good possibilities for economically favourable installation. The annual savings are larger at high COPs which imply that the rapid increase of the payback period starts at lower fuel prices for a heat pump with high COPs. A performance indicator that is commonly used to define the performance of a conventional drier is the specific moisture extraction rate (SMER). It is defined as the ratio between the amount of water extracted from the dried material ( kWh):
mwater
, expressed in kg) and the energy input to the dryer (
SMER
drier mwater drier Einput
Einput
, expressed in
(kg / kWh) (4.2)
This parameter depends on the heat pump dehumidification capacity, the compressor and blower power inputs, and the total water quantity removed from the dried material. In the case of drying heat pumps, the modified specific moisture extraction rate (MSMER) defined as the
Industrial Drying Heat Pumps
21
ratio between the amount of water extracted from the dried material by the heat pump ( , expressed in kg) and the energy input to the heat pump (compressor and blower) ( expressed in kWh):
MSMER
hp mwater EChp B
hp mwater
EChp B ,
(kg / kWh) (4.3)
The specific energy consumptions of the drier (SEC) and heat pump (MSEC), defined as reciprocals of SMER and MSMER respectively, can be used to compare energy efficiency of different type of dryers and heat pumps. The performance indicator MSMER depends on the annually running time of the dryer. Industrial applications require at least 2000 hours of operation per year to provide low cost per litre of water removed. Because heat is extracted from the drier humid air instead exhaust it outside, the heat pump driers offer higher MSMERs. These efficiency indicators range between at least 1.5 and 4, while those of conventional convection driers are of 0.12 to 1.3 for hot air convective driers, and 0.7 to 1.2 for vacuum driers. Air
2
C
2'’
CD Blower
SC
1
3 HR
m
1a
Back-up heater
5 LR
d
b
EX
4 LV
6
EV 1'
i
i Air
Air damper
Drying enclosure Mass enthalpy h (kJ/kg)
ha a 5
2s 2
2'’
pCD
3 2'
Pressure
4
6
i m
d
hb
pEV
b
1' 1a
1
Enthalpy (kj/kg) Refrigerant cycle in p-h diagram
Absolute humidity, kg/kg
hi = hd
Ta Tm Ti Td Tb Dry bulb temperature, °C Air cycle in Mollier diagram
Figure 4.1. Schematic of a drying heat pump, and refrigerant-side and air-side thermodynamic cycles.
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4.3. Payback Period The payback period is defined as the total investment cost (TIC, expressed in $) divided by the annual net savings in energy costs (ANSEC, expressed in $/year) due to its installation, which in the case of a heat pump is the net saving in fuel or energy cost:
Payback period
TIC ANSEC
(4.4)
The capital costs are higher than that of conventional dryers and require additional maintenance (compressors, blowers, lubricant, air filters, etc). These costs include design, project management, installation and commissioning, as well as compressors, motors, control equipment, ductwork, fans, heat exchangers, procurement, fabrication and structure supports. The total investment cost of heat pump driers is made up of fixed and variable costs. Fixed costs are unrelated to the amount of moisture removed from the product over the years. The drier cost is the primary element, but it includes also the electrical demand charges. Specific installation costs are very site-specific and they decrease as the unit size becomes larger. Thus, the absolute increase in payback period at, for instance, 50% higher installation costs compared to the assumed values, will be low if the payback period is short, and high if the payback period is long. It is important to decrease the total investment cost for a heat pump installation, i.e. the heat pump itself, its installation and other associated costs, not just the cost of the equipment alone. Because the payback period is directly proportional to installation costs, it is important to decrease these costs. It is sometimes argued that the heat pump component cost will decrease when the number of installations increases. Although this is generally true, it must be noted that only a part of the total cost (normally, between 25% and 50%) is related to the drying heat pump itself. The rest is due to installation, piping, control devices, engineering, etc. It is thus important to decrease the associated costs, e. g. by developing better knowledge of heat pumps, advanced compressors, heat exchangers, etc. Energy prices also vary, and the cost of energy saving equipment is highly specific to each application. The simple payback periods of initial investments are generally shorter if more product moisture is available. Higher MSMER can be interpreted as lower operating cost, making payback period for the initial capital cost shorter. To estimate the costeffectiveness of a drying heat pump, it must establish the input and output air temperatures and humidity, and estimate the amount of heat which can be recovered. The payback period of the electric motor-driven closed compression cycles is strongly influenced by the electricity price and the COP, and by the temperature lift. If it is assumed that the useful heat generated by drying heat pump replaces heat from an existing boiler with thermal efficiency B , the payback period can be also expressed using the equation [2]:
Payback period
1 FEP EEP * 8760 AMC COP B
(4.5)
Industrial Drying Heat Pumps
23
Where:
FEP is the fuel energy price ($/kWh), EEP - electrical energy price ($/kWh), COP - coefficient of performance of the heat pump, AMC - annual maintenance cost of the heat pumps ($/kWh/year). The operating costs of electricity-driven heat pumps increase as the electricity price rises, so the payback period increases. The larger the electricity consumption, the greater is its influence. Hence, the drying heat pumps having high COP will be affected to a lesser degree. The fuel price also has an impact on the payback period due to its influence on the absolute savings. The payback periods of the electrically-driven heat pumps are very sensitive to fuel prices. At low fuel prices, the annual savings approach zero, and the payback period increases rapidly. They decrease at higher fuel prices, due to the larger absolute savings at high fuel prices, although the electricity price also increase (the ratio between electricity and fuel prices is kept constant). A high fuel price also favours this type, due to the larger absolute savings at high fuel prices. The payback period varies from industry to industry and from country to country and has the advantage of focusing attention on the first years of the life of the equipment. It depends on the type of drier. Acceptable payback periods are normally between two and three years.
