A numerical optimisation of a steam ejector refrigeration plant have been carried out, based on the Complex optimisation method. The plant is treated as an open system exchanging thermal power with three water flow rate. Ejector is a two stage with annular primary in the second stage. The heat exchangers are shell and tube with external water in the tubes. For the evaporator the solution with shell and tube is compared to that with flash chamber. Better performances are obtained for the plant with flash evaporation. In this plant configuration, a very little evaporation area and, therefore, short heat exchanger dimensions are obtained. However, the numerical flash evaporation model used requires high solving time and gives worse statistical distribution of simulation results.