Dynamic simulation of refrigeration systems needs computer routines for calculation of the thermodynamic properties of refrigerants. These routines must be simple, and computationally fast with accuracy adequate for commonly encountered refrigeration condition. An approach, widely used, is proposed by Cleland in the form of curve-fitted equation. The execution speed of these equations is fast, but their accuracy levels decrease as the duration of simulation increases. This paper proposes a solution of this problem based on new system of equations. The proposed solution enables to obtain more accuracy and a fast execution speed. A more recent thermodynamic database is used in the equations curve-fits. The equations system proposed enables a modelling of the refrigeration systems with a fast execution speed and a better accuracy than that obtained by a Cleland's equations.