4.4. Optimization Requirements The principal advantage of heat pumps dryers emerges from the hot and humid air as well as from their ability to control the drying air temperature and humidity. However, it is necessary to optimize the system components and design to increase the energy efficiency of drying heat pumps.
4.4.1. Background Optimum operation of drying heat pumps requires a number of components and configurations to provide appropriate thermodynamic conditions. Furthermore, for a given dehumidification capacity of the heat pump, it is mandatory to determine the required quantity of the dried material to provide high energy performances. A brief review of published studies shows what design features and/or information are missing to accurately operate laboratory and/or industrial-scale heat pump dryers. It was observed that many research studies on heat pump drying technology don‘t specify the heat pump dryer configuration neither the quantity of dried materials when reporting high energy performance. They include general statements like ―the drying conditions are controlled by adjusting the capacity of the heat pump components”, without indicating ―how‖ capacity control was performed in each particular case. Many works also agree that the heat pump technology allows better control of temperature and relative humidity (RH) of the drying process. However, few of them explain how the temperature and RH control is achieved to provide reported relatively high dehumidifying performances [6]. A number of published
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configurations, as shown in Figures 4.3a and 4.3b either use parallel condensers [7] or refrigerant desuperheaters [8] to reject the system excess heat outdoors. The most significant lack in such configurations is related to the air ducts across the heat pump evaporator and condenser. As can be seen in both examples, the air flows at the same rate through the evaporator and the condenser. Now this is not an optimum situation because a number of problems may occur. For example, if the heat input (sensible and latent) or the air temperature at the heat pump evaporator inlet are too low, or if there is poor air distribution through the coil, the unit may trip off on ―low‖ suction pressure control. If the air flow rate through the condenser is too low, the unit trips off on ―high‖ pressure control. Also, if the air temperature at the condenser inlet is too high, as when the back-up steam or electrical coils are starting, the compressor may shut-down on ―high‖ limit pressure. Another concept combining a heat pipe heat recovery heat exchanger with a drying heat pump (Figure 4.3c) [9] is also questionable. Because the heat pipe simultaneously reduces the heat pump‘s evaporating temperature and increases the condensing temperature, the claimed 12%-20% reduction in the heat pump energy consumption therefore seems rather unlikely. In addition, it could be that the overall effect on the COP is not high enough to justify the additional investment for the heat pipe device.
Heat rejection coil Internal heat exchanger (condenser)
Heat rejection coil Air
Internal heat exchanger (condenser) EX EV
Dryer
EV
Air
F
F Dryer (b)
(a) Heat pipe Condensation
Evaporation EV Dryer
C
EX
C
EX
C
Air F
Back-up heater CD (c) Figure 4.2. Published configurations of heat pump dryers.
Industrial Drying Heat Pumps
25
Table 4.1. Short review of published studies on drying with heat pumps. Product (reference) -
Initial product mass -
Moisture contents Initial
Final
Drying temperature (RH) -
Moisture extraction rate -
Drying time -
Drying heat pump Capacity
MSMER
WC -
kg
% w.b.
