A PC quickbasic program was developed in the aim to design a jet-pump device for a water refrigeration cycle. At given conditions of generator, evaporator and condenser, with a defined cooling power, the design is automatic, yielding to a configuration which allows the maximum pressure gain the system can achieve. The physical model used is the classical one-dimensional with constant-area mixing. A second stage is used when the single one cannot achieve the necessary compression ratio,due to boundary condition. An annular primary flux is used for the second stage So a compact structure is obtained leading to an efficient utilization of kinetic energy of the secondary stream. The failure of perfect gas model for steam is discussed so a third stage was excluded. The comparison with some experimental literature data shows a good agreement for single stage model. The multistage annular jet-pump appears as a very promising device in the field of refrigeration.