The Effects of Oil on the Thermodynamic Properties of Dichlorodifluoromethane (R-12) and Tetrafluoroethane (R-134a)

The presence of oil in refrigeration and air conditioning systems reduces the vapor pressure of pure refrigerants and significantly affects the thennodynamic properties of the mixture. This paper presents equilibrium pressure-temperature-concentration data experimentally measured for four refrigerant-oil mixtures. Baseline data are obtained for mixtures of R-I21naphthenic mineral oil and R-I2/paraffmic mineral oil. These data are compared with results obtained for mixtures of R-134a/ester oil and R-134a/polyalkylene glycol (PAG). The solubility effects of synthetic lubricants in R-134a are less than for R-12 and mineral oils. Using the experimental data, empirical models are developed to predict the solubility relations for each mixture at equilibrium. Raoult's Rule and Flory-Huggins theory are also used to predict mixture behavior. Idealized mixture enthalpies are calculated from pure component enthalpies and used to detennined evaporator capacity at a given inlet quality, pure refrigerant saturation temperature, and apparent degree of superheat at the evaporator outlet. Assuming standard operating conditions in a domestic refrigerator and mobile air conditioner, the reduction in evaporator capacity is consistently higher for mixtures of R-12 and mineral oil. Finally, these equilibrium models are also used to detennine the amount of liquid refrigerant held in solution as the refrigerant in an evaporator boils. In both evaporator simulations, the properties and perfonnance of pure R-I34a are affected less by the addition of oil.Air Conditioning and Refrigeration Center Project 1