Rat liver [10] Cheese
n/a
Vegetable seeds [11] Paddy [12] Cellulose [13] Shredded radish Sludge
°C (%)
-
hrs
kW
kW
kg/kWh
90
% w.b. 20
n/a
0.75 %/h
95
n/a
n/a
3.2-4.5
n/a
80
20
n/a
120
n/a
n/a
n/a
n/a
30
6
n/a
n/a
n/a
n/a
3.1-4.5
1 200
n/a
n/a
n/a
4.2
2.0
n/a
n/a
8 – 15.9 kg/h n/a
15.5
n/a
0; 4; 8; 12 (n/a) 30 (55); 50 (50) 42 & 46 (26 & 14) -15 & 20 (n/a)
n/a
n/a
n/a
0.28; 4
200
90
16
40 (n/a)
6.3 kg/h
25
15
n/a
n/a
2 - 3.5
46.8
5.42
60 (36)
0.71 kg/cycle
5.16
n/a
0.17
n/a
On the other hand, the required mass of the dried materials is not mentioned in several studies, even when the initial and the final moisture contents are indicated (Table 4.1). Experimental results as drying curve (i.e. moisture content vs. time) for rat liver [10] or cheese and MSMER ratios (kgwater/kWh) as a function of the air temperature at the drying chamber inlet, are given without specifying nor the material initial mass, nor the heat pump dehumidification capacity or – at least – the compressor power input. Without this information it is difficult, if not impossible, to validate the pertinence of reported drying performances. It is also not indicated how the air relative humidity has been kept constant respectively at 40% or 50% (inlet) and 60% (outlet), and how the moisture removal rate has been controlled. In the case of cheese drying at low temperatures between 0 and 12°C it is not indicated how the drying system has been stabilized before starting-up the heat pump. High differences exists between the drying air temperature leaving the heat pump evaporator (36.5°C) and the evaporating temperature 15°C [11] may prove low input heat to the evaporator or lack of refrigerant charge. Up to 54% higher COPs with two-stage trans-critical CO2 heat pumps compared to those of single-stage drying heat pumps are also reported. That gives MSMER ratios up to 43% higher, but the authors didn‘t specify the duration of drying cycles, quantities of vegetable seeds dried and volume of the water removed, and the energy consumed. With much complete information [12], as the compressor nominal power input, quantity and initial/final moisture contents of the dried product (paddy), average drying time, total water volume removed and electrical energy consumption, it is much easier to validate the reliability of the experimental work and of the system drying performances. In other study [13], the sulphate and sulphite cellulose seem being dried with an ammonia two-stage heat pump system, but the schematic layout of the fluidized dryer represents a single-stage heat pump. Moreover, no information is given about the quantity of the dried material, drying time, volume of water removed, compressor input power, electrical energy consumption and the heat pump nominal
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dehumidification capacity. In return, data on MSMER values at different drying temperatures are provided. The concept of fluidized bed heat pump drying of bovine intestine [14] at temperatures varying from -10 to 25°C also don‘t give information concerning the heat pump cooling capacity and the material initial quantity and properties. Other works propose dimensioning techniques for codfish heat pump dryers, including the parameters for both heat pump and air drying circuits [15]. However, the air thermodynamic diagram represents a mixing air process but isn‘t represented on the drier air flow diagram. Also, it is not specified the refrigerant for which the dehumidification capacity and MSMER and COP parameters have been achieved.
4.4.2. System integration The selection of the drying mode depends on the product drying characteristics and the required loading capacity. Heat pump driers can operate in batch and continuous modes where heat pumps are compact units installed outside the drying chamber. In the batch mode, the product is placed on a tray that is positioned in the drying chamber and removed once the desired product moisture content is reached (Figure 4.4a). The drying air can flow parallel or perpendicular to the product surface. Batch drying is generally suitable for smaller production rates but provides higher labour costs. In the continuous mode, the product is placed on a tray positioned on a conveyor belt system of which speed can be varied (Figure 4.3b). Continuous systems involve faster loading and unloading of drying products and are less labour-intensive. In the case of multiple drying systems involving two drying chambers with different conditions of temperature and humidity, heat pumps with two evaporators can be used (Figure 4.4). Such multiple (two or more) evaporator refrigeration systems are commonly used in commercial and industrial refrigeration applications. Two air streams with different drying conditions, i.e. high and low temperatures and humidity come from two independent drying chambers. The overall energy efficiency of the drying process can be thus improved. The subcooled refrigerant liquid is split into two streams at the exit of the condenser. One stream enters the expansion valve EX1 of the high-temperature evaporator EV1 and the second, the expansion valve EX2 of the low-temperature evaporator EV2. Liquid refrigerant flows through the thermostatic expansion valves to the low and high temperature evaporators. Because there are two evaporating temperatures, a device must be used to keep one of the evaporators at a higher low-side pressure. A two-temperature valve in the suction line keeps the low-side pressure of the refrigerant in evaporator EV2 at a higher pressure than in the evaporator EV1. The evaporator temperature is governed by the evaporating pressure. A check valve is located in the suction line coming from the colder evaporator EV1. It prevents the warmer, higher pressure low-side vapour from entering the colder evaporator EV1 during the off cycle. The vaporized refrigerant is returned to the compressor where it is compressed and becomes high-pressure and high-temperature superheated vapour. To further enhance the performance of conventional heat pump driers, several other technologies can be incorporated to enhance the drying rates while reducing the thermal load of the heat pump itself. Infrared-assisted heat pump dryers could be used for fast removal of surface moisture during the initial stages of drying, followed by intermittent drying over the rest of the drying process. Heat for drying is generated by radiation from infrared generators (Figure 4.5a). The infrared driers present advantages as high heat transfer rates (up to 100kW/m2), easy to direct the heat source to the surface, quick response times allowing easy and rapid process control.
