A performance optimization system has been studied for refrigeration plants based on vapor compression cycles. The water temperature at the evaporator exit is maintained at the requested value when the refrigeration capacity varies. Minimum power consumption is also achieved. The method is based on the calculation of valve opening set point law. Valve openings are related to the external quantities: water mass flow in the condenser and evaporator, temperature of the water at the condenser inlet, compressor and electric motor serviceability, fouling of heat exchangers, etc. Results of an investigation carried out on a 200 kW refrigeration capacity plant are given and discussed.