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on the pinched condition, the temperature difference between the superheat region and the air stream is reduced, which decreases the overall heat transferred weighted average temperature difference. 3.4 Ideal system performance The analysis to this point has considered the ideal cycle where the required system capacity is met by adjusting the refrigerant mass flow rate via a variable displacement compressor. In both heat pump and air conditioning mode the evaporator air temperature determines the most extreme operating conditions: warm outdoor conditions in air conditioning mode and cold outdoor conditions in heat pump mode require the highest compressor displacement to maintain capacity. In a typical residential application, it is reasonable to base the maximum compressor displacement on the load requirements at the 45oC outdoor operating condition. In Figure 3.10 the ideal cycle capacity in heat pump mode is plotted for a supply air temperature of 21oC based on a fixed compressor capacity corresponding to the normalized compressor capacity required for a 12oC evaporating temperature and a 45oC sink temperature (0.54 m3/hr per kW cooling capacity for R410A and 0.21 m3/hr per kW cooling capacity for R744). The capacity of R744 is significantly higher than for R410A, which has significant practical implications in terms of overall heating efficiency. Because of the reduction in capacity inherent in heat pumps, heat pump systems require some sort of low-efficiency supplementary heating. The increased capacity of R744 at lower outdoor temperatures reduces the dependence on supplementary heating, which increases the overall heating efficiency. In Figure 3.11, the cycle efficiency corresponding to Figure 3.10 is shown. 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 R744 R410A -20 -15 -10 -5 -0 5 10 Evaporating Temperature (C) Figure 3.10 Effect of fixed displacement compressor on ideal heat pump cycle capacity 20 Capacity (kW)PDF Image | Comparison of R744 and R410A
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