Industrial Drying Heat Pumps
27
Incorporating infrared into an existing heat pump dryer is simple and capital cost is low. This mode of operation ensures a faster initial drying rate and offers advantages of compactness, simplicity, ease to control and low equipment cost. The are possibilities of significant energy savings and enhanced product quality due to reduced residence time in the dryer chamber. LV
HR
EX
CD
EV
HR
LV
M
D1
EX
CD
M
EV
D1
D2
C
D2
F
C
Condensate Air vent
Hot & humid air
Fresh air
F
Condensate Inlet wet product
Dryer fan
Product to be dryed
Hot & humid air
Dry air Drying air Conveyor belt
Product outlet
Product inlet
Outlet dry product
Product tray
(b)
(a)
C: compressor; CD: condenser; HR: heat rejection coil; EX: expansion valve; EV: evaporator; D: air damper; M: damper motor; F: fan Figure 4.3. Batch (a) and continuous (b) driers with compact drying heat pumps.
HR2
LV2
EX2 M
LV1
HR1
EV2 Condensate Two-temp valve EX1 M
CD C
CV SA
EV1
Air vent
F1 Dryer fan
Hot & humid air
Product inlet Low-temperature drying chamber #1 Fresh air F2
Dry air
Dryer fan
Product outlet High-temperature drying chamber #2
Figure 4.4. Drying heat pump with two evaporating levels (for legend, see Figure 4.3).
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Materials that are difficult to dry with convection heating alone (as ceramics and glass fibre) because of poor heat transfer characteristics can be candidates for radio frequencyassisted heat pump dryers. A radio frequency-assisted heat pump dryer comprises a vapour compression heat pump retrofitted with a radio frequency generating system capable of imparting radio frequency energy to the drying material at various stages of drying processes (Figure 4.5b). This arrangement can overcome the limitation of heat transfer of conventional hot air drying systems, particularly during the falling period. The radio-frequency generator generates heat volumetrically within the wet material by the combined mechanism of dipole rotation and conduction effects. Radio frequency heats all parts of the product mass simultaneously and evaporates the water in situ at relatively low temperatures usually not exceeding 82°C. Since the water moves through the product in the form of a gas rather than by capillary action, migration of solid is avoided. Warping, surface discoloration and cracking (caused by the stress of uneven shrinkage) associated with conventional drying methods are also avoided. Radio-frequency drying is a rapid drying process whereby the heat necessary to dry a product is generated within the product itself. Product containing moisture is subjected to an alternating electric field which causes the dipole water molecules to rotate in response to the changing polarity of the field. This rotation causes molecular friction and at radiofrequency drying frequencies (1 to 30 MHz) the frictional heat is sufficient to produce temperatures exceeding the boiling point of water. In the drying chambers the alternating field is created between two large metal-plate electrodes which are connected to a high frequency generator. The material is dried by placing or passing it between these electrodes. The advantage of using radio-frequency for small product items is that they can be dried to uniform moisture content in a matter of minutes without developing defects, especially in the case of wood. M EV
C
EV
CD
EX M C
LV
HR
EX
HR LV
Blower
Radio-frequency generator
Infrared generators
CD Infrared drier
(a)
Blower
Dried material
(b)
Figure 4.5. Infrared (a) and radio-frequency (b) assisted drying heat pumps (for legend, see Figure 4.3).
Compared to drying at atmospheric pressure, vacuum drying reduces the vapour pressure and allows moisture to evaporate faster at the same temperature (or at the same speed at a lower temperature). This process accelerates the drying rate and produce good quality product. However, the capacity of driers is very low in relation to the cost of equipment.
Industrial Drying Heat Pumps
29
Consequently, the volume of product processed over a given period of time is substantially less in comparison to that obtained from conventional driers of equal cost.
