Effect of dents in condenser fins on air-conditioner performance.
The performance of residential and commercial air-conditioners has a large influence on electrical energy consumption in the United States. In 2004, households in the U.S. used 216.8 billion kWh of electricity for air-conditioning, accounting for 6.4 percent of all electricity consumed by residences. (1) Commercial buildings in 2004 consumed an additional 169.9 billion kWh of electricity for air-conditioning. In 2001, 80.8 million households within the U.S. were air-conditioned. (2) In 2003, 3.6 million commercial buildings were air-conditioned. (3) Most of those were in the Southwest, south Atlantic, Midwest, and middle Atlantic states. (2), (3) The number of air-conditioners found in any locale can be attributed to two primary factors: population and climate.
Manufacturers rate the performance of their products according to strict standards; however, there are many factors that can affect air-conditioning equipment performance after installation. One common condition found in the field is dents within 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 Rocky Mountains, particularly the southwestern and midwestern states, are especially prone. In those areas, warm moist air from the Gulf of Mexico collides with cold air from the north, creating conditions that favor hail. Figure 1 is a map of the United States depicting locations of all reports of hail, 3/4 inch (19.1 mm) in diameter and larger, from 1981 to 1990.4 Included on that same map is a breakdown by geographical region of the number of residences and commercial buildings with air-conditioning. From the map, it is readily apparent that hail-prone regions have many air-conditioned buildings.
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Hail's effect on crops, roofing, automobiles, and aircraft is well documented but there does not appear to be much understanding of its impact on air-conditioning equipment performance. As far as the authors are aware of, no studies have been published regarding hail effects on air-conditioners, specifically. A related study reported by Dooley (5) indicated that fouling over time had minimal affect on efficiency and capacity. Other than the common practice of combing dents in repairing and maintaining air-conditioners, there does not appear to be published data quantifying effects dents in fins have on performance.
Hail physically alters air-conditioning systems by denting their condenser coil fins. Typically, condenser coil assemblies comprise punched and pressed aluminum fins fitted onto copper coils. Fin spacing generally varies between 8 and 20 fins per inch (3.1 and 7.9 fins per cm). The thin cross sections of the fins serve well to conduct heat from the refrigerant in the coils and transfer it to air passing by them, but these fins are easily bent. Figure 2 is a photograph of a condenser with fins deformed by hail. Not much has been reported on the extent air-conditioning system performance becomes degraded when fins are folded over in this way.
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A systematic study of the effect of fin dents on air-conditioning system performance is performed. Hail creates a wide range of dent shapes, sizes, and depths in condenser fins. Dent characteristics are governed by many variables including properties of the impacting hailstones (size, hardness, and speed), orientation of the condenser to wind, and shielding features surrounding the condenser. The present study does not fully replicate actual field conditions but is limited in scope in dealing only with a controlled, laboratory situation to help in understanding the more complex denting scenarios in practice. The study does incorporate several defining aspects of dents caused by hail such as a random distribution of the dents, a discrete size of each dent, and a dent characterized by fins folded on top of one another. The limited goals of this study are to determine the effects that dents have on air-conditioning system performance and to determine to what extent conventional fin repair methods can restore performance.
