1、336 2010 ASHRAEABSTRACTExperiments demonstrated performance degradation inair-conditioning systems resulting from dents in the fins of theircondensers and the extent to which conventional fin repairmethods restore performance. Two different, commerciallyavailable residential-type air-conditioner con
2、densing unitswere tested. Flattening substantial areas of the fins caused areduction in capacity and efficiency of the cooling systemstested. Performance dropped appreciably after 60% of the finareas were pressed flat. With 100% of the fin area flattened,system capacity decreased by 27% and system S
3、EER by 34%for one of the systems. Combing the dented fins after theextreme case where all fins were flattened restored capacity towithin 1% and SEER to within 6% of the undented condition.Similar results were obtained for the second condenser unittested.INTRODUCTIONThe performance of residential and
4、 commercial air-conditioners has a large influence on electrical energyconsumption in the United States. In 2004, households in theU.S. used 216.8 billion kWh of electricity for air-condition-ing, accounting for 6.4 percent of all electricity consumed byresidences.1Commercial buildings in 2004 consu
5、med anadditional 169.9 billion kWh of electricity for air-condition-ing. In 2001, 80.8 million households within the U.S. were air-conditioned.2In 2003, 3.6 million commercial buildings wereair-conditioned.3Most of those were in the Southwest, southAtlantic, Midwest, and middle Atlantic states.2,3Th
6、e numberof air-conditioners found in any locale can be attributed to twoprimary factors: population and climate.Manufacturers rate the performance of their productsaccording to strict standards; however, there are many factorsthat can affect air-conditioning equipment performance afterinstallation.
7、One common condition found in the field is dentswithin fins of condensers. There are various causes of dents;one is impact from hail; others include strikes from wind-borne debris, and brushes of various items against the fins.Much of the continental United States experiences hail.Areas east of the
8、Rocky Mountains, particularly the south-western and midwestern states, are especially prone. In thoseareas, warm moist air from the Gulf of Mexico collides withcold air from the north, creating conditions that favor hail.Figure 1 is a map of the United States depicting locations of allreports of hai
9、l, inch (19.1 mm) in diameter and larger, from1981 to 1990.4Included on that same map is a breakdown bygeographical region of the number of residences and commer-cial buildings with air-conditioning. From the map, it is readilyapparent that hail-prone regions have many air-conditionedbuildings.Hails
10、 effect on crops, roofing, automobiles, and aircraftis well documented but there does not appear to be muchunderstanding of its impact on air-conditioning equipmentperformance. As far as the authors are aware of, no studieshave been published regarding hail effects on air-conditioners,specifically.
11、A related study reported by Dooley5indicatedthat fouling over time had minimal affect on efficiency andcapacity. Other than the common practice of combing dents inrepairing and maintaining air-conditioners, there does notappear to be published data quantifying effects dents in finshave on performanc
12、e.Hail physically alters air-conditioning systems by dentingtheir condenser coil fins. Typically, condenser coil assembliesEffect of Dents in Condenser Fins on Air-Conditioner PerformanceMatthew J. Sitzmann, PE Frank K. Lu, PE Steve R. SmithStudent Member ASHRAEMatthew J. Sitzmann is a senior engine
13、er and Steve R. Smith is an engineer at Haag Engineering, Co., Irving, TX. Frank K. Lu is a professorin the Mechanical and Aerospace Engineering Department at the University of Texas at Arlington, Arlington, TX.OR-10-037 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers
14、, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 337comprise punched and press
15、ed aluminum fins fitted ontocopper coils. Fin spacing generally varies between 8 and 20fins per inch (3.1 and 7.9 fins per cm). The thin cross sectionsof the fins serve well to conduct heat from the refrigerant in thecoils and transfer it to air passing by them, but these fins areeasily bent. Figure
16、 2 is a photograph of a condenser with finsdeformed by hail. Not much has been reported on the extentair-conditioning system performance becomes degradedwhen fins are folded over in this way.A systematic study of the effect of fin dents on air-conditioning system performance is performed. Hail creat