4.4.3. Refrigerant side Significant improvements are available by modifying the cycle and adding new components. First of all, efficiency improvement is possible by minimizing the refrigerant pressure drop in pipes and within heat exchangers. Pipe diameters have to be selected so the pressure drop is small, even if larger pipes contribute to an increased first cost. In the compressor suction pipe, the vapour velocity must be sufficient high to ensure proper oil return to the compressor. The main features of optimized drying heat pumps are shown in Figure 4.1. On the refrigerant side, the liquid valve LV allows controlling the refrigerant migration during the on/off cycling and standby periods. On the other hand, the suction vapour always has lower temperature than the condensed refrigerant is. It can be used to sub-cool the liquid condensate and, at the same time, superheat the suction vapour and increase the compressor work input. The internal sub-cooler/superheater SC improves the overall efficiency of the heat pump thermodynamic cycle by increasing the evaporator capacity and effectiveness. The heat pump output thus increases without any increase in the compressor work. Typical improvements of the COP and the capacity are approximately 1% per degree Kelvin of sub-cooling. The amount of sub-cooling of the liquid refrigerant at the condenser outlet is generally of about 5°C. If the suction gas entering the sub-cooler (state 1a) (see Figure 4.1) is dry, the heat capacity of the suction gas is less than the heat capacity of the liquid. For this case, the sub-cooler heat transfer effectiveness is defined as:
SC
T1 T1a T4 T1a
(4.7)
If the suction gas entering the sub-cooler is saturated (wet), the heat capacity of the liquid is less than the heat capacity of the suction gas. For this case, the sub-cooler heat transfer effectiveness is defined by:
SC
T4 T5 T4 T1a
(4.8)
The conservation of energy for the sub-cooler is represented by:
h1 h1a h4 h5
(4.9)
The sub-cooler heat transfer rate (kW) is determined by the energy conservation equation:
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Q SC m R (h1 h1a )
(4.10)
4.4.4. Air side On the drying air side, a bypass circuit around the evaporator is recommended to vary the air flow through the evaporator (see Figure 4.1). Firstly, in order to control the dehumidification rate during the whole drying process. Secondly, in order to mix the air leaving the evaporator at 80%-90% of relative humidity with the air leaving the dryer. This supplies a sufficient air flow rate through the condenser to avoid excessive compressor discharge pressures and temperatures. At the same time, the mixing process allows the condenser to reach the desired air supply temperature without over-designing the heat transfer
surface. The correlation between the evaporator air flow rate ( m EV ) and the condenser air
flow rate ( m CD ) is:
m EV m CD
(4.11)
where the bypass factor ( ) may be less than or equal to 1. Calculations are based on the dry air mass flow rate through the system. The volumetric flow rate (m3/s) of the duct air blower at other conditions than nominal (i.e. at ambient temperature) varies according the following fan low:
V air V n (vm / vamb ) 0.5
v
(4.12)
v
Where m and amb are the specific volumes of the air leaving the blower and the ambient air, respectively. Consequently, the air mass flow rate (kg/s) will be:
m air air V air
(4.13)
The electrical power input of the air blower is converted to heat:
Q blower Wblower
hd hm
Q blower
m air
(3.12)
The continuity equations of water vapour and energy of the by-pass mixing process respectively are represented by:
Industrial Drying Heat Pumps
m b (1 )i hm hb (1 )hi
31
(4.14)
Where is the air absolute humidity (kgwater/kgdry air). The heat pump evaporator EV may also be provided with a variable speed fan to improve the control of the material drying rate. The combined action of the bypassing air and the fan variable speed provides optimum air flow rate through the evaporator, independently to the air flow rate through the condenser.
5. WOOD DRYING HEAT PUMPS 5.1. Wood drying As for other materials, wood drying is a coupled heat and mass transfer process consisting in moisture movement from the internal zones to the wood surface. Drying process is an essential step in the manufacturing of most wood products. Its objective is to minimize the development of defects to achieve a low and uniform moisture content before machining, gluing and finishing are carried out [16]. The most widely used method for removing moisture from wood is drying under controlled conditions of temperature and humidity. Trees are classified under two categories: deciduous trees, known as hardwoods and coniferous trees known as softwoods. Wood cells are composed of thin walls of wood substance surrounding cavities. More than 90% of the volume of a piece of softwood is composed of vertically-aligned fibre cells. They are arranged in regular rows and perform the dual function of conducting liquid and providing mechanical support for the tree. Softwoods have both vertical and horizontal enlarged cavities surrounded by cells serving for the movement of moisture. The structure of hardwoods has large-diameter cells forming long vertical tubes through which liquids can pass.