Tests were performed to determine the capacity and seasonal energy efficiency ratio (SEER) of two separate air-conditioning condensers as increasingly larger portions of their condenser fins were dented. All tests were conducted according to Air-Conditioning and Refrigeration Institute (ARI) standard 210/240. (6), (7) In all cases, the indoor enthalpy method was used for reporting performance, a schematic of which is shown in Fig. 3. Figures 4 and 5 are photographs of the laboratory setup. The Outdoor Enthalpy Method was used as a check of the accuracy of the indoor enthalpy method. Duct traverses with pitot tubes mounted in accordance with ANSI/ ASHRAE 41.1-2000 measured the airflow. (7)
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Figure 3 shows that a complete air-conditioning system was constructed using an outdoor condensing unit and an indoor air handler. The condensing unit and air handler were mounted in separate rooms (psychrometric chambers), and auxiliary equipment controlled the environments of both rooms. A duct attached to the air handler discharge captured the airflow leaving it. Pressure, temperature, and relative humidity sensors measured properties of the air entering the air handler, leaving the air handler, and entering the condenser. Pitot tubes with differential pressure sensors measured the airflow rate, and a power analyzer measured electricity consumption. Change in enthalpy of the air across the air handler and the airflow rate combined to give the cooling capacity of the systems. Dividing the cooling capacities by electricity consumption produced the efficiencies. (6-12)
Air-conditioner capacities were measured with the indoor chamber temperature held at 80[degrees]F (26.7[degrees]C) dry bulb, 67[degrees]F (19.4[degrees]C) wet bulb, and the outdoor chamber temperature held at 95[degrees]F (35.0[degrees]C) dry bulb. Data for calculating the seasonal energy efficiency ratio (SEER) were measured with the indoor temperature at 80[degrees]F (26.7[degrees]C) dry bulb, 67 [degrees]F (19.4[degrees]C) wet bulb, and the outdoor temperature at 82[degrees]F (27.8[degrees]C) dry bulb. In addition to the parameters required by ARI 210/240, the pressure of the refrigerant leaving the condenser and temperatures at multiple locations on the evaporator and condenser were monitored throughout the tests. Additional ducts, instrumentation, and a fan were added to the condenser outlet for usage in the Outdoor Enthalpy Method. For complete specifications on the protocol for measuring air-conditioning system performance, refer to ARI 210/240. (7)
Two commercially available condensing units made by different manufacturers were tested. As will be evident, testing two different condensing units allow a general performance degradation trend to be identified with increasing dented fin area. Each condensing unit tested had a nominal rated capacity of 30,000 BTU/h (8.8 kW) and was coupled with a matched air handler. Manufacturer recommended charges of R-22 refrigerant were used.
Table 1 lists specifications for both condensing units tested. Figure 6 is a set of photographs showing the geometry of the fins of each condenser. Unit 1 had rippled plain fins. Unit 2 had slit fins. Further fin specifications can be found in Table 1.
Table 1. Condensing Unit Specifications Specification Unit #1 Unit #2 Capacity, BTU/h (kW) 29,600 (8.68) 28,600 (8.38) SEER 10.0 10.0 EER 9.5 - Type Reciprocating - Reciprocating--single single speed speed Compressor Run Load Amps 13.7 13.7 Locked Rotor 75 75 Amps Fan Flow, SCFM 2,200 2,500 Full Load Amps 1.5 1.13 Fan Motor HP 1/4 1/8 Fin type Rippled Plain Slit Fin Fin Rows 1 1 Fin Density, 19 18 fins/in. Coil 18 20 Number of tubes Tripod: 6,6,6 Tripod: 8,8,4 configuration configuration Tube O.D., in. 3/8 3/8 Face Area, 9.4 8.3 [ft.sup.2]
Denting Strategy and Repair Method
A systematic approach for denting the fins was adopted so as to gain insights into the effect of dents on performance. Such an approach, moreover, avoids large-scale statistical analysis and other details, representing a first step toward further understanding of dents on air-conditioning system performance.
A wooden dowel with a hemispherical head on one end, 1/2-inch (12.7 mm) radius, was used to dent the fins. Welded wire with a 1-inch (25.4 mm) mesh stretched over the front of the condensers provided a grid for locating the dents. Each square of the welded wire grid was assigned a letter and a number. A random number generator selected the pattern in which the fins were to be dented.
Dents were formed by manually pressing the denting tool (denter) against the fins. The denter was pressed inward until it flattened fins against the coil. Regions of fins affected by pressing the denter inward were elliptical in shape. The fins were flattened completely in a circular portion of the affected area. Figure 7 is a photograph of a typical dent where the elliptical affected area and circular flattened area are marked for clarity. On unit 1, flattened areas for each dent averaged 0.40 in. (2) (258 [mm.sup.2]). On unit 2, flattened areas for each dent averaged 0.46 in. (2) (297 [mm.sup.2]). The denter was utilized until ultimately every square in the guiding grid had been dented.
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As built, steel louvers surrounded the condensers of both units tested. The units were tested initially with their louvers installed, then the louvers were removed to expose the condensers and the units tested again. Next, dents were made randomly in the aluminum coil fins. Successive tests were performed with increasing numbers of dents until every square of the wire grid had been dented. With every grid square dented, 41 to 45% of the fin areas were flattened. The remainder of the fin area was deformed but not flat or unaffected altogether. To increase the flattened fin area beyond the denter's capability, entire columns of fins were folded over mechanically so that no gaps remained between fins 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 and the welded wire grid while Fig. 9 shows several dented conditions for which tests were run. To reverse the effects of denting, conventional fin combs were passed through the condensers to straighten the fins. Tests measured the capacities and efficiencies of the systems for each condition. Finally, the fins were combed and the performances measured again.
