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This analysis assumes the presence of an internal heat exchanger in the R744 cycle, but not one in the R410A cycle. It was noted that the presence of an internal heat exchanger provided a benefit to the R744 cycle, where no benefit would be realized in the R410A cycle. In Figure 4.10 the cycle efficiency is plotted as a function of outdoor temperature for a fixed evaporating temperature of 12oC for both refrigerants with and without an internal heat exchanger. From the figure it is evident that the efficiency improvement using a suction line heat exchanger with R410A in cooling mode is minimal, and would probably be negative if pressure drop in the heat exchanger were considered. In the following heat pump cycle analysis, an internal heat exchanger is included in the R744 cycle, but not in the R410A cycle. 6 5 4 3 2 1 34 36 38 40 42 44 46 Ref. Exit Temperature from Gas Cooler (C) Figure 4.10 Effect of internal heat exchanger on cooling cycle efficiency 4.4 Heat pump cycle 4.4.1 Cycle analysis The analysis in Section 3.2 of the ideal heat pump cycle neglected compressor inefficiencies and found that R410A was more efficient than R744 at lower supply air temperatures, and approximately equal at higher supply air temperatures. When compressor efficiency is included in the analysis, shown in Figure 4.11, R744 becomes more efficient at supply air temperatures above about 40oC, depending on the evaporating temperature. This relative improvement for R744 results from the fact that the compression ratios required for a high supply air temperature are much lower than for R410A, which translates into higher efficiency for the R744 compressor. Additionally, because R744 has a higher refrigerant side heat transfer coefficient, the evaporating temperature could be one-half degree higher for the same outdoor ambient temperature. This would result in approximately a 2% increase in the heating COP of R744 as compared to R410A for the same outdoor temperature (based on an evaporating temperature of 2.3oC, a supply air temperature of 40oC, and a refrigerant exit temperature of 21oC). Figure 4.12 R410A--w/ IHX R410A--No IHX R744--w/ IHX R744--No IHX Evaporating Temperature: 12 C 31 Cooling COPPDF Image | Comparison of R744 and R410A
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