Summary

Phase-out of halocarbons refrigerants due to their contribution to ozone layer depletion and global warming gave rise to a new era in refrigeration. Nowadays, in addition to aspects related to the performance in the thermodynamic cycle, refrigerants must also be safe and environmental-friendly. Because of their unique characteristics, natural refrigerants such as R-600a (isobutane) and R-744 (carbon dioxide), are powerful candidates for substitution of CFC, HCFC and HFC refrigerants. Thermodynamics of refrigerants and lubricant mixtures is currently a very active research field. However, despite the number of experimental data and advanced models available in the open literature, there is still a dearth of specific studies regarding the combination of natural refrigerants with synthetic oils such as polyol ester (POE) and alkylbenzene (AB). In the present work, thermophysical properties (solubility, viscosity and density) of R- 744 with two synthetic oils (POE ISO 56 and AB ISO 32) were determined experimentally at pressures and temperatures ranging from 5 to 120 bar and 12 to 75°C, respectively. A mixture of R-600a and a polyol ester (POE ISO 7) at pressures from 1 to 10 bar and temperatures from 10 to 60°C was also investigated. For all mixtures, the solubility was determined through a gravimetric analysis and the liquid density was measured using a Coriolis Effect mass flow transducer. Mixture viscosity was determined only for the R-600a/POE ISO 7 system using a commercial oscillating piston viscometer. The experimental data were analyzed and correlated in the light of thermodynamic models, such as excess properties and the Peng and Robinson (1976) equation of state. Viscosity was correlated using the Eyring Theory (Glasstone et al., 1941) for the Excess Energy of Activation for Viscous Flow. Empirical relationships were proposed when critical properties and molecular masses of the lubricants were not available. In general, the behaviour of the R-600a/POE mixture was close to that of an ideal solution under the conditions of the present study. On the other hand, significant departures from ideality were observed for R-744/POE and R-744/AB mixtures.

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