Efficient refrigeration technologies with low electrical energy consumption such as the Ejector Refrigeration System (ERS) can contribute to the sustainability goals. With the aim of assessing the ERS thermodynamic performance, a decoupled numerical algorithm is proposed. The model integrates simultaneously the operation of each part of the ERS composed of three heat exchangers (generator, condenser and evaporator), a steam ejector, an expansion valve and a recirculation pump; being capable of predicting the global behavior of the cycle by describing mass, momentum and energy transport in each subsystem. The solution of the model is carried out in the Python framework, with the use of CoolProp libraries for the calculation of refrigerant properties, NumPy for mathematical operations and SciPy for numerical solutions of initial value problems. The algorithm solves each subsystem of the ERC in a decoupled manner and uses the results of each subsystem as input parameters of the next subsystem. Initially, the generator model is solved, assuming the primary mass flow. Then, with the results of the generator and assuming the pressure and temperature at the evaporator outlet, the ejector model is solved. With these results, the model of the condenser, pump and valve is assessed. Finally, the evaporator model is solved with the valve and ejector results. This process is repeated until reaching convergence with respect to the cooling conditions required in the evaporator and the agreement of both primary and secondary mass flow rates. After validating the numerical approach by comparison to experimental data from the literature, the behavior of the ERS is simulated for different operational conditions, obtained from changes in generator heat input. The proposed numerical model allows to obtain system coefficient of performance (COP) and ejector entrainment ratio (ER) as a function of ERS operational conditions, showing the system sensitivity to the changes in conditions.