USE OF AN OSCILLATING ELECTRIC FIELD TO

Experimental Heat Transfer, 22:257–270, 2009 Copyright © Taylor & Francis Group, LLC ISSN: 0891-6152 print/1521-0480 online DOI: 10.1080/08916150903098936

USE OF AN OSCILLATING ELECTRIC FIELD TO MITIGATE MINERAL FOULING IN A HEAT EXCHANGER L. D. Tijing,1 H. Y. Kim,2 D. H. Lee,1 C. S. Kim,1 and Y. I. Cho3 1 Division

of Mechanical and Aerospace System Engineering, Chonbuk National University, Jeonbuk, Korea 2 Department of Precision Mechanical Engineering, Chonbuk National University, Jeonbuk, Korea 3 Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, Pennsylvania, USA

This article presents an investigative study on the efficacy of a new physical water treatment technology using an oscillating electric field to mitigate mineral fouling in heat exchangers. Parallel graphite electrode plates immersed in water were used to generate the electric field directly in water. Artificial hard water at 500 ppm hardness was used in all fouling tests. The inlet temperatures were maintained at 23.5 ˙ 0.5ı C and 85 ˙ 0.5ı C for cold- and hot-water sides, respectively. The results at a cold-water side velocity of 0.3 m/s showed a 16–60% drop in fouling resistances from the baseline test, depending on the frequency of the electric field for the physical water treatment-treated cases. Keywords resistance

mitigation of fouling, physical water treatment, oscillating electric field, fouling

INTRODUCTION Mineral fouling is defined as the deposition of inverse solubility salt crystals found in water on heat transfer surfaces as temperature is increased. The formed fouling layer decreases the thermal efficiency of the heat exchanger and produces a higher pressure drop, which, in turn, increases the operating cost [1–15]. Fouling entails billions of dollars annually for cleaning and maintaining the equipment in world industries [2–4]. Studies show that a 1-mm limestone deposit could double the energy consumption in a heat power plant [3]. Fouling is also considered to be one of the major factors in the design and operation of the heat transfer equipment [4]. Nesta and Bennet [4] stated that proper design strategy could well mitigate the fouling problem by taking into account a maximized shear stress and controlled wall temperature. Various methods are available to clean clogged pipes and heat exchangers. The use of chemicals are commonly practiced in industries, but their high cost and the pollution concern require less costly and environmentally friendly cleaning or mitigating means [7, 16]. The use of non chemical or physical cleaning devices is a good alternative that Received 15 June 2008; accepted 5 June 2009. Address correspondence to Dong Hwan Lee, Division of Mechanical and Aerospace System Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Korea. E-mail: [email protected] 257

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NOMENCLATURE Ao cp f m P Q Rf Ti To U

outside surface area of the copper tube (m2 ) specific heat of water (J/kg K) frequency (Hz) mass flow rate of water (kg/s) heat transfer rate (W) fouling resistance (m2 K/W) inlet temperature of water (ı C) outlet temperature of water (ı C) overall heat transfer coefficient (W/m2 K)

!TLMTD

Subscripts c clean fouled h

log-mean-temperature difference (ı C)

cold side clean state using distilled water fouled state using hard water hot side

is safe and effective [7, 17, 18], and one example is physical water treatment (PWT), which uses electric or magnetic fields, catalytic surfaces, ultrasound, or sudden pressure changes. Several studies [2, 6, 8–10, 19, 20] showed the efficacy of permanent magnets and solenoid coils in mitigating fouling in heat transfer equipment. Catalytic alloys and heat-treated titanium balls [14, 15] also decreased the fouling resistances in counterflow heat exchangers. The PWT-treated cases also produced a calcite form of scales that are known to be soft and easily removed through flow shear forces. In the present study, we used two graphite electrode plates oriented in parallel to each other and connected to a power supply where we controlled both the output voltage and frequency over a wide range. In cooling tower systems, calcium carbonate (CaCO3 ) is one of the most common mineral fouling crystals [2, 6]. Hence, we used hard water of 500 ppm of CaCO3 hardness, which was artificially made from the mixture of calcium chloride (CaCl2 ) and sodium bicarbonate (NaHCO3 ) in the right proportions. The objectives of the present study were as follows: (1) to investigate whether the present PWT using oscillating electric fields could mitigate mineral fouling (i.e., CaCO3 fouling) in a double-pipe counter-flow heat exchanger and (2) to determine the most effective frequency of the oscillating electric field at 10 V that produces maximum performance in mineral fouling mitigation. Present PWT The present PWT device utilized oscillating electric fields that were produced directly in water using two graphite electrode plates submerged in water. The power supply connected to the graphite electrode plates provided a controlled voltage and frequency for the PWT device. Since the new PWT produces oscillating electric fields directly to water, the strength and frequency of the electric field in water were substantially higher than the previous indirect methods such as permanent magnetics, solenoid coils, and electrostatic device. These devices produce induced electric fields indirectly, where the production of the electric field is governed by basic physics law such as Lorentz’s law or Faraday’s law. Due to the induction process, the strength and magnitude of the induced electric field is in the order of 1 mV at 3,000 Hz or less. The higher frequency beyond 3,000 Hz is desirable for more efficient operation of the PWT, but

