Experimental research on instability of expansion valve–dry evaporator refrigeration system

Abstract

The instability of the refrigeration cycle makes it difficult to control the system stably and decreases the system efficiency. In this article, the stability of expansion valve-dry evaporator (EVDE) refrigeration cycle has been systematically studied near “0 superheat”. First, a special-designed sensor had been developed to measure “0 superheat”. This method could distinguish the refrigerant flow states. Therefore, the values measured by the thermal sensor could be used as a feedback signal for the EVDE system. Second, there were liquid-vapor and bubble-vapor flows at the inlet of the evaporator, and a transition region occurred between them. When the compressor frequency was 70 Hz and the expansion valve opening was equal to 27%, the two flow patterns alternated, which leaded to the system periodic oscillation (for example: evaporating temperature, suction temperature and discharge temperature). In addition, the increase of refrigerant flow rate would reduce the periodic oscillation. Moreover, when the compressor frequency was 70 Hz and the expansion valve opening was less than 22%, the flow pattern was dominated by superheated vapor flow at the outlet of the evaporator. When the expansion valve opening was more than 23%, the flow pattern was dominated by misty “wet” vapor flow. The two flow patterns alternated, which leaded to suction temperature fluctuation. And a transition region also occurred between them. The results showed that EVDE refrigeration cycle could be controlled stably nearby “0 superheat”, and it could coordinate the efficiency of the dry evaporator and the safety of the compressor to optimize the system.