1、OR-05-1 -3 Impact of Fouling and Cleaning on Plate Fin and Spine Fin Heat Exchanger Performance Bock Choon Pak, PhD ABSTRACT Eckhard A. Groll, PhD Member ASHRAE An experimental study was conducted to investigate the impacts of air-side fouling and cleaning on theperformance of various condenser coil
2、s used in unitary air-conditioning systems. A total of six condenser coils with diferentjn geom- etries and depths were tested. Performance tests were conducted at three different conditions: clean-as-received, after fouling, and after cleaning. In all cases, it was observed that fouling was mostly
3、conjned to the frontal face of the heat exchanger as reported in previous investigations. The amount of deposited dust was more dependent on jn geometry for single-row heat exchang- ers than for double-row heat exchangers. The predominant effect of fouling was to cause a more signijcant increase in
4、air-side pressure drop than degradation in heat transferperformance. For the single-row heat exchang- ers, the pressure drop increased by 28% to 31 %, while the heat transferperformance decreased by 7% to 12% at the standard air face velocity of 1.53 m/s depending onjn shape. For the double-row heat
5、 exchangers, the pressure drop increased by 22% to 3 7%, and heat transferperformance decreased by only 4% to 5% at the same air face velocity. Once the contaminated coils were cleaned according to a manufacturer-spec$ed cleaning procedure, the original performance of the heat exchangers was recover
6、ed almost completely. The pressure drop was restored to within I % to 7% and the heat transferperformance could be recovered to within 1 % to 5% of the originally clean heat exchangers. Therefore, it is concluded that aperiodic application ofspec$ed cleaning procedures will be efective in maintainin
7、g the thermalperfor- mance of condenser coils. INTRODUCTION James E. Braun, PhD, PE Member ASHRAE Fouling may be defined as the formation of deposits on heat transfer surfaces. It is well known that fouling impedes heat transfer and increases pressure drop for a given flow rate over the heat transfe
8、r surface (Taborek et al. 1972; Suitor et al. 1977; Bott 198 1). Engineers working in the heat transfer area have a particular interest in fouling since it significantly impacts the performance and lifetime of heat transfer equip- ment. If a deposition occurs on a solid surface in the presence of a
9、dirty gas stream, the process is called gas-side fouling. This may be encountered in energy-intensive industries such as the food, textile, pulp and paper, chemical petroleum, primary metal, cement, and glass industries. Contamination of a condenser in refrigeration and air-conditioning systems by a
10、irborne dust and debris is also an example of air-side fouling. It causes a decrease in the heat exchanger capacity and even- tually a decrease in system efficiency. Gas-side fouling has received considerable attention owing to increased interest in heat recovery from exhaust gas streams, which was
11、originally stimulated in large part by increasing fuel costs in the early 1970s. Marner (1990) conducted an extensive review of studies that involved various aspects of gas-side fouling during the period from 1970 to 1990. The mechanisms that led to gas-side fouling were explained in detail, and ana
12、lytical and experi- mental studies were listed. The paper presented an excellent overview of fouling studies but focused on gas-side fouling in boilers and gas turbines at very high temperatures. In contrast, the gas-side fouling of extended surface heat exchangers, as typically used in refrigeratio
13、n and air-conditioning applica- tions, which is the topic of the current study, has received very Bock Choon Pak is an associate professor of mechanical engineering at Chonbuk National University, Korea. Eckhard A Groll is an associate professor and James E. Braun is a professor at Purdue University
14、, West Lafayette, Indiana. 496 02005 ASHRAE. little attention so far. Only a few studies are found in the liter- ature. These studies are summarized here. Cowell and Cross (198 1) investigated the effects of gas- side fouling on pressure drop and heat transfer characteristics in 22 automobile and in
15、dustrial engine radiators. The authors concluded that the increase in pressure drop is substantially greater than the reduction in heat transfer and that the fouling by dust was confined almost totally to the front face of the heat exchanger core. The authors also stated that the hydraulic diameter
16、of the basic orifice in the front face of the radiator determines the effect of fouling, and high-performance louvered fins are best for resistance to fouling by dust. Bott and Bemrose (1983) carried out a systematic study of air-side fouling in finned tube bundles using fin densities of 354 to 433
17、fins/m, fin heights of 12.7 to 15.9 nun, a fin array of one, two, three, and four rows of a staggered, equilateral layout, and air velocities of 1.85 to 5.99 m/s. The authors concluded that air-side fouling has a pronounced effect on the air-side pressure drop (ffoui = 1.4 to 2.5 fciean) and that he
