A departure-function approach to calculate thermodynamic properties of refrigerant-oil mixtures

A method based on the theory of departure functions coupled with the Peng and Robinson (1976) equation of state was used to calculate the thermodynamic properties (internal energy, enthalpy, entropy and specific volume) of refrigerant-oil mixtures. The method is exemplified with isobutane (R-600a)/alkyl benzene ISO 5 and isobutane/polyol ester ISO 7 mixtures. Pressure-enthalpy, pressure–volume and temperature–entropy diagrams are presented for overall oil concentrations ranging from 0.1% to 1%. The influence of the oil type on the working fluid properties is discussed. For a given overall oil concentration, the higher solubility of alkyl benzene ISO 5 (in comparison with polyol ester ISO 7) gives rise to larger deviations from the pure refrigerant behavior.