Summary

In mechanical vapor compression cooling systems, the working uid is a mixture of refrigerant and lubricant oil, as a fraction of the latter is pumped by the compressor together with the refrigerant to the other system components (condenser, evaporator and expansion device). The main functions of oil are sealing and reducing friction and wear of solid parts inside the compressor. The solubility of refrigerant oil must be guaranteed to ensure the return of the circulating oil to the compressor. Similarly, inside the compressor, a high solubility contributes to the decrease of the equalization pressure (i.e., the pressure attained by the system at the end of the compressor off-cycle), which reduces the torque at the motor start-up and the amount of copper in the electric motor. Although the oil is essential in order to ensure good performance and reliability of compressor, its dissolution in the refrigerant flowing through the system components decreases the vapor pressure of the working fluid, thus reducing the specific refrigerating effect, the cooling capacity and the coeffcient of performance. Additionally, the thermophysical properties of the mixture must be accurately known since, for example, due to the large difference between the viscosities of pure oil and pure liquid refrigerant, even small amounts of refrigerant dissolved in the oil can significantly alter the viscosity of the mixture and reduce the lubrication of bearings and piston-cylinder clearance, which increases mechanical losses and compromises the reliability of the compressor. Therefore, a detailed knowledge about the phase equilibrium of mixtures of refrigerant and lubricant oil and the effect of oil solubility on the thermodynamic and thermophysical properties of the refrigerant is essential to assess the effect of the oil circulation ratio in the system performance. The objective of this thesis is to investigate by means of experimental and theoretical analyses the thermodynamics of refrigerant-oil mixtures and the absorption of refrigerant in lubricating oil. The mixtures evaluated in this study involve refrigerants of low environmental impact, such as isobutane (R-600a) and carbon dioxide (R-744). Models based on cubic equations of state (Peng-Robinson, SRK) and on the PC-SAFT equation of state were implemented, and the interaction coefcients were determined from experimental data collected in an experimental apparatus that allows for the simultaneous determination of solubility, viscosity and density of refrigerant-oil mixtures. The thermodynamic models were incorporated into methods for calculating the viscosity of mixtures based on the Eyring Theory of activation energy for viscous ow and on the f-Theory, which showed good agreement with experimental data (RMS errors of the order of 1% for the R-600a/LAB ISO 5 mixture and 3% for the R-744/POE ISO 68 mixture). Regarding the prediction of thermodynamic properties and the parameters of the standard vapor compression refrigeration cycle, thermodynamic models based on cubic equations of state have been used in conjunction with the theory of departure functions to determine the enthalpy, entropy and specific volume of the refrigerant-oil mixtures. An algorithm was developed for calculating the condensing and evaporating pressures and the coeffcient of performance as a function of the oil circulation ratio, the temperatures of the cold and hot sources, the overall thermal conductances of the heat exchangers and compressor geometric parameters. The