5.1.1. Moisture movement in wood Moisture occurs in wood as free water (liquid and vapour) in the cell cavities, and as hygroscopic or bound water contained in the cell wall saturated structure. Drying air has a strong affinity for moisture and eventually moisture begins to leave the walls of the surface cells. But, since cell walls also have an affinity for moisture, they will attract replacement moisture from areas of lower affinity, i.e. the interior zones. Moisture is therefore drawn from the interior of wood. As it reaches the surface, it is again absorbed by the dry air and the drying process continues. Moisture can move through wood within the cell cavities and through the pit chambers from cell to cell, through the ray cells, through intercellular spaces and through transitory cell wall passageways. The space available for moisture movement ranges from 25% to about 85% of the total volume of wood depending on its density. To maintain a constant drying rate, the water molecules inside the wood must absorb additional heat in order to increase their kinetic energy. The forces that move moisture through wood include capillary action (adhesion and cohesion), vapour pressure and moisture content differences, and diffusion.
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Capillary action causes the free water to flow through the cavities, spaces and pits from one cell to another. It is due to the simultaneous operation of adhesion and cohesion. Adhesion is the attraction between water molecules and the walls of cells and pit chambers, and the cohesion is the attraction of the water molecules to each other. When green wood commences to dry, free water evaporation will occur from the surface cells. Evaporation and the cohesion of water molecules exert a pull on the water in the adjoining cell cavities. As drying continues, and the free water in the cell cavities is progressively removed, other drying forces become operative. Capillary action moves into the wood core and gradually disappears as the moisture content of the core cells approaches the fibre saturation point. When most of the capillary action ceases, the majority of the cell cavities will contain only air and water vapour, and this establishes a vapour pressure. The higher the vapour content, the higher the pressure. A pressure differential between cells occurs because the amount of vapour in a given volume of air decreases as the surface of the boars is approached. This vapour pressure gradient causes moisture in the vapour state to move from areas of high vapour pressure (interior) to areas of lower vapour pressure (surface of the board). As moisture begins to leave the cell walls near the surface of a board, a moisture content gradient develops between the surface and interior cells. However, since wood has an affinity for moisture, the drier surface cell walls will absorb moisture from cell walls of higher moisture content, i.e. moisture moves from the wetter interior cells to the drier surface cells. A combination of vapour pressure gradients and cell wall moisture gradients operates simultaneously in the diffusion process. A water molecule moves through a cell wall by moisture gradient, across the cell cavity and through openings by vapour pressure gradient, and again through a drier cell wall by moisture gradient until it finally reaches the wood surface. Internal diffusion of moisture takes place longitudinally and laterally, and it is the major factor controlling the drying rate of wood. The main consequence of wood drying is the change in dimensions that occurs with a change in moisture content. Although wood is capable of holding large quantities of water above fibre saturation, its hygroscopic nature becomes evident below the fibre saturation point. The fibre saturation point (FSP) is the stage in the drying process where the cell walls are saturated with water and the cell cavities are free of liquid water. For practical purposes it is assumed that the fibre saturation point at which shrinkage begins is 25% moisture content. When wood is at or above FSP, the cell wall is in a non-shrunken condition. Below FSP, as water is removed from the cell wall by diffusion, the cell shrinks. As an approximation, volumetric change in wood below FSP is directly proportional to the volume of water gained or lost from the cell wall. This relationship is expressed by the fallowing equation:
Rv Db ( MC) where: Rv is the total possible volumetric shrinkage (green volume basis), %; Db - basic specific density of wood (oven dry weight, green volume basis), kg/m3; Ψ – fibre saturation point, %; MC – moisture content (dry basis) below FSP, %
(2.9)
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In the living trees, the cell wall is moist. When water is removed from the cell wall, it may shrink resulting in reduction in cell diameter. Only the moisture contained within the cell wall structure has an effect on shrinkage. Consequently, all water in the cell cavity must be removed before any shrinkage of the cell wall can occur. As wood dries, shrinkage continues until the equilibrium moisture content is reached. In calculating shrinkage allowances, it is essential to know the wood density and the final moisture content to which the wood will be dried.
