A control method using adaptive setting of electronic expansion valve for water chiller systems equipped with variable speed compressors

Abstract

Abstract Efficient control methods are essential for refrigeration systems that utilize variable speed compressors. In this paper, stability and performance of a water chiller system equipped with a variable speed compressor and an electronic expansion valve (EEV) were investigated. We focused on the system response in terms of both the degree of superheat (DS) and the temperature of evaporator water (product water) with three control methods. These methods are single control loop (SCL) with adaptive EEV opening, two-control loops (TCL) with adaptive DS setting, and constant DS setting. The SCL method takes into account the coupling effect between the compressor speed and the EEV opening. For the TCL method, the DS and the product water temperature are separately controlled. We tested a proportional fuzzy (PF) controller with the three methods in addition to conventional PI and PID controllers. The results showed that the application of appropriate control method may have a significant influence on the coefficient of performance (COP) and system stability rather than the type of controller. The operation with fixed superheat setting yields the lower COP compared to the SCL and the TCL method. In comparison to the PI and PID controllers, the PF controller reduces the fluctuation in the DS. The SCL method produces the maximum COP as well as good stability for both the DS and the product water temperature. Compared to the constant DS operation, the SCL method enhanced the COP by 27.5%, 18.3%, and 19.7% for the PF, PID, and PI controllers, respectively.