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In heating mode the combination of the heat exchanger area and airflow rate determines the evaporating temperature (assuming that the exit quality is fixed by a suction accumulator and the refrigerant flow rate is determined by the indoor capacity requirements). As the airflow rate is increased the evaporating temperature increases which results in higher cycle efficiency. In Figure 4.20 the relationship between airflow rate, cycle and system efficiency is shown for the 8.3oC outdoor condition with a supply air temperature of 40oC. In Figure 4.21 the required heat exchanger area as a function of airflow rate is shown. In cooling mode, the refrigerant pressure in the outdoor coil becomes an variable in addition to the airflow rate over the outdoor coil. In Figure 4.22 the optimum discharge pressure in terms of cycle efficiency is plotted as a function of airflow rate for the 45oC outdoor cooling condition with an indoor evaporating temperature of 12oC. The effect of airflow rate on system efficiency is also shown. For the transcritical R744 cycle, because the optimum discharge pressure is above the minimum possible, increasing the air flow rate does not effect the optimum discharge pressure. Therefore, the cycle efficiency is constant. For the subcritical R410A cycle, however, the optimum pressure corresponds to the minimum possible pressure. As a result, as the airflow rate is increased the optimum pressure decreases and system efficiency improves. 7 6 5 4 3 2 1 0.0 R744 Cycle R410A System 0.1 0.2 0.3 0.4 0.5 R410A Cycle R744 System Airflow Rate (kg/s per kW heating cap.) Figure 4.20 Effect of airflow rate on heating cycle and system efficiency for 8.3oC outdoor heating condition, 40oC supply air temperature 39 Heating COPPDF Image | Comparison of R744 and R410A
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