A review on the operational instability of vapor compression system

Operational instability, or hunting, is widely observed in vapor compression systems (VCSs), negatively affecting the operational safety and efficiency. As one of the most significant challenges present in VCS operation and control, operational instability has been extensively studied for decades. This paper reviews the current research state on the operational instability of VCSs, including the mechanisms for triggering hunting, the measures to mitigate hunting and the related unsolved issues. Two different views, i.e. the inherent characteristics of two-phase evaporating flow and dynamic characteristics of expansion (EV)-evaporator control loop, were classified for explaining the causes of hunting. As the typical characteristics of two-phase evaporating flow, the present of slug flow upstream in an evaporator, the sudden variation of heat transfer mechanism or two-phase flow instability would trigger the unexpected change of refrigerant temperature at evaporator exit. Consequently, operational instability will be resulted in. The dynamic behaviours of the EV-evaporator control loop in terms of its nonlinearities were considered as another essential factor that would cause system instability. Superheat setpoint adjustment and adaptive superheat control were suggested as two effective measures for mitigating hunting. With the increasing applications of vapor compression cycle for electronic cooling and the multi-evaporator VCSs, more effects should be done to investigate their operational instability with the consideration of two-phase flow instability and the coupling influences of each EV-evaporator control loop.