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R046, Page 7 The results clearly show that the lower the GC outlet temperature is (i.e. the lower gas cooler pressure), the more obvious will the concave shape of the temperature profile be. As mentioned above, the concave shaped temperature profile will influence the heat exchanger size (Area). Therefore, the lower GC outlet temperature is, the bigger the required heat exchanger size will be to transfer a certain amount of heat. However, when selecting the GC’s outlet temperature, one also needs to consider its influence on the system COP. As showed in figure 9, the lower the GC’s outlet temperature is, the higher the system COP will be, due to the reduction of throttling losses in the expansion valve by reducing the GC’s outlet temperature. 4.5 4 3.5 3 2.5 2 1.5 30 34 38 42 46 Tgas cooler outlet (°C) Figure 9. Carbon dioxide transcritical refrigeration cycle’s COP vs. GC outlet temperature at different evaporation temperature 6. CONCLUSIONS In this paper, a basic carbon dioxide transcritical refrigeration cycle has been analyzed to show the influence of supercritical carbon dioxide specific heat’s sharp variation on the heat exchanger performance. The results calculated in EES and Refprop 7 show that due to the sharp variation of supercritical carbon dioxide’s specific heat, the temperature profile in the gas cooler will show a concave shape. Due to the shape of the temperature profile, the temperature difference, which is the “driving force” for heat transfer to take place, will be much smaller inside the gas cooler than at its ends. Therefore, • To remove a certain amount of heat from the gas cooler, the required heat exchanger surface will be much bigger than the one without such a shape of temperature profile. • The logarithmic mean temperature difference, which is calculated by the measured temperature difference at the heat exchanger ends, will over predict the real temperature difference for the heat exchanger (gas cooler). • The UA value, which is calculated by the measured logarithmic mean temperature difference, will be under estimated. It is also found that the gas cooler’s outlet temperature has a crucial influence on the value of optimum gas cooler pressure and consequently the temperature profile’s shape in the gas cooler. The higher the gas cooler outlet temperature is, the less concave shape the temperature profile inside the gas cooler will be, which is an advantage from the heat exchanger design viewpoint. However, higher gas cooler outlet temperature will also leads to a lower COP for the system. COP at - 5 °C evaporation temperature COP at 0 °C evaporation temperature COP at 5 °C evaporation temperature IHX GC Cp Internal Heat Exchanger Gas Cooler specific heat (-) Subscripts (-) a-f cycle route point (kJ/kg•k) g-h points for air properties NOMENCLATURE International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006 COPPDF Image | Analysis of Supercritical CO2 Heat Exchangers in Cooling
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