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air/refrigerant temperature difference reducing the required heat exchanger area. For R410A, as the optimum discharge pressure is dropped by increasing the airflow rate, the air refrigerant temperature difference decreases and the required heat exchanger area increases. This is shown in Figure 4.23, which shows the effect of heat exchanger area on system COP. 4 3 2 1 Airflow Rate: R744: Solid R410A: Open 0.05 kg/s 0.15 kg/s 0.25 kg/s 0.35 kg/s 0.45 kg/s 0 0 2 4 6 8 10 12 Area (m2/kW cooling capacity) Figure 4.23 Effect of heat exchanger area on system cooling COP, 45oC outdoor cooling condition, 12oC evaporating temperature 4.6 Real cycle conclusions By specifying the isentropic compressor efficiency, the thermodynamic cycle can be specified with only an evaporating temperature, a condensing pressure and a refrigerant exit temperature from the gas cooler. Additionally, if heat transfer coefficient correlations are included then estimated can be made regarding the size of heat exchanger required for a given cycle efficiency. In cooling mode, the compression ratios and compressor efficiencies, of the two cycles are comparable. As a result, relative cycle efficiencies are the same as for the ideal cycle, and R410A shows a considerable advantage. If the air flow rates over the heat rejecting coil are matched at reasonable levels, however, then the efficiency advantage of the R410A cycle over R744 is reduced by nearly half. In heating mode, because of the lower compression ratios for R744, above supply air temperatures of about 40oC the efficiency of R744 is higher than the efficiency of R410A, approximately 10% higher at a supply air temperature of 60oC. For supply air temperatures below 40oC R410A has higher efficiency than R744, approximately 8% higher for a supply air temperature of 35oC. For systems with compressors sized such that the cooling capacity is equal, R744 has higher capacity in heating at low outdoor temperatures. In a typical application, 41 Cooling System COPPDF Image | Comparison of R744 and R410A
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