Retrospective on Cubic Equation of State for R134a Refrigerant Used in Automotive Application 2013-26-0061
The need for a consistent and reliable calculation of thermodynamic property of refrigerants has been a topic of research since the past decade. This paper reports a study of various cubic equations of state for a refrigerant being used in automotive air-conditioning applications. The thermodynamic property of refrigerant 1,1,1,2 tetrafluoroethane (commercially known as R134a) is estimated for this purpose. A comparative analysis is made on three sets of equations of state. They are Redlich Kwong equation (RK), Peng Robinson equation (PR) and Patel Teja equation. It is found that the Patel-Teja and Peng-Robinson equations are accurate in the operating region of automotive air-conditioning system. Using these literature based equations and Maxwell correlations, thermodynamic models are developed. They estimate thermodynamic properties of saturated liquid/vapor, sub-cooled liquid and superheated vapor phases. Simulation was carried out to generate saturation curves (pressure-density, pressure-enthalpy and temperature-entropy). They show excellent agreement with established values from the NIST database. Our studies show accuracy close to 96% up to a temperature of 4 K below the critical point. The percentage errors in entropy and enthalpy are in general, far less than the percent errors in density for same temperature and pressure. These models are computationally less expensive and hence are advantage. Future work involves using these thermodynamic models for the dynamic simulations of refrigeration cycles.