A Study On The Constraining Effect Of Expansion Device In Heat Pump System

With the development of national economy and the improvement of people's living standard, the heat pump system is widely being put into use. Therefore, Further improve the heat pump'efficiency and widen its running condition become an important research project in HVAC field. When the condensing temperature is too low in refrigeration system, there is a behavior showing that the cooling capacity and the evaporation pressure of the system will drop down rapidly. Aiming at explaining this behavior, some researches found the reason is that the pressure difference of expansion device is too small, and then lead to the refrigerant flow'reduce rapidly when the valve opens completely. That means the performance of the refrigeration system is constrained by expansion device. As an extension of the subject described above, the target in this study is to research the expansion device'constraint characteristics in heat pump system under variable conditions. And some appropriate solutions to this problem are presented.In this paper, a steady-state distributed parameter model of small heat pump water heater is built with MATLAB software. And simulations about constraining characteristics of the thermostatic expansion valve and capillary are conducted. In contrast to practical expansion device, an ideal expansion device is adopted in heat pump system. And analysis about the energy consumption negative impact to heat pump system which the expansion device exerts is presented.The results show that: The heat pump system whose expansion device is expansion valve running in evaporator side variable condition, the system's heat output increases with the rising of the heat source temperature, but after the heat source temperature reaches a certain level, the heat pump's heat output is on longer increasing. And the reason is that: As the heat source temperature rises too high, the expansion valve opens up completely. Then the expansion valve has no capacity for adjusting the refrigerant flow. Therefore, the refrigerant flow is becoming stable, superheat degree is rising, and evaporator'utilization ratio decreases rapidly. As the heat pump running in condenser side variable condition, the system's heat output keeps stable with the decreasing of the water temperature in condenser'inlet, but after the inlet water temperature decreases to a certain level, the heat pump's heat output drops down rapidly. And the reason is that: the pressure difference of the expansion device keeps decreasing when the condenser temperature is dropping down, which leads to the decreasing of the refrigerant flow. As to the ideal expansion device, it can widen the system'running condition and improve its efficiency.The heat pump system whose expansion device is capillary running in evaporator side variable condition, the system's heat output increases with the rising of the heat source temperature, but heat output's increasing rate is slowing down with the rising of the heat source temperature. And when the heat source temperature is relative higher, the heat pump with shorter capillary shows better efficiency; As the heat pump running in condenser side variable condition, the system's heat output increases with the decreasing of the water temperature in condenser'inlet, and when the inlet temperature is relative lower, the heat pump with shorter capillary shows better efficiency.