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the aforementioned basic physics laws prevent using a higher frequency in the previous PWT devices. It is hypothesized that the oscillating electric fields in water precipitate the dissolved mineral ions, such as calcium and magnesium, to mineral salts in bulk water. As the treated water enters the heat exchanger, the temperature of water increases, the solubility of the calcium ions decreases, and calcium ions precipitate and adhere on the solid surface of heat exchanger. At this moment, the suspended particles produced by the PWT compete with the heat transfer surface, such that the dissolved mineral ions adhere to the surfaces of the suspended particles rather than to that of the heat exchanger. As the dissolved ions continue to precipitate and adhere on the suspended particles, the particles grow in size and eventually form a soft sludge on the heat transfer surface, which is often known as particulate fouling [2, 8–10, 14, 15]. It is known that particulate fouling is a soft deposit that is more easily removed than the deposit produced from the chemical precipitation of minerals directly on the heat transfer surface. The present PWT method utilized a low voltage, not only to provide safety in equipment and to personnel, but also to minimize the electric power consumption for energy efficiency.

EXPERIMENTAL FACILITY The present experimental system (see Figure 1) was composed of a hard-water reservoir (200-L capacity), a pump, two flow meters, two static mixers, a PWT device using two graphite electrode plates, a heat transfer test section, a chiller, a hot-water bath circulator, four thermocouples, LabView data acquisition system (DAS; SCXI-1000, National Instruments, Austin, Texas, USA), and a personal computer. The main fouling system loop connections were made up of both copper tubes and expandable hoses with

Figure 1. Schematic diagram of the present experimental set-up: (1) artificial hard-water reservoir, (2) water pump, (3) rotary flow meter, (4) PWT device, (5) heat transfer test section, (6) hot-water circulator bath, (7) chiller, (8) power source for PWT device, (9) LabView DAS, (10) PC, (11) bypass line, (12) water sampling port, (13. drain.

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metal clips. A bypass flow was maintained throughout the tests to help keep the water temperature at the reservoir constant. Copper tube coils inside the hard-water reservoir were connected to the chiller serving as the cooling heat exchange system. Two drain valves were positioned at the bottom of the reservoir tank for the purpose of both draining and sampling the hard water. Heat Transfer Test Section Figure 2 shows the heat transfer test section, which was composed of copper tube, quartz crystal, Teflon heads, and fittings that were arranged in a double-tube counter-flow configuration. The inner copper tube had dimensions of 1.385 cm ! 1.6 cm ! 49.9 cm (di !do !L) and the quartz crystal had dimensions of 2.48 cm ! 2.81 cm ! 49.9 cm (di ! do ! L). The cross-sectional areas for the copper tube and the annulus formed were 1:51 ! 10!4 m2 and 2:82 ! 10!4 m2 , respectively. Both tubes were fitted in Teflon heads with connection fittings. The Teflon heads minimized the axial heat transfer losses in the test section. O-rings were provided inside the Teflon heads to prevent leakages. Hot water flowed through the inner copper tube, while cold water (i.e., hard water) flowed in the opposite direction through the annulus gap between the two tubes. The hot-water temperature was constantly maintained by a hot-water circulator bath. Static mixers were set in place at the outlet points of the hot- and cold-water sides. Four copper-constantan type T thermocouples (Sentech, Korea) were used to measure the inlet and outlet temperatures of both hot- and cold-water sides in the heat transfer test section. These thermocouples were calibrated using a thermocouple bath calibration unit (Venus Calibrator Series 965, Isothermal Technology Ltd., Southport, UK), a precision thermometer (Model F250, Automatic Systems Laboratories, Milton Keynes, UK), and the Measurement and Automation Explorer (MAX Version 4.0, National Instruments, Austin, Texas, USA). The precision thermometer served as the reference temperature with a precision of ˙ 0.01ı C. Styrofoam was covered to the heat transfer test section to reduce any heat transfer losses to the surroundings. PWT Device As shown in Figure 3, two rectangular graphite plate electrodes were placed in parallel to each other inside a stainless-steel casing. The graphite plate electrodes had identical dimensions of 7.65 mm ! 35 mm ! 155 mm (t ! h ! L). The distance D