17、esa wide range of dent shapes, sizes, and depths in condenserfins. Dent characteristics are governed by many variablesincluding properties of the impacting hailstones (size, hard-ness, and speed), orientation of the condenser to wind, andshielding features surrounding the condenser. The presentstudy
18、 does not fully replicate actual field conditions but islimited in scope in dealing only with a controlled, laboratorysituation to help in understanding the more complex dentingscenarios in practice. The study does incorporate severaldefining aspects of dents caused by hail such as a randomdistribut
19、ion of the dents, a discrete size of each dent, and adent characterized by fins folded on top of one another. Thelimited goals of this study are to determine the effects thatdents have on air-conditioning system performance and toFigure 1 Locations of hailstorms with 3/4-in. (19.1 mm) and larger dia
20、meter hail from 1981 to 1990.4Overlaid on the hail mapis the number of residences and commercial buildings with air-conditioning by region. 2,3Figure 2 Condenser fins of a rooftop air-conditioner thatwere dented by hail. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers
21、, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 338 ASHRAE Transactionsdetermine to what extent c
22、onventional fin repair methodscan restore performance.EXPERIMENTAL METHODTests were performed to determine the capacity andseasonal energy efficiency ratio (SEER) of two separate air-conditioning condensers as increasingly larger portions oftheir condenser fins were dented. All tests were conducteda
23、ccording to Air-Conditioning and Refrigeration Institute(ARI) standard 210/240.6,7In all cases, the indoor enthalpymethod was used for reporting performance, a schematic ofwhich is shown in Fig. 3. Figures 4 and 5 are photographs ofthe laboratory setup. The Outdoor Enthalpy Method was usedas a check
24、 of the accuracy of the indoor enthalpy method. Ducttraverses with pitot tubes mounted in accordance with ANSI/ASHRAE 41.1-2000 measured the airflow.7Figure 3 shows that a complete air-conditioning systemwas constructed using an outdoor condensing unit and anindoor air handler. The condensing unit a
25、nd air handler weremounted in separate rooms (psychrometric chambers), andauxiliary equipment controlled the environments of bothrooms. A duct attached to the air handler discharge capturedthe airflow leaving it. Pressure, temperature, and relativehumidity sensors measured properties of the air ente
26、ring theair handler, leaving the air handler, and entering the condenser.Pitot tubes with differential pressure sensors measured theairflow rate, and a power analyzer measured electricityconsumption. Change in enthalpy of the air across the airhandler and the airflow rate combined to give the coolin
27、gcapacity of the systems. Dividing the cooling capacities byelectricity consumption produced the efficiencies.6-12Air-conditioner capacities were measured with the indoorchamber temperature held at 80F (26.7C) dry bulb, 67F(19.4C) wet bulb, and the outdoor chamber temperature heldat 95F (35.0C) dry
28、bulb. Data for calculating the seasonalenergy efficiency ratio (SEER) were measured with the indoorFigure 3 Schematic of test setup for measuring the performance of an air-conditioning system using the Indoor EnthalpyMethod. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engin
29、eers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 339temperature at 80F (26
30、.7C) dry bulb, 67 F (19.4C) wetbulb, and the outdoor temperature at 82F (27.8C) dry bulb.In addition to the parameters required by ARI 210/240, thepressure of the refrigerant leaving the condenser and temper-atures at multiple locations on the evaporator and condenserwere monitored throughout the te
31、sts. Additional ducts, instru-mentation, and a fan were added to the condenser outlet forusage in the Outdoor Enthalpy Method. For complete specifi-cations on the protocol for measuring air-conditioning systemperformance, refer to ARI 210/240.7Air-Conditioning EquipmentTwo commercially available con
32、densing units made bydifferent manufacturers were tested. As will be evident, testingtwo different condensing units allow a general performancedegradation trend to be identified with increasing dented finarea. Each condensing unit tested had a nominal rated capacityof 30,000 BTU/h (8.8 kW) and was c
33、oupled with a matched airhandler. Manufacturer recommended charges of R-22 refrig-erant were used.Table 1 lists specifications for both condensing unitstested. Figure 6 is a set of photographs showing the geometryof the fins of each condenser. Unit 1 had rippled plain fins. Unit2 had slit fins. Furt
34、her fin specifications can be found in Table 1.Denting Strategy and Repair MethodA systematic approach for denting the fins was adopted soas to gain insights into the effect of dents on performance.Such an approach, moreover, avoids large-scale statisticalanalysis and other details, representing a f
