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5 – Experimental Results reheating gas cooler was mainly determined by the discharge gas tempe- rature from the compressor, the outlet water temperature from the DHW preheating gas cooler and the water flow rate (i.e. the CP-value2). Due to the large heat transfer surface of the DHW preheating gas cooler, the outlet water temperature dropped off only 3 K when the mass flow rate was increased from 0.53 l/min at 8 MPa high-side pressure to 0.90 l/min at 9.5 MPa. The cooling effect of the increasing water flow rate was more or less offset by the rise in the discharge gas temperature from the compressor, and the outlet CO2 temperature from the DHW reheating gas cooler T2 was therefore practically constant at varying high-side pressure. 5.1.3.3 Hot Water Heating Only The Maximum COP vs. the Optimum High-Side Pressure With reference to Figure 3.10 in Section 3.2.4.1, Principle System Design, the space heating gas cooler (B) was by-passed, and heat was delivered solely from the DHW preheating (A) and reheating (C) gas coolers units in the DHW mode. Whereas the power input to the compressor was virtually proportional to the high-side pressure, there was a significant change in the total gas cooler heating capacity when the high-side pressure was increased. Hence, for each measuring series with fixed set-point for the hot water tempera- ture, there was a maximum COP which corresponded to an optimum high- side pressure. This is demonstrated in Figure 5.27, where the heat rejec- tion process in the two gas cooler units is displayed by means of T-Q dia- grams. For the selected measuring series, the evaporation temperature was -5oC and the hot water temperature was 70oC. At 9 and 9.5 MPa high-side pressure, the heat transfer process was con- siderably hampered by a pinch point inside the gas cooler units, which in turn limited the water flow rate in the hot water circuit and led to a very large temperature approach. By increasing the high-side pressure to 10 MPa, the temperature fit between the CO2 and the water was greatly improved, and the temperature approach of the DHW preheating gas cooler unit dropped off to 3.5 K. Since an additional 0.5 MPa rise in the high-side pressure only had a marginal effect on the temperature approach and the total heating capacity of the gas cooler units, the maximum COP of 3.58 was measured at 10 MPa high-side pressure. 2 The product of the specific heat capacity and the mass flow rate. 134PDF Image | Residential CO2 Heat Pump System for Combined
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