18、at transfer performance, as measured in the form of the j-factor, decreased only slightly with time. They also reported that the first and last rows of the heat exchanger coils were fouled more heavily than the middle rows. Zhang et al. (1990) tested particle fouling of a diesel charge air cooler. T
19、he test parameters included particulate concentration, particle size, and ternperaturc gradient. The authors found that there were two parts that contribute to foul- ing: a delay in which fouling was not apparent and a rapid exponential fouling process. High velocities and small parti- cles were fou
20、nd to accelerate fouling. The fouling layer was generally soft and easy to remove. There are a few studies that addressed the effects of condenser fouling on overall air-conditioning system perfor- mance. Breuker and Braun (1 998) showed a 5% loss in capac- ity and an 8% loss in COP when about 25% o
21、f the condenser coil was blocked due to fouling. Bultman et al. (1993) reported a 7.6% decrease in system COP for a 40% reduction in condenser airflow for an air conditioner. Despite the longstanding problems of gas-side fouling of extended surface heat exchangers, there have been remarkably few sys
22、tematic studies of the phenomenon. In summary, quite apart from considerations of improved design techniques, more research should be done in determining the effects of gas-side fouling on hydrodynamic and thermal performances of heat exchangers such as condensers and evaporators having complicated
23、extended surfaces. Figure 1 Schematic diagram of the experimental facilities. EXPERIMENTAL EQUIPMENT AND METHOD Test Facility and Procedure Figure 1 shows a schematic of the test facility, which consists of an open and rectangular air duct, a closed water loop, a dust-injecting system, and the test
24、coils. The cross- sectional dimensions ofthe air duct are 48.8 cm tall by 90.2 cm wide. A variable-speed blower draws room air into the preheating section of the air duct, which was connected to the test section inlet. Four 5 kW electric heaters were located directly downstream of the blower to cont
25、rol the inlet temper- ature. The air duct is capable of delivering air coil face veloc- ities from 0.2 to 2.0 m/s and dry-bulb temperatures extending from room temperature to 55C. In order to measure the static pressure drop across the test coil, a total of eight pressure tap holes with a diameter o
26、f 1.6 mm were mounted circumferen- tially (four holes upstream and four holes downstream of the test coil). Temperature grids with nine K-type thermocouples were mounted upstream and downstream of the test coil. Airstream mixers were installed in front of each temperature grid to provide uniform air
27、 temperature distributions. Immediately after the test section, a final filter was inserted into the air duct to collect the dust passed through the test coil. Two relative humidity meters were also installed inside the air duct upstream and downstream of the test coil in order to measure the relati
28、ve humidity of the airstream. A nozzle apparatus, constructed according to ASHRAE Standard 33-1978 (ASHRAE 1978) was located downstream of the test coil and allowed measurement of the volumetric airflow rate. The discharge of the nozzle apparatus was connected to the outdoors to vent the air. This t
29、est loop was sealed with duct tape so that air leakages at places that would influence the capacity measurements did not exceed 1 .O% of the test airflow rate. The test coil itself was directly connected to the closed water flow loop. The main components of the water loop include a flexible impeller
30、 pump, a mass flow meter, a surge The study presented here investigated the effect of parti- cle contamination and subsequent cleaning on the perfor- mance of various condensers used in unitary air-conditioning systems. The importance of this research lies in its potential to promote further underst
31、anding of the fouling process, the effects of fouling on different coil types, and to provide better information for heat exchanger field service procedures. tank, and an electric heating system. A variable-speed control- ler connected to a motor was used to adjust the flow rate. A surge tank was al
32、so inserted between the pump and the flow meter to minimize possible pressure fluctuations caused by the pump. Galvanized steel tubes of 1.27 cm diameter were used to connect the components of the water loop. Hot water flowed inside the test coil tubing to supply heat to the air flowing in ASHRAE Tr
33、ansactions: Symposia 497 Coil Fin No. of No. Type Rows HXOl plate i HX02 plate i HX03 spine 1 HX04 date 2 Coil Surface Description 22 finslin., plain fins 22 finslin., louvered fins spine fins (standard) 22 finslin., one-by-one, louvered fins Figure2 Photograph of heat exchanger (HX 01) after circui
