The operational stability of a refrigeration system having a variable-speed compressor (VSC)

Abstract

Variable-speed compressors (VSCs) are increasingly used for better capacity control in refrigeration systems. In response to system load changes, compressor speed may have to be continuously altered from time to time for the purpose of capacity control. However, the refrigerant mass flow rate provided by a thermostatic expansion valve (TEV) or an electronic expansion valve (EEV) may not be able to simultaneously match the amount of refrigerant as determined by the compressor speed variation to cope with the load changes, so that certain system operational parameters such as the degree of refrigerant superheat (DS) would likely oscillate. Such unstable operational phenomena, which have been well known historically as hunting in constant-speed compressor refrigeration systems, have been already observed in a refrigeration system having a variable-speed compressor. This thesis firstly presents an experimental study on qualitatively determining the relationship between minimal stable superheat (MSS) and the cooling load imposed on a refrigeration system. A series of steady-state experiments have been designed and carried out in an experimental VSC direct-expansion (DX) air conditioning (A/C) plant where a TEV and an EEV were installed in parallel, to evaluate the MSS at different cooling loads. The experimental results showed that the so-called MSS line did exist in both TEV- and EEV-controlled VSC DX A/C systems. Based on the experimental results, a modified MSS line which possessed a piece-wise function shape instead of a monotone function shape as previously suggested by others, having a minimum and a maximum MSS value, has been proposed. Secondly, the thesis reports on an analysis on the operational stability of a VSC-EEV DX A/C system due to the changes in its compressor speed for the purpose of capacity control, using the modified MSS line and supported by a series of specifically designed experiments. The analysis showed that if the DS setting was lower than the maximum MSS value, increasing compressor speed may lead to the hunting of DS if the final operating point was in the unstable region. On the contrary, if the DS setting was greater than the maximum MSS value, no hunting of DS would occur. On the other hand, decreasing compressor speed may cause hunting of the DS during transit from the original to the new compressor speed, but no hunting of DS was expected after the system reached a steady state operation at the new compressor speed. It was also demonstrated that it was beneficial to operational stability to limit the magnitudes of speed reductions, but potentially at the expense of a lower sensitivity for a capacity controller involving varying compressor speed. Finally, a further theoretical analysis of the operational stability in an EEV-evaporator control loop using the classical control theories including Transfer Function and the Nyquist stability criteria, is reported. A dynamic model of the EEV-evaporator control loop has been developed on a basis of mass and energy balance at the superheated region of the evaporator. A number of factors influencing the operational stability of the EEV-evaporator control loop including the flow coefficient of an EEV, the proportional gain and the integral time to be used in a PI controller of the EEV were studied. The analysis suggested that a smaller flow coefficient of an EEV, a smaller proportional gain and a larger integral time in the PI controller settings would lead to a better operational stability for an EEV-controlled refrigeration system.