Investigation on improving the heating performance of a heat pump using a rotary compressor with vapor and two-phase injection

Refrigerant injection technique can be applied to heat pumps in cold regions to enhance performance and reliability, but currently this technique is mainly used in scroll compressors, and relatively few applications are for rotary compressors. In this paper, the performance of a vapor and two-phase injection (VTI) heat pump with a rotary compressor is experimentally studied under different injection pressures and compressor frequencies. Meanwhile, a numerical model with satisfactory reliability and practicality is developed to predict the heat pump performance. According to the prediction model, the optimal injection pressure for maximal heating capacity and efficiency is analyzed as a function of the injection ratio under different working conditions and the risk of wet compression is analyzed under various operating conditions. The optimal heating capacity and COP are obtained simultaneously under an injection pressure equaling the geometric mean of the evaporating pressure and the condensing pressure and an injection refrigerant state is near the saturated gas state, which improve by 23.76-42.80% and 4.22-8.96% over the non-injection compressor respectively at evaporating temperatures between 0 degrees C and-30 degrees C. For fixed-frequency compressors, a fixed injection port area is found to achieve optimal heating performance under any operating conditions, for example, it is about 2.4 mm2 under 50 Hz. Further, it is easier to form a two-phase injection under a higher injection pressure or lower compressor fre-quency, and a lower injection quality helps to decrease the discharge temperature but to increase the risk of wet compression.