Analysis of optimal intermediate temperature and injection pressure for refrigerant injection heat pump systems with economiser

Abstract

A heat pump cycle with refrigerant injection is one of the effective means for enhancing the heating performance in low-temperature climates. The intermediate temperature and injection pressure have been proven to be important parameters influencing the operating performance of a heat pump system. Previous studies on optimal intermediate pressure were mostly based on empirical formulas, and the calculation result greatly depended on the available scope of the empirical formula, especially in low-temperature conditions. In this study, the optimal intermediate temperature of the refrigerant injection cycle, including the vapour injection cycle and two-phase injection cycle, was analysed through theoretical derivations. This method is based on a thermodynamic analysis of the system and the thermodynamic properties of the R134a refrigerant. The aim of the theoretical analysis was to determine the intermediate temperature and injection pressure corresponding to the maximum COP of the system. First, the factors influencing the optimal intermediate temperature of the two injection cycles were studied using the theoretical calculation model. The optimal intermediate temperature was more sensitive to the system subcooling than the suction superheat. Increasing the heat exchange efficiency of the economiser is conducive to improving the heating COP. For the same operating conditions, the optimal injection pressure was higher than the empirical value. Second, an experimental system of refrigerant injection with an economiser was built to verify the accuracy of the method. The results indicated that the optimal injection pressure calculated by the theoretical derivation method was in good agreement with the experimental values for both the vapour and two-phase injection cycles. The maximum error of the injection pressure was within ±3%.