5.1.2. Evaporation rate Heat is the source from which the water molecules in wood acquire the kinetic energy necessary for evaporation. The rate of evaporation is dependent upon both the amount of energy supplied per unit time and the ability of the heating medium (air) to absorb moisture. The moisture movement rate is governed by the capacity of the surrounding air to absorb moisture from the surface of the wood. It largely depends on the temperature and relative dryness of the air and wood. Water moisture evaporates from the wood surface because of the difference in partial vapour pressures between the drying air and the surface moisture. The evaporation rate depends on the amount of energy supplied, the mass transfer coefficient and the air temperature, flow rate and capacity to absorb humidity. Under stable conditions, the convective evaporation rate (or drying velocity) ( anh N M wood
kg water /h ) may be expressed as follows: d ( MC ) d
(2.10)
M anh
wood is the wood anhydrous mass (kg) and MC - the moisture content (%). where When the humid wood boards are dried under stable conditions, the moisture content decreases at first linearly. In the case of pure convective drying, the wood surface is always saturated with water, and consequently the boundary layer reaches the air wet temperature. This first step is followed by a non-linear process where the moisture content decreases until the wood board attains hygroscopic equilibrium. If the drying velocity is known as a linear or non-linear equation (N = f (MC)), the total time required to reduce the moisture
content from
dry MCindry to MC fin (dry basis) may be calculated with the following equation:
tot A
anh M wood d ( MC ) dry MCin Aev N MC dry fin
(2.10)
where ev is the evaporation surface, m2. If the temperature of the drying air is constant, the rate of evaporation will gradually decrease to maintain a steady drying rate, the water molecules in the wood must acquire additional energy, or the vapour pressure of the drier atmosphere must be reduced. This is achieved by either increasing the temperature (more energy) or reducing the relative humidity (lower vapour pressure).
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When the vapour pressure of the moisture in the wood is equal to the vapour pressure of the surrounding air and neither gain nor loses moisture at a given temperature and relative humidity, the equilibrium state is achieved. Air velocity at the wood surface must be high enough to produce rapid air change, avoid the formation of death zones and provide uniform drying. Circulation of air is required to carry heat to the wood being dried, and to carry away evaporated moisture. For effective drying, this circulating air must be continually conditioned for temperature and humidity before it passes through the wood stack. The air velocity must be high enough to provide a rapid exchange of air. Higher air velocities also minimize dead zones and promote more uniform drying throughout the wood stack. It is essential to remove moisture from the boundary layer as rapidly as possible to maintain the desired drying rate. This is achieved by controlling the circulation rate of the main air stream, usually through the use of multiple-speed fan motors. Initially, when the wood is wet, the temperature differential between the air stream and wood surface will be equal to the wet-bulb depression. Large quantities of heat are required to vaporize the free water brought to the wood surface, and the rate of heat transfer is at a maximum. Higher circulation rates are beneficial during the early stages of drying when wood is wet and the requirements for surface evaporation and moisture removal is high. Later, as the fibre saturation point is reached at progressively deeper levels in each piece of lumber, the wood temperature approaches the air stream temperature, and the rate of heat transfer diminishes. As drying progresses below the fibre saturation point, the rate of heat transfer is further reduced and, since this affects the drying rate, the dry-bulb temperature must be increased to maintain an acceptable drying rate. At this stage, high air velocity has little effect on drying rate and its continuation serves no useful purpose. As the fibre saturation point approached, diffusion to the surface becomes the limiting factor in rate of moisture removal, and air velocity should be reduced accordingly. Any conventional wood drier (continuous, batch, fluidized bed, rotary, etc.) that uses convection as the primary mode of heat input can be fitted with a suitably designed heat pump. Optimum wood drier and heat pump integration, and appropriate thermodynamic design and control strategies are needed to provide safe operation of the system and good quality of wood.
5.2. Wood Driers 5.2.1. Generalities Heat pump dryers can provide efficient and cost-effective drying of low-and high grade wood. The advantages of wood heat pump driers include efficient utilization of recovered heat, controlled drying rates resulting in low energy cost, equivalent or reduced drying time, enhanced productivity, and improved wood quality. When high quantity of water has to be removed from large volumes of wood, the efficiency of drying increases and shorter payback periods can be obtained. The economics of the installation of wood drying heat pumps depend on how the heat pump is integrated in the process. Identification of feasible installation alternatives (i.e. split or compact) is of crucial importance.
