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1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 234567 Compression Ratio 7 cylinder swashplate, 33 cc (1999) Single cylinder recipricating semi-hermetic (Richter, 2000) 10 cyl swashplate, 20.8 cc, ext. driven (1996) Single-stage pre-series R744 compressor (Neksa, 1999) Figure4.2 R744compressorefficiency The linear fits used are: P P R410A: ηisentropic = 0.8134 − 0.05296 ⋅ dis ηvolumetric = 1.0214 − 0.0338 ⋅ dis (4.1) (4.2) The linear assumption breaks down at very low compression ratios, so this analysis is limited to conditions having a compression ratio greater than two. This limitation is not overly restrictive on the air conditioning cycle, because such conditions are encountered only momentarily at the beginning of the pulldown process when an air conditioner is first turned on. In the heat pump cycle, this assumption limits the lowest operating pressure of the condenser or gas cooler, and can constrain the theoretical minimum supply air temperature. 4.2.2 Variable vs. fixed displacement Use of a variable displacement compressor is advantageous in that the flow of refrigerant can be adjusted to match the load condition. With a fixed capacity compressor, because efficiency depends on temperature (pressure) lift, the cycle has maximum capacity at the smallest indoor/outdoor temperature difference where the heating or cooling load is the smallest. As a result the load and capacity curves are “mirrored” from each other, with the highest load having the least capacity. To illustrate this, Figures 4.3 and 4.4 show the capacity and efficiency of theoretical R744 and R410A systems with a fixed capacity compressor described by Equations 4.1 and 4.2. A linear relationship between the outdoor temperature and the required load is assumed. The efficiency shown is the maximum cycle efficiency, and, as such, the heat exchangers are assumed to be infinite. In heating mode, the evaporating temperature is assumed to be equal to the outdoor temperature and the results for two different supply air temperatures are shown. In cooling PP suc suc P P R744: ηisentropic=0.8536−0.0577⋅ dis ηvolumetric =1.028−0.097⋅ dis PP suc suc 25 Isentropic EfficiencyPDF Image | Comparison of R744 and R410A
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