Research on Cooperative Control and Stability of Variable Refrigerant Flow Refrigeration System

Abstract

Variable refrigerant flow (VRF) refrigeration systems use variable flow regulation technology to better match the system load. But the frequency of the compressor and the opening of the electronic expansion valve (EV) directly affect the system's stability and performance. To study cooperative control and stability of performance parameters (superheat, refrigeration capacity, and coefficient of performance (COP)), a small variable frequency rolling rotor refrigeration system of R32 was set up. Experimental research on the stability of the system performances was studied with the effect of superheat, then collaborative control of the fitting model was established. The test campaign was carried out on the R32 variable refrigerant flow refrigeration system with a cooling capacity of 5 kW, compressor frequencies from 40 Hz to 60 Hz, pressure ratio from 2.93 to 3.2, and expansion valve opening from 20% to 80%. Results showed that (1) with the increase of refrigerant mass flow, the system came into the unstable oscillation range of the minimal stable signal (MSS) line, and then the system performances (e.g., superheat) started to oscillate. So, the superheated vapor flow and mist flow alternated, which led to the change of the heat transfer mechanism. (2) There was one inflection point (minimum stable superheat point) for the refrigeration capacity at the certain superheat for the different compressor frequencies. When superheat was near the minimum stable point, it was the best choice to balance refrigeration capacity (or coefficient of performance) and the control difficulty. (3) When the compressor frequency was from 40 Hz to 60 Hz under constant pressure ratio, the increase of the compressor frequency had a more direct impact on the compressor power consumption compared with refrigeration capacity. Therefore, there was a negative correlation between the COP and compressor frequency. (4) When the expansion valve opening was between 28% and 32% and the compressor frequency was from 44.5 Hz to 46.5 Hz, it was more difficult to control superheat. The relationship between the compressor frequency and the EV opening could reduce the phenomenon of overshoot or hysteresis. And, the system could reach a stable state quickly with the method of the synchronous control.