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As a result, a large decrease in efficiency produces very little increase in the maximum supply air temperature. However, in the system with an internal heat exchanger the exit temperature from the compressor is not fixed. Since the effectiveness of the suction line heat exchanger is by definition one in the ideal cycle, an increase in refrigerant flow that raises the exit temperature of the refrigerant from the gas cooler also raises the refrigerant inlet temperature to the compressor. A higher inlet temperature to the compressor results in a higher exit temperature, and the pinch- point is moved. This accounts for the greater increase in maximum supply air temperature in the system with an internal heat exchanger for the same drop in efficiency as a system without an internal heat exchanger. By correlating the heating COP for a given operating condition with the maximumsupply air temperature, the energy cost of comfort can be quantified. The ideal cycle performance of R744 and R410A is shown in Figure 3.6 as a tradeoff between the heating COP and maximum supply air temperature. This plot is normalized to a system capacity of 1 kW, for an evaporating temperature of 8oC, a return air temperature of 21oC, a zero approach temperature and both cycles running with an internal heat exchanger (IHX). Under these conditions, for supply air temperatures greater than about 46oC, R744 and R410A have approximately the same ideal cycle efficiency. In the R744 cycle, the suction line heat exchanger begins to increase the cycle efficiency slightly when the supply air temperature is above 43oC, up to 6% when the supply air temperature is 80oC. In the R410A cycle, the suction line heat exchanger increases the cycle efficiency less than 2% over the supply air temperature range plotted. This illustrates that, at relatively low supply air temperatures with a properly controlled system, the internal heat exchanger could be bypassed with little effect on ideal cycle efficiency. However, as previously shown, the suction line heat exchanger diminishes the effect of a non-zero approach temperature on cycle efficiency. 20 18 16 14 12 10 8 6 4 2 Refrigerant/Evaporating Temperature: R410A, -8.3 C 20 30 40 50 60 70 80 90 100 110 Maximum Supply Air Temperature R744, 8.3 C R744, 1.7 C R744, -8.3 C R410A, 8.3 C R410A, 1.7 C Figure 3.7 Effect of decrease in evaporating temperature on heating cycle efficiency The effect of decreasing the evaporating temperature is shown in Figure 3.7. As the evaporating temperature decreases, so does the evaporating pressure. More work is required by the compressor to move refrigerant across the larger pressure difference, which is reflected in reduced heating COP. In all cases the efficiency of R744 is higher at the higher supply air temperatures. The breakeven supply air temperature increases 17 Heating COPPDF Image | Comparison of R744 and R410A
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