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Figures 4.15 and 4.18 can be correlated for the 40oC supply air case to estimate the reversible performance of an indoor coil of specified air-side area. Based on these results, an airside area of 2 m2/kW capacity would meet the heating capacity within 5% of maximum efficiency, and the cooling capacity within 5% of minimum airflow rate for both R410A and R744. As a result, the indoor coil sizing considerations are basically the same for both refrigerants. For a supply air temperature of 60oC, the airflow rate required would approximately half per kW capacity as the 40oC supply air case (assuming an air inlet temperature of 21oC). Therefore, assuming a heat transfer coefficient dependence on Re0.8, the heat transfer coefficient for a supply air temperature of 60oC would be 57% of that for a supply air temperature of 40oC. For R744 the airside area would need to be increased nearly 70% (based on the assumptions in Table 4.1) to compensate for the reduction in heat transfer area; for R410A the area would need to be increased by over 60%. As a result, for supply air temperatures of 60oC and reasonable efficiency, heating performance of the indoor coil would be the primary design consideration. Alternatively, indoor coil designs that adjust depending on if they are heating or cooling may need to be developed. A concept of this is shown in Figure 4.19. Heat Pump Air Conditioning Figure 4.19 Conceptual diagram of indoor coil for reversible air conditioning and heat pump operation (side view). Slats at either end could rotate to increase the area for airflow by a factor of the number of passes, maintaining the air side heat transfer coefficient for large differences in flow rate. 4.5.2 Outdoor coil sizing Because the airflow rate over the outdoor coil is not constrained (as it is for the indoor coil based on comfort) there is an additional variable in terms of required heat exchange area. Since the airflow rate effects the required heat exchanger area, the effect of fan power on system COP needs to be included in order to get an accurate assessment of the effect of heat exchanger size on system performance. In order to incorporate the effect of fan power on system COP the following relationships are used: W =v& ⋅∆P air outdoor fan COP = system (4.5) The fan efficiency (ηfan) is assumed to be 0.5. Similar to the assumptions regarding the air-side heat transfer coefficient in Table 4.1, the air side pressure drop over the outdoor coil (∆Poutdoor) is assumed to be 45 Pa at an airflow rate of 0.15 kg/s, and is assumed to vary as Re2. η fan Qindoor (4.4) Wfan +Wcompressor 38PDF Image | Comparison of R744 and R410A
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