35、irst step towardfurther understanding of dents on air-conditioning systemperformance.A wooden dowel with a hemispherical head on one end,-inch (12.7 mm) radius, was used to dent the fins. Weldedwire with a 1-inch (25.4 mm) mesh stretched over the front ofthe condensers provided a grid for locating t
36、he dents. Eachsquare of the welded wire grid was assigned a letter and anumber. A random number generator selected the pattern inwhich the fins were to be dented.Dents were formed by manually pressing the denting tool(denter) against the fins. The denter was pressed inward untilit flattened fins aga
37、inst the coil. Regions of fins affected bypressing the denter inward were elliptical in shape. The finswere flattened completely in a circular portion of the affectedarea. Figure 7 is a photograph of a typical dent where the ellip-tical affected area and circular flattened area are marked forclarity
38、. On unit 1, flattened areas for each dent averaged 0.40in.2(258 mm2). On unit 2, flattened areas for each dent aver-aged 0.46 in.2(297 mm2). The denter was utilized until ulti-mately every square in the guiding grid had been dented.As built, steel louvers surrounded the condensers of bothunits test
39、ed. The units were tested initially with their louversinstalled, then the louvers were removed to expose thecondensers and the units tested again. Next, dents were maderandomly in the aluminum coil fins. Successive tests wereFigure 4 Indoor chamber with air handler, duct, flowmeasuring device, and i
40、nstruments.Figure 5 Outdoor chamber with condenser, duct, flowmeasuring device, fan, and instrumentsarranged for the Outdoor Enthalpy Method. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part
41、1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 340 ASHRAE Transactionsperformed with increasing numbers of dents until every squareof the wire grid had been dented. With every
42、 grid squaredented, 41 to 45% of the fin areas were flattened. The remain-der of the fin area was deformed but not flat or unaffected alto-gether. To increase the flattened fin area beyond the denterscapability, entire columns of fins were folded over mechani-cally so that no gaps remained between f
43、ins in those areas.Columns of flattened fins were added so that 60, 80, and 100%flattening resulted. Figure 8 is a photograph of the denter andthe welded wire grid while Fig. 9 shows several dented condi-tions for which tests were run. To reverse the effects of dent-ing, conventional fin combs were
44、passed through thecondensers to straighten the fins. Tests measured the capaci-ties and efficiencies of the systems for each condition. Finally,the fins were combed and the performances measured again. Data Acquisition and AccuracyData were acquired using a portable data acquisitionsystem comprising
45、 of a laptop and National Instruments dataacquisition system with a LabVIEW interface. The data wereacquired at 15 samples per second per channel. Fifty samplesfor each data point were taken to provide statistical estimatesper accepted protocol.13,14A typical set of averaged, standarddeviation and i
46、nstrument published accuracies is shown inTable 2.The duration of each test was 30 minutes as established byARI 210/240. Temperature, relative humidity, discharge staticpressure, and condenser coil static pressure readings weretaken once every minute during each test, with 50 samples perreading. Flo
47、w measurements were taken once every tenminutes, also with 50 samples. Averages and standard devia-tions listed in Table 2 were calculated using the average valueof each 50 sample reading over the course of the 30 minute test.Therefore, those columns demonstrate the average conditionsat which the te
48、st was run over the course of the 30 minutes andthe extent to which conditions varied during, even while stay-ing within tolerances mandated by the ARI standard. The uncertainty, defined as the ratio of the sample stan-dard deviation to the sample average, are listed in Table 2. Theuncertainties wer
49、e of the same order as the instrumentspublished accuracies. Accuracies of instruments used werewithin limits set by the standard.For every test condition, we performed four tests (twocapacity tests and two efficiency tests) and used two differentmeasurement methods (the Indoor Enthalpy Method and theOutdoor Enthalpy Method) to confirm result validity. ARI standard 210/240 does not provide a method forcalculating the accuracy attained in measuring the capacity.and SEER of a system. However, the standard dictates, forreporting purposes, the capacity of air-conditioning
