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Assuming the same evaporator geometry and sensible and latent heat transfer for R410A and R744, the required evaporating refrigerant to wall temperature difference can be determined from the refrigerant side heat transfer coefficient alone. Based on the heat transfer coefficients listed in Table 4.1 (2500 W/m2 K for R410A and 10,000 W/m2 K for R744), the refrigerant/wall temperature difference in the R744 evaporator would be need to be 25% of that needed for R410A. Because the same amount of heat could be transferred across a smaller temperature difference for R744, the cycle efficiency of R744 would be improved by operating at an evaporating temperature higher than R410A. This is shown in Figure 4.7 for a range of sensible heat ratios based on the assumptions listed in Table 4.1. The effect that evaporating temperature has on cycle efficiency is shown in Figure 4.8 assuming two different outdoor temperatures: 35oC and 45oC. Over this range of outdoor temperatures the pressure ratios of R410A and R744 are both less than three, so the relative performance of the two cycles is changed little by considering the effects of a real compressor. As discussed in the ideal cycle, the higher efficiency for R410A results primarily from the fact that in Figure 4.8 the maximum outdoor airflow rate is unconstrained. In Figure 4.9 the effect of finite airflow rate is shown, assuming that the airflow rates over the outdoor coil are equal and determined by the pinched condition for R744. It is evident from Figures 4.8 and 4.9 that the operating point determined by the comfort constraint reduces significantly the overall efficiency of the system. For examp le, reducing the evaporating temperature to 12oC from 23oC reduces the efficiency by nearly half for most of the cases shown. 17 15 13 11 9 7 5 3 1 R744 R410A 0.6 0.7 Inlet air: 27 C, 50% RH 0.8 0.9 1.0 Sensible Heat Ratio Figure 4.7 Dependence of evaporating temperature on sensible heat ratio 29 Evaporating Temperature (C)PDF Image | Comparison of R744 and R410A
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