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Data Acquisition and Accuracy
Data were acquired using a portable data acquisition system comprising of a laptop and National Instruments data acquisition system with a Lab VIEW interface. The data were acquired at 15 samples per second per channel. Fifty samples for each data point were taken to provide statistical estimates per accepted protocol. (13), (14) A typical set of averaged, standard deviation and instrument published accuracies is shown in Table 2.
Table 2. Measurement Uncertainties from a Typical Test Measurement Manufacturer Model # Measurement Unit Average Pitot pressure Omega PX655-01DI in. [H.sub.2]O 0.3342 at the duct gauge wall Pitot pressure Omega PX655-01DI in. [H.sub.2]O 0.5954 near the duct gauge centerline Air-conditioner Omega PX655-01DI in. [H.sub.2]O 0.6260 discharge static gauge pressure Condenser coil Ashcroft K17M0242F2 psig 272.4 static pressure 500# Indoor Omega P-L-1/10-1/ [degrees]F 80.16 temperature 8-6-1/2-T-3 Outdoor Omega P-L-1/10-1/ [degrees]F 94.98 temperature 8-6-1/2-T-3 Air-conditioner Omega P-L-1/10-1/ [degrees]F 62.11 discharge 8-6-1/2-T-3 temperature Indoor relative Vaisala HMT333 % 51.4 humidity Outdoor relative Vaisala HMT333 % 22.7 humidity Air-conditioner Vaisala HMT333 % 78.4 discharge relative humidity Measurement Std. Dev. % Uncertainty Instrument (Std. Dev/ Published Average) Accuracy Pitot pressure at the duct 0.0089 [+ or -]2.7 [+ or -]0.0025 wall Pitot pressure near the 0.0038 [+ or -]0.64 [+ or -]0.0025 duct centerline Air-conditioner discharge 0.0043 [+ or -]0.67 [+ or -]0.0025 static pressure Condenser coil static 0.8 [+ or -]0.29 5.0 pressure Indoor temperature 0.16 [+ or -]0.76 [+ or -]0.08 Outdoor temperature 0.16 [+ or -]0.17 [+ or -]0.09 Air-conditioner discharge 0.11 [+ or -]0.18 [+ or -]0.07 temperature Indoor relative humidity 0.4 [+ or -]0.78 [+ or -]1 Outdoor relative humidity 0.3 [+ or -]1.3 [+ or -]1 Air-conditioner discharge 0.4 [+ or -]0.51 [+ or -]1 relative humidity
The duration of each test was 30 minutes as established by ARI 210/240. Temperature, relative humidity, discharge static pressure, and condenser coil static pressure readings were taken once every minute during each test, with 50 samples per reading. Flow measurements were taken once every ten minutes, also with 50 samples. Averages and standard deviations listed in Table 2 were calculated using the average value of each 50 sample reading over the course of the 30 minute test. Therefore, those columns demonstrate the average conditions at which the test was run over the course of the 30 minutes and the extent to which conditions varied during, even while staying within tolerances mandated by the ARI standard.
The uncertainty, defined as the ratio of the sample standard deviation to the sample average, are listed in Table 2. The uncertainties were of the same order as the instruments' published accuracies. Accuracies of instruments used were within limits set by the standard.
For every test condition, we performed four tests (two capacity tests and two efficiency tests) and used two different measurement methods (the Indoor Enthalpy Method and the Outdoor Enthalpy Method) to confirm result validity.
ARI standard 210/240 does not provide a method for calculating the accuracy attained in measuring the capacity. and SEER of a system. However, the standard dictates, for reporting purposes, the capacity of air-conditioning units such as those that were tested for this study to be rounded to the nearest 200 BTU/h. SEERs are to be rounded to the nearest 0.05.
RESULTS AND DISCUSSION
Figure 10 shows that there was no significant decrease in capacity with increase in dented area, even with 41% of the fin areas completely flattened. Beyond the discrete dents, folding columns of fins so that 60% of the condenser areas were flattened caused the capacities to drop by about 4%. Flattening all of the fins, thereby causing a large drop of flow across the condensers, reduced the two tested unit capacities by 23 and 27% from undented levels. Figure 10 also shows that combing the dents from the fins after the extreme cases where all fins were flattened completely restored capacities to within 1 and 4% of the undented capacities. The original undented data with the louvers on the units as shipped are shown on the upper right of the plot.
