Minggu, 27 Januari 2013

A Comparison of an R22 and an R410A Air Conditioner Operating at High Ambient Temperatures

R22 and R410A split air-conditioning systems were tested and compared as outdoor temperature ranged from 27.8 °C (82.0 °F) to 54.4 °F (130 °F). The R410A system tests were extended to 68.3 °C (155.0 °F) ambient temperature with a customized compressor. Capacity and efficiency of both systems decreased linearly with increasing outdoor temperature, but the R410A system performance degraded more than the R22 system performance. Operation of the R410A system was stable during all tests, including those with the customized compressor extending up to the 68.3 °F (155.0 °F) outdoor temperature and resulting in a supercritical condition at the condenser inlet. No noticeable changes in noise level or operation of the system was noted.
Operation of an air conditioner at elevated ambient temperatures inherently results in a lower coefficient of performance (COP). This conclusion comes directly from examining the Carnot cycle. The COP relation, COP=Tevap/(Tcond-Tevap), indicates that the COP decreases when the condenser temperature increases at a constant evaporation temperature. This theoretical indication derived from the reversible cycle is valid for all refrigerants. For refrigerants operating in the vapor compression cycle, the COP degradation is greater than that for the Carnot cycle and varies among fluids. The two most influential fundamental thermodynamic properties affecting this degradation are a refrigerant’s critical temperature and molar heat capacity. (e.g., McLinden 1987, Domanski 1999). For a given application, a fluid with a lower critical temperature will tend to have a lower COP.
The lower critical temperature of R410A versus that of R22 (70.1 °C (158.1 °F) vs. 96.2 °C (205.1 °F)) indicates that degradation of performance at high ambient temperature should be greater for R410A than R22.
Wells et al. (1999) investigated split system air-conditioning units in the 12 to 13 SEER range.Their test results were normalized with respect to cooling capacity at the 35.0 °C (95.0 °F) outdoor condition. The R22 system cooling capacity decreased by 14 % at an outdoor temperature of 51.7 °C (125.0 °F). The R410A system cooling capacity decreased nonlinearly by 22 % at the same condition. EER at 51.7 °C (125.0 °F) decreased by 35 % and 42 % for the R22 and R410A systems, respectively. Their study showed that performance varied between units equipped with a TXV, short tube, or capillary as the expansion device.
Motta and Domanski (2000) simulated the performance of R22 and four of its alternatives in an air conditioner as the outdoor air temperature ramped from 25.0 °C (77.0 °F) to 55.0 °C (131.0 °F). When the performance of the R410A system was normalized with respect to the performance of the R22 system over the entire temperature range, the R410A system EER was approximately 2 % lower at 25.0 °C (77.0 °F) and 6.5 % lower at 55.0 °C (131.0 °F). Their simulations also included the addition of a liquid line to suction line internal heat exchanger. It was shown in the simulations that the 6.5 % loss in COP (EER) could be reduced to 3.2 % by the addition of an internal heat exchanger.

ANSI/ASHRAE Standard 37-1988. Methods of testing for rating unitary air-conditioning and heat pump equipment. American Society of Heating, Refrigerating and Air-Conditioning Engineers. 1791 Tullie Circle NE, Atlanta, GA, USA.
McLinden, M.O., Klein, S.A., Lemmon, E.W., and Peskin, A.P. 1998. NIST thermodynamic and transport properties of refrigerants and refrigerant mixtures – REFPROP (Version 6.01), National Institute of Standards and Technology – Physical and Chemical Properties Division (Boulder, Colorado). 

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