The optimum discharge pressure of CO2-based refrigeration cycles operating under subcritical and transcritical conditions

Abstract

In the present study a simple, single stage CO2-based vapour compression refrigeration system cycle, consisting of a compressor, condenser/gas cooler, expansion valve and an evaporator is studied to find the effect of various design and operating parameters on the separation of optimal pressure line from saturation and pseudocritical pressure lines. The results show that the optimum discharge pressure line deviates markedly from the pseudocritical pressure line for the transcritical cycle. In the subcritical cycle, away from the critical point, the optimum discharge pressure is the same as the saturation pressure corresponding to the condenser exit temperature. However, near the critical point, the optimum discharge pressure is different from the saturation pressure. With the help of this study, an optimum discharge pressure equation is developed which is found to be valid for subcritical as well as transcritical CO2 cycles. Highlights The Operation of CO2-based refrigeration cycle across a critical point is analysed. Optimum discharge pressure for sub-and transcritical conditions is obtained. The effects of evaporator temperature and compressor efficiency are studied. A correlation for optimum discharge pressure valid for sub- and transcritical cycles is suggested.