Refrigerant-Oil Mixtures and Local Composition Modeling

Abstract

The solubility and miscibility refrigerant-oil mixtures are important in understanding
\nthe performance of refrigeration and air-conditioning systems. The objective of this study
\nwas to provide useful information on the thennophysical properties of new refrigerant-oil
\nmixtures and to evaluate candidate mixture models.
\nIn this report, the effect of a polyol ester lubricant on equilibrium pressure, liquid
\ndensity, and viscosity is presented for R-22, R-125, R-134a, and AZ20 (50% R-125, 50%
\nR-32) at varying temperatures and concentrations. Deviations from ideal mixture behavior
\nare interpreted using activity coefficients and may be manifested in three ways: positive,
\nnegative, or mixed deviations from Raoult's law. The departure from Raoult's law is
\nrelated to molecular size differences and intennolecular forces in the mixture. Six different
\nmodels are used to correlate the experimental data: Wilson's relation, the Heil equation, the
\nNon-Random Two Liquid (NRTL) model, the Universal Quasi-Chemical (UNIQUAC)
\ntheory, and two different extensions of Wilson's relation. Interaction parameters for all six
\nmodels are developed from the experimental data (and data from a previous ACRC research
\nproject). A comparison of the pressure predictions (activity coefficients) for each model
\nover a wide range of mixture compositions and temperatures shows that the Heil equation
\nhas the best overall penonnance. The Heil equation correlates the saturation behavior of
\nthese mixtures with an average error less than 6%. The report is organized as follows:
\nChapters 1 & 2: Background material and research objectives.
\nChapter 3: A detailed description of the apparatus and test procedure. A
\ncomparison of measurements from this apparatus to ASHRAE data.
\nChapter 4: Experimental data: mixture pressure as a function of liquid mass
\nfraction and temperature; liquid density as a function of liquid mass
\nfraction and temperature; viscosity as a function of temperature and liquid
\nmass fraction. Oil density data and limited miscibility results (see also
\nAppendix D).
\nChapter 5: Data analysis background (see also Appendix A).
\nChapter 6: Mixture modeling background (see also Appendices B, C, &E).
\nChapter 7: Interaction parameters for all mixtures and models are provided,
\nand a detailed comparison of model performance is given.
\nChapter 8: A very brief summary of conclusions and recommendations.