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Incremental increases in the area dented caused incremental increases in power consumed by the systems. This is displayed in Fig. 11. The power consumption of one system increased by 5% and the other by 3% when the entire condenser surface was covered with discrete dents (the 41% flat condition). With all of the fins flattened, power consumption increased from the undented condition by 15% for unit 1 and by 19% for unit 2.
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Denting the fins also reduced air-conditioning equipment efficiency. With discrete dents on the entire coil surface (the 41% flattened condition), the SEER of one unit decreased by 2% and the SEER of the other unit dropped by 5%. In the extreme condition of the coil fins being flattened completely, the SEER of both units was reduced 34%. Combing the dents from the fins after the extreme cases where all fins were flattened completely restored SEER's to within 4 and 6% of the undented efficiencies. Effects of denting on SEER are shown in Fig. 12.
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Experiments were conducted to determine the effects on air-conditioning system performance caused by denting condenser fins and the extent to which conventional fin repair methods restore performance. Two different condensing units were tested. With discrete dents spread over the entire surface of the units (41% of the fin area flattened), there were little to no decreases in capacity, 3 and 5% increases in power consumption, and 2 and 5% decreases in SEER. In the extreme cases where all the fins were flattened completely, one unit lost 23% of its capacity while the other lost 27% of its capacity. Power consumption increased 15% for one unit and 19% for the other. The SEER of each unit decreased 34%. Combing dents from the fins after the extreme cases where all fins were flattened completely restored the systems to within 1 and 4% of their undented capacities and to within 4 and 6% of their undented efficiencies.
We would like to thank the University of Texas at Arlington and Haag Engineering for their financial and technical support. Special thanks go to Albert Ortiz for his programming expertise.
(1.) Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy. 2006. 2006 Buildings Energy Data Book. Washington D.C., 2006.
(2.) Energy Information Administration, 2001 Residential Energy Consumption Survey: Housing Characteristics Tables; http://www.eia.doe.gov/emeu/recs, accessed May 7, 2008.
(3.) Energy Information Administration, 2003 Commercial Buildings Energy Consumption Survey: Building Characteristics Tables, revised June 2006; http://www.eia.doe.gov/emeu/cbecs, accessed May 7, 2008.
(4.) National Climatic Data Center, Hail Events Map- ESRI ArcExplorer 1.1, http://cdo.ncdc.noaa.gov/climaps/hail4614.pdf.
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(6.) U.S. Department of Energy. "10 CFR--Part 430, Appendix M to Subpart B, Uniform Test Method for Measuring the Energy Consumption of Central Air-Conditioners and Heat Pumps," Washington, D.C.;
(7.) Air-Conditioning and Refrigeration Institute. "Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment," ARI Standard 210/240, 2006, Arlington, Virginia; http://www.ari.org/Content/FindaStandard_218.aspx?Listing_PK=160, accessed May 7, 2008
(8.) ASHRAE, "Standard Methods for Laboratory Airflow Measurement," ANSI/ASHRAE Standard 41.2-1987 (RA 92), 1992, Atlanta, Georgia.
(9.) ASHRAE, "Methods of Testing for Rating Electricity Driven Unitary Air-Conditioning and Heat Pump Equipment," ANSI/ASHRAE Standard 37-2005, 2005 Atlanta, Georgia.
(10.) ASHRAE, "Standard Methods for Measurement of Moist Air Properties," ANSI/ASHRAE Standard 41.6-1994 (RA 2006), 2006, Atlanta, Georgia.
(11.) ASHRAE, 2005 ASHRAE Handbook - Fundamentals, I-P Edition. 2006, Atlanta, Georgia.
(12.) ASHRAE, "Standard Method for Temperature Measurement," ANSI/ASHRAE Standard 41.1-1986 (RA 2006), 2006, Atlanta, Georgia.
(13.) Kline, S. J. and McClintock, F.A., "Describing Uncertainties in Single-Sample Experiments," Mechanical Engineering, Vol. 75, No. 1, 1953, pp. 3-8.
(14.) Rood, E.P. and Telionis, D.P., "Editorial," and "Policy on Reporting Uncertainties in Experimental Measurements and Results," Journal of Fluids Engineering, Vol. 113, No. 3, 1991, pp. 313-314.
Matthew J. Sitzmann is a senior engineer and Steve R. Smith is an engineer at Haag Engineering, Co., Irving, TX. Frank K. Lu is a professor in the Mechanical and Aerospace Engineering Department at the University of Texas at Arlington, Arlington, TX.