Figure 2. Sketch of the present heat transfer test section.


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Figure 3. Schematic of the present PWT graphite plate electrode device.

between the two plate electrodes was fixed at 20 mm. In order to ensure that the plate electrodes did not touch any metal parts in the case, a cylindrical Teflon block with a square cross-section, which was tightly fit between the casing and the electrodes, was used. The present device was also specially designed so that the plate electrodes could be screwed at opposite sides from the outside of the device through screw ports, which served as the connections for the power supply, thus eliminating any direct contact of water to the electric wire for power supply. The present PWT device used a stainless-steel cylinder casing where diverging and converging sections were used at the inlet and outlet of the casing for smooth transitions of flow, respectively. The detachable heads were used in the main cylinder casing with screw-type locks with rubber seals for service. For a volume flow rate of 8:33 ! 10!5 m3 /s (300 L/h), the equivalent water velocity in the graphite electrode device was 0.12 m/s. The power supply was composed of a programmable function generator and a voltage source (100 mA, ˙ 15 V). The function generator could generate square wave signals in a frequency range of 1 to 50 MHz. The power supply was connected to the graphite electrode through alligator clips attached on the screws. The present study used a fixed voltage of 10 V and at varying frequency levels. EXPERIMENTAL PROCEDURE Pre-Fouling Tests Hot water was first passed through the inner tube of the test section for about 10–20 min to check for any leakage in the inner tube. No water was flowing at the annulus at this stage. After making sure that there was no leakage in the inner tube, the cold water was then passed through the annulus of the test section, and, likewise, was checked for any leakages. When no leakage was found, both the hot- and cold-water sides were kept flowing in opposite directions in the heat transfer test section for at least 1 h to stabilize them at the desired temperatures and flow rates. Distilled water was used. The stabilized system at the desired parameters was continuously run to test for heat exchanger efficiency. The heat transfer test section was found to have an error of ˙ 8%, which was good enough to carry on fouling tests.

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L. D. TIJING ET AL. Table 1. Water analysis of distilled water Electrical conductivity ("S/cm) pH Total hardness (ppm) Total alkalinity (ppm)

7.7 6.2 1.2 0.6

Preparation of Artificial Hard Water One of the most common forms of scale is calcium carbonate, which possesses inverse solubility characteristics (i.e., its solubility decreases when temperature increases). In the present study, we wanted to have calcium carbonate fouling, so hard water was artificially prepared for more controlled water chemistry by mixing chemical reagents of calcium chloride (CaCl2 ) and sodium bicarbonate (NaHCO3 ) in distilled water. Table 1 presents the water chemistry for the distilled water used. The mixture of both chemicals resulted in the following chemical reaction: CaCl2 C 2NaHCO3 ! CaCO3 C 2NaCl C H2 O C CO2 .

