A thermodynamic model for the design of ejectors is described, validated and applied for conditions prevailing in a multiple effect desalination system with thermal vapour compression. The model, which calculates all the dimensions of the ejector, incorporates several innovations. Firstly, it uses polytropic efficiencies to account for irreversibilities during the acceleration and deceleration processes thus taking into consideration the effect of the pressure ratio on the corresponding losses. Secondly, the proposed parametric relations between the pressure at the fictive throat and at the primary nozzle outlet constitute a generalization of particular geometries and operating conditions treated in previous studies. Thirdly, it considers mixing processes which take place with simultaneous pressure and area changes. The model has initially been used to determine the ejector dimensions and fluid properties for a base case. Parametric studies have then been conducted to demonstrate the existence of design choices which minimize the size of the primary nozzle and the length of the constant diameter duct (C) 2015 Elsevier B.V. All rights reserved.