Theoretical And Experimental Study On A5kW Quasi-two Stage Compression Heat Pumping System

The quasi-two stage heat pumping system is able not only to overcome thedifficulties that conventional heat pump faced when operating at low ambient temperature,but also to satisfy the needs of heating in cold regions. Designing and operation of heatpump which employed vapor injection techniques are much more complicated than that ofconventional heat pumping system, due to its additional injection rout and injection port incompressor. The location of injection port and the impact on the system performanceposed by the variation of refrigerant injection ratio were studied.The theoretical thermodynamic model of the quasi-two stage heat pumping systemwas built based on its features and the injection process analysis. Then the systemperformances were calculated under different evaporating temperature of-25to-5℃andcondensing temperature of41℃, with different vapor injection port location. Resultsshowed that the optimum volume specific ratio for first-stage compression was1.1, thusthe optimum injection port location was found. Besides, it was also found that theperformances of quasi-two stage heat pump were better than those of conventional heatpump. Moreover, the actual thermodynamic model which involved the calculations of heatexchangers and expansion valves was built. System performances were calculated withfixed injection port location and different vapor injection ratio based on this model.Results showed that when the vapor injection ratio increased, heating capacity increasedthen decreased as well as the heating COP, and the electrical power decreased and thenincreased. Therefore the optimum injection ratio was found in the range of25%~30%. Aprototype was installed and tested. The experimental results were found to be accordancewith the calculation results to some extent.The study showed that the refrigerant injection techniques can improve the systemperformance significantly. And the results can be used as a valuable reference for designand application of quasi-two stage heat pump system.