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5.2.2. Types of wood driers Wood driers are particular types of industrial equipments. Their selection depends on several factors such as capital investment, energy sources, production capacity and drying efficiency. If wood is not dried accordingly to proper procedure and methods, many losses may result. Wood drier can be divided in conventional and specialized using solar energy, infrared, high frequency, vacuum and heat pumps as dehumidifiers. The specialized techniques are usually more expensive and oriented to particular high-value end products. Air-drying reduces the moisture content of construction wood to a minimum of about 15%. Wood to be air-dried is flat-piled in open yards with stickers between each layer of board to allow free circulation of air. Drying rate is uncontrolled and, depending on the ambient temperature, relative humidity and wind, drying times varies throughout the drying season. The capital cost is low and the advantages and disadvantages depend on location, annual production, drying season, wood species and dimensions, and cost of inventory. In cold climates, outdoor driers have en effective drying season of about 8 months. Accelerated air-drying is similar to that of normal air-drying with the exception that air is forced through the wood piles by large multi-blade fans. Forced-air helps reducing the drying time, but one must consider the added cost of sheds, fans, motors and electrical power. To minimize power consumption, controllers of relative humidity are installed to stop the fans when higher air velocities are undesirable or ineffective. The efficiency of accelerated airdrying is greatest with high moisture contents and thin wood boards. Low-temperature driers operate at temperatures of 20 to 45°C and are designed to accommodate large quantities of wood (236 to 700 m3). Interior fans produce air velocities of 2.5 to 3.6 m/s. The main objective is to produce large quantities of air-dried wood in a comparatively short time as accelerated air-drying method, but with minimal capital expenditure in building materials and equipment. Low-temperature driers are most advantageous as pre-driers, particularly for slow-drying hardwoods which are pre-dried to 25 or 30% of moisture content prior to conventional kiln drying. Conventional wood drying is a fully controlled drying procedure operating at temperatures below 100°C. Heat losses and inefficiencies in heat transfer usually limit temperatures to between 82°C and 93°C. Steam is the most common of heat for conventional driers, but oil and electrical heating are also used. Humidity is controlled through the operation of steam or water spray valves and drier vents. Spray valves, vents and heating systems are regulated by means of controllers to maintain selected wet and dry-bulb temperatures according to a certain drying schedule. Fan systems are designed to give reasonably uniform air circulation at velocities from 1.5 to 3.0 m/s with or without variable speed motors. Air circulation is periodically reversed to obtain uniform heating and drying. The main advantage of conventional driers over pre-driers is the reduction in drying time obtained with higher temperatures. High-temperature wood drying uses temperatures above 100°C and is an efficient method for drying softwood construction lumber. Driers and equipment are similar to conventional driers, but are designed for temperatures approaching 149°C. In addition to increased heating capacity, there are additional requirements for drier insulation and maintenance for highpressure steam boilers. The capital investment per unit capacity is higher, but drying times and energy consumption are reduced by up to 25% and 50% respectively compared to conventional driers. The reduced energy is primarily the result of the large reduction in drying time, to better heat transfer to the wood and improved insulation of the drying enclosure.
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Figure 5.1 shows a compact drying heat pump where all heat pump components (compressor, evaporator, condenser, expansion valve, refrigerant piping, fans and controls) are installed outside the drying chamber in a closed cabinet. The hot and humid air from the drying enclosure enters the evaporator EV and the condenser CD. A mixing process using motorized air dampers and constant or variable speed heat pump blowers is achieved prior reheating the air through the condenser CD. The back-up heating coil supplements the heat that the drying atmosphere receives from the condenser. When the dryer dry-bulb temperature drops under its setting point, it raises the air temperature before it is returned to the wood stacks being dried. The back-up heating source may be provided by natural gas, bark or oil-fired boilers. A multi-blade dryer fan circulates the air through the wood stack. Its rotation direction periodically changes every 3 hours at the beginning, and every 2 hours at the end of the dehumidification process. The air vents open when the dryer fan changes rotation direction in order to avoid air implosion hazards, and also when the dryer dry-bulb temperature exceeds its setting point, to avoid excessive superheating. Split drying heat pumps also consist in two chambers thermally isolated from one another: the drying enclosure with remote condenser CD and the mechanical room where all temperature-sensitive components (compressors, evaporators, variable speed blowers, electronic expansion valves with microprocessor-based process controllers) and air ducts are thermally separated from the drying area (Figure 5.2). In this case, the air flow trough the evaporator EV can be controlled only by using variable speed blowers. The air flow rate through the condenser CD is provided by the drier fans. It is normally constant and designed according to the condenser thermal capacity.
Air vent Dryer fan
Back-up heating coil
Mechanical room Air outlet
SV
EV
CV
C
Wood stack Heat pump
SC
Blower
SA LR
CD
Back-up heat source
Air inlet
LV EX
Figure 5.1. Compact wood drying heat pump.
Drying enclosure
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Air vent
Dryer fan
Mechanical room
CD
Back-up heating coil
Blower
LR
Air supply
C
SV
Wood stack LV EV
Air inlet
Back-up heat source
SC SA EX
Drying enclosure
Figure 5.2. Split wood drying heat pump.
5.2.3. Ecological aspects Condensates produced by wood drying heat pumps do not contain suspended particles, but contain toxic quantities of volatile organic compounds (VOC) including formaldehyde and acetaldehyde that are harmful to trout and daphnia [17]. The volumes of water extracted from wood by drying site are generally low (