34、ting tubes and installing flanges. HX05 HX06 had been tested in clean and fouled conditions, all test coils were cleaned and each coil was re-tested in cleaned condi- tions. The condenser coils were cleaned using the following cleaning procedure obtained from the coil manufacturer: plate 2 22 finsli
35、n., continuous, louvered fins spine 2 spine fins (woven) Step 1: Step 2: Step 3: Step 4: Step 5: Clean the contaminated condenser coil with compressed air for five minutes (single-row coils) or ten minutes (double-row coils). Prepare a dilute solution of one part heat exchanger cleaner (obtained fro
36、m the coil manufacturer) with five parts of water, according to the instructions of the heat exchanger cleaner, and spray the solution onto the condenser coil. Allow the condenser coil to soak for approximately four minutes. Carefully rinse the condenser coil with water. Continue rinsing for five mi
37、nutes (single-row coils) or ten minutes (double-row coils). During the rinsing of the spine finned condenser coils, care was taken to not damage the spine fins. Completely dry the condenser coil. During the experiments, the inlet air temperature was fixed at 35C and the inlet water temperature was f
38、ixed at 54OC. At clean conditions, the water-side temperature drop through the coil was adjusted to be 8C by controlling the water mass flow rate through the coil. Once the water mass flow rate was set, it was kept constant for the remaining tests of the coil. Data Reduction The outputs from the the
39、rmocouples, thermistors, pres- sure transducers, and mass flow meter were collected by a data acquisition system, which sent the signals to a personal computer using appropriate data acquisition software. In addi- tion, the net amount of the deposited dust onto the condenser coil was determined from
40、 a mass balance using the measured 498 ASHRAE Transactions: Symposia injected dust and dust collected on a high-efficiency down- stream filter. As mentioned before, a total of 300 g of ASHRAE stan- dard dust was sprayed into the airstream during a three-hour interval. Some amount of the dust was dep
41、osited onto the test coil surface, while the remaining portion of the dust passed through the test coil and was collected by the final (down- stream) filter. The relative amounts of dust deposited on the condenser coils (in percentage) during the loading was deter- mined by the following equation: w
42、, = loox(wt- w1 (%) During the tests, the air-side pressure drops of each heat exchanger were measured directly by a differential pressure transducer. The air-side pressure drop, Ma, under the three different test conditions, was measured as a function of air face velocity and values compared to eac
43、h other. The air-side capacity of the condenser coil was calculated using the following equation: where the enthalpy h at the inlet and outlet of the test coil and the density p at the nozzle throat are calculated from property relations as a function of temperature, pressure, and humidity ratio. Th
44、e water-side capacity of the condenser coil was calcu- lated by Equation 3. Q, m,c,(T, 0 - T, i) (3) The average heat exchange capacity is then defined as (4) Once the on-line measured air-side and water-side capac- ities fell within 5%, experimental data were collected for 30 minutes and averaged.
45、The heat transfer rate of a two-fluid heat exchanger is given by e, = (uA),fpT, 3 (5) where indicates the log mean temperature difference of a counterflow heat exchanger. Since the heat exchangers were tested in cross-flow configuration, the UA value obtained by Equation 5 includes an F-factor and,
46、thus, the term i/( U!),is called the effective overall thermal resistance of the heat exchanger. Using the effective overall thermal resistance, the following expression can be set up: tance, respectively. The last two terms, i.e., Rm and ll(hA), on the right-hand side of Equation 6 can be assumed c
47、onstant since the water flow rate was kept constant during the exper- iments. However, the other two terms on the right-hand side cannot be calculated separately unless the thickness of the dust deposit on the heat exchanger is uniform. The combined effects of these two terms due to fouling and clea
48、ning will be expressed as the variation of the l/(UA),value in the results section of this paper. Uncertainty Analysis The measurement error of the condenser coil pressure drop results from the uncertainty of the differential pressure transducer. The differential pressure transducer ranged from O Pa
49、 (O in. H20) to 1246 Pa (5 in. H20) with an accuracy of *l.O% of full scale. The uncertainty of the airflow rate measurement results from uncertainties in the differential pressure transducer that measures the static pressure drop across the nozzle, a second differential pressure transducer between the atmosphere and the nozzle throat that measures the absolute pressure at the nozzle throat for a given barometric pressure, a IC-type ther- mocouple that measures the temperature at the nozzle throat, and a humidity transmitter that measures the relative humidity entering the nozzle a
