Optimum control of the gas overheat in the evaporator of a vapour compression refrigeration system when the refrigerant is polluted

Results concerning the response of a refrigeration system simulation software to an increase in the amount of oil flowing with the refrigerant are presented. It is shown that there are optima of the apparent overheat value, for which either the exchanged power or the refrigeration coefficient of performance (COP) is maximised: consequently, it is not possible to optimise both the refrigeration COP and the evaporator power. However, in most of the cases (i.e. contents of circulating lubricant), the optimal values of the apparent overheat are below the values that the expansion valves usually impose.