(1)

To get 500 ppm of CaCO3 water hardness, the following chemical reagents were added to a tank reservoir with 0.175 m3 of distilled water: Calcium chloride (CaCl2 : Mw D 110.98 g/mol) " 97:02 g; Sodium bicarbonate (NaHCO3 : Mw D 84.01 g/mol) " 146:89 g: The CaCl2 was first added to the distilled water and was mixed through stirring with a stick and recirculating in the reservoir for around 20–30 min using a by-pass line of the system loop. Then, NaHCO3 was added and was also stirred and recirculated making sure that almost all chemical powders were dissolved in water. After 30 min of recirculating the mixture water solution, the test was started. At this time, hard water from the circulating loop was sampled, and the water chemistry was analyzed using Hanna test kits, a portable electrical conductivity meter, and a portable pH meter. Water Analyses The water hardness and alkalinity tests were conducted using Hanna instruments test kits, HI 3812 and HI 3811 (Hanna Instruments, Hungary), respectively. The pH readings was measured using a portable pH meter (HI 8424 microcomputer pH meter, Hanna Instruments, Portugal), and the electrical conductivity was measured with a portable conductivity meter (WTW Cond 340i, Germany). The water-hardness level was determined with the use of ethylene-diamene-tetraacetic acid (EDTA) titration, and hydrochloric acid titration was used to measure the alkalinity of water. Water analyses were conducted on the hard water within an hour of water sampling. It is noted that there were no additional chemicals (CaCl2 or NaHCO3 ) added in the system during the whole duration of the fouling tests. In other words, the hardness level was allowed to gradually decrease with time due to fouling deposit in the present study.


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Fouling Tests The inlet temperatures of the hot- and cold-water sides of the heat transfer test section were maintained throughout the tests at 85˙0:5ıC and 23:5˙0:5ıC, respectively. The inlet–outlet temperature difference in the hot-water side was maintained at 2ı C– 4ı C to provide uniform fouling conditions along the test section. The cold-water flow rate was maintained at 8:5 ! 10!5 m3 /s, which was equivalent to a flow velocity of 0.3 m/s at the heat transfer test section (cold-water side). The hot-water side had a constant flow velocity of 1.1 m/s (1:67 ! 10!4 m3 /s). The mean temperatures for both hot- and cold-water sides served as the reference temperatures for specific heat and density values. The mass flow rates for the hot- and cold-water sides were maintained constant at 0.162 kg/s and 0.084 kg/s, respectively. The temperature readings were measured every 5 min with a real-time DAS using the LabView program (National Instruments). Both the heat transfer test section and the PWT device were electrically grounded. The duration for each fouling test was about 30–36 h. After each fouling test, the artificial hard water was drained out of the system loop. Since the hard water was drained right after the test, the remaining scales in the hard-water reservoir as well as in the system loop could be relatively easily removed. We cleaned the reservoir tank with a brush and flushed it with a high-pressure water jet. Then, tap water was recirculated through the loop continuously for 2–5 h. At this point, the system was considered to be ready for the next test, and new distilled water of 0.175 m3 was poured into the reservoir. A new, smooth, and clean copper tube was installed in the heat transfer test section for each fouling test. Prior to this cleaning process, the heat transfer test section was removed from the system loop, and the scaled pipe was dried off for inspection and scanning electron microscopy (SEM) photography. The heat transfer rate from the hot-water side was estimated using the following equation [2, 8–10, 14, 15]: Qh D m P h cph !Th :

(2)

The heat transfer rate at the cold-water side can be described in a similar manner: Qc D m P c cpc !Tc :

(3)

The discrepancy in heat transfer rates between the hot- and cold-water sides was found to be ˙ 8%. The heat transfer rate in the cold-water side Qc was used to calculate the overall heat transfer coefficient, considering that heat losses in the hot-water side might have incurred to the surroundings although the hot-water side was insulated [2, 8–10, 14, 15]. The fouling resistance was calculated using the following equation [2, 8–10, 14, 15]: Rf D

1 1 # ; Ufouled Uinitial

(4)

where Ufouled is the overall heat transfer coefficient for the fouled case, while Uinitial is the overall heat transfer coefficient for the initial clean condition. This overall heat transfer coefficient was calculated using the following equation [2, 8–10, 14, 15]: Q D UAo !TLMTD ;

(5)

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where !TLMTD is the log-mean-temperature difference, which was determined from measured temperatures at the inlet and outlet of the hot and cold water, which is given as [2, 8–10, 14, 15]: !TLMTD D

.Th;o # Tc;i / # .Th;i # Tc;o / ! " : .Th;o # Tc;i / ln .Th;i # Tc;o /

(6)

The surface area used to calculate the overall heat transfer coefficient was based on the outer diameter of the inner copper tube and an effective heat transfer length of 0.6 m. The !TLMTD was maintained around 56ıC–58ıC in all tests. EXPERIMENTAL RESULTS AND DISCUSSION The fouling tests were conducted in two cases: (a) the baseline (no-treatment) case and (b) the PWT-treated cases. The baseline test served as the reference test, where the PWT-treated cases were compared to view any effects of PWT in the fouling resistances and water chemistry through time. The preparation and cleaning procedures, water chemistry, experimental conditions (such as inlet temperatures and velocities), and the equipment used were the same for all tests. A fixed voltage of ˙ 10 V was used at four different frequencies for the PWT-treated cases: 13.56 MHz, 1 MHz, 100 KHz, and 1 kHz. Fouling Resistance Figure 4 shows the fouling resistances versus time for both the no-treatment and PWT-treated cases. All fouling resistances for both cases did not show any induction period. An induction period is the time wherein a stable crystal nucleation takes place and slowly spreads out laterally until it completely covers the heat transfer surface with scale deposits [6, 11]. It is indicated by a straight line in the beginning of the fouling test. The high water hardness, high heat transfer surface temperature, and low flow velocity must have affected the negligible induction periods [11]. The presence of a vast number of calcium ions in bulk water at a low flow velocity (i.e., low flow shear rate) should have been the primary factor in the rapid scale deposition in the first few hours of the test, as indicated in the drastic increase of fouling resistance. The fouling resistance for the no-treatment case increased exponentially to its maximum value of 4:7 ! 10!4 m2 K/W after t D 12 h of test, and then it gradually decreased and reached 4:2 ! 10!4 m2 K/W at the end of t D 36 h. The whole copper tube surface was already fully covered with white scale deposits after t D 6 h when checked visually. At the end of the test, the dried scale deposits on the copper tube showed rough deposit surfaces (see Figure 5b). The PWT-treated cases showed lower fouling resistances as compared to the notreatment case. In the case of PWT-treatment at f D 13:56 MHz, the fouling resistance decreased by 60% from the no-treatment case at the end of the test. It reached its asymptotic value of 1:65 ! 10!4 m2 K/W at t D 34 h. The fouling deposit layer on the copper showed a smoother surface (Figure 5c) as compared to the no-treatment case. It was obviously thinner when checked with visual inspection, though no thickness


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Figure 4. Fouling resistance versus time for both no-treatment and PWT-treated cases (10 V) at 0.3 m/s cold-water side velocity.

measurement was done. The scale deposits had a bluish-green color, as opposed to the white-colored scales for the no-treatment case. For the case of PWT-treatment at f D 1 MHz, the fouling resistance had an asymptotic value of 2:7 ! 10!4 m2 K/W after t D 36 h, which was a 36% decrease from the no-treatment case. The fouling deposits were white in color and with a rough surface, though lesser in the degree of roughness than that of the no-treatment case (see Figure 5d). At much lower frequencies of 100 kHz and 1 kHz, the fouling resistances decreased by about 25% and 16%, respectively, from the no-treatment case after t D 36 h. Both fouling deposits had white-colored scales (Figures 5e and 5f). It can be seen that in the fouling resistances, the first 10–12 h of the test is crucial in crystal growth and adhesion for crystallization fouling using artificial hard water. The effect of frequency on the mitigation of fouling in the present study had better performance as frequency was increased. After each test, the surfaces of the graphite electrode plates did not show any scaling. SEM Photographs The SEM photographs for both the no-treatment and treated cases are shown in Figure 6. The specimens with fouling deposit for SEM photography were prepared by drying the fouled tubes thoroughly after each test at room temperature. An approximately 5-mm by 5-mm specimen (i.e., copper tube with scale on surface) was cut from the fouled tube using a cutting saw. The SEM results in Figure 6 show different crystal structures between the no-treatment and the treated cases. The no-treatment case (Figure 6a) showed sharp and needle-like crystals depicting an aragonite form of scale. The treated case at f D 13:56 MHz (Figure 6b) showed a blunt crystal tip, and the edges and were a bit thicker in width. The treated cases at f D 1 MHz, f D 100 kHz, and f D 1 kHz

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Figure 5. Photographic images of clean and scaled copper tubes: (a) new and clean, (b) no-treatment, (c) treated at f D 13:56 MHz, (d) treated at f D 1 MHz, (e) treated at f D 100 kHz, and (f) treated at f D 1 kHz.

(Figures 6c, 6d, and 6e, respectively) showed flower-like crystal structures and agglomerated scales conveying a calcite form of scale. Aragonite crystal structures are known to be harder to remove as compared to the calcite crystal structures. Thus, the PWT-treated results suggest easier to remove scales. Tests for Water Chemistry The water chemistry in the fouling tests was also measured and analyzed. The water analyses were done within an hour from water sampling. It should be noted that the initial values of water quality for all tests were similar, and no chemical reagents were added once each test was started. The electrical conductivity, pH, total hardness, and total alkalinity of water (refer to Figure 7) were analyzed every 6–10 h with a sample volume of 100–200 cm3 . Electrical Conductivity. Figure 7a shows the electrical conductivity through time of both no-treatment and PWT-treated cases. The initial conditions were kept similar at 5,000–5,200 microSiemens/cm. The results showed a similar decreasing trend for all tests. The no-treatment case dropped by 10% after t D 30 h from its initial value. The electrical conductivities for the PWT-treated cases also similarly dropped


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Figure 6. SEM photographs of the fouling deposits for both no-treatment and treated cases at a constant voltage of 10 V and at different frequencies (5,000" magnification): (a) no treatment, (b) treated—13.56 MHz, (c) treated—1 MHz, (d) treated—100 kHz, (e) treated—1 kHz.

from 8.6–12.3%, depending on the frequency applied. The electrical conductivity is directly related to the number of conductive ions in the solution; thus, with the continuous deposition of CaCO3 scales, electrical conductivity decreased through time. pH. The acidity or alkalinity of a solution portends its precipitating capability. In the present study, the pH of the hard water was measured from the similar initial value for all cases until t D 36 h. The results shown in Figure 7b signifies that as mineral crystals are depositing on heat transfer surfaces, the alkalinity of the solution decreases. For the case of the PWT-treated case at f D 13:56 MHz, there was a 4% decrease in pH after 32 h as compared with a 9% decrease for the no-treatment case. The results for the other PWT-treated cases varied from a 6–8% drop in pH from their initial values. Total Hardness. In the present study, distilled water with negligible water hardness was used to mix the solution for producing artificial hard water at 500 ppm. Thus, the main component of water hardness was calcium ions. Figure 7c shows the water hardness versus time for the present study. The no-treatment case decreased by 22% after the fouling test. The PWT-treated case at f D 13:56 MHz showed the lowest percentage decrease at 9.6%, and as the frequency was lowered, the percentage decrease also increased to 18.6%, 21%, and 26% for f D 1 MHz, f D 100 kHz, and f D 1 kHz, respectively. Calcium ions combine with carbonate ions found in water to form the scales, so that the more deposition that takes place, the lesser will be the water hardness through time. Total Alkalinity. Alkalinity is the measure of the carbonate and bicarbonate concentrations in water. The alkalinity results in Figure 7d show a decrease in the range of 12–25% from their initial values for the no-treatment and PWT-treated cases.

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(a)

(b) Figure 7. Water analyses for both no-treatment and PWT-treated cases with respect to time: (a) electrical conductivities, (b) pH, (c) hardness, and (d) alkalinity. (continued)

CONCLUSION The objectives of the present study were to investigate the efficacy of a new PWT device for the mitigation of mineral fouling in heat exchangers. The PWT device used oscillating electric fields that were generated using two graphite plate electrodes submerged in water. It was operated at 10 V and in a frequency range of 1 kHz–13.56 MHz. Artificial hard water of 500 ppm of CaCO3 hardness was utilized in all fouling tests, with cold-water flow velocity of 0.3 m/s.


269

(c)

(d) Figure 7. (Continued).

The present results showed that the PWT device performed better with increasing frequency. In the present study, the maximum frequency tested was 13.56 MHz, where the best performance was obtained for the mitigation of mineral fouling. At this frequency, the fouling resistance decreased by 60% from the no-treatment case. The SEM photograph at this frequency also depicted a calcite form of scale that is easier to remove with enough flow shear forces. One may obtain a better performance at a frequency greater than 13.56 MHz. However, since 13.56 MHz is the frequency allowed for industrial applications by the Federal Communications Commission (FCC), it may be concluded that the present test showed the maximum performance of the present PWT operation.

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