Effect of sudden changes in evaporator external parameters on a refrigeration system with an evaporator controlled by a thermostatic expansion valve

Abstract

A theoretical model is used to investigate the effect of sudden changes in the evaporator external parameters on the behaviour of a refrigeration system with a dry-expansion evaporator controlled by a thermostatic expansion valve. The analysis showed that even under stable operation, sudden changes in chilled water inlet conditions may result in unstable system for a certain period of time. Results were obtained for the 50 and 25% partial loads. The inlet chilled water temperature and mass flow rate were suddenly increased or decreased with factors F t and F m respectively. Instability of the system parameters such as evaporator and superheat temperatures, and TEV-outlet mass flow rate followed the sudden changes. The bulb temperature remained stable. Since the evaporator and the bulb temperatures are control parameters of the TEV-outlet mass flow rate, then the instability of the latter must be mainly due to the evaporator temperature instability. The unstable region vanished completely for the 50% partial loads, and continued for the 25% partial loads. The evaporator may get flooded depending on the factors applied. For example, with the 50% partial load, the evaporator has more potential to flood when sudden decrease is applied. The new stable values showed to be different than the initial ones due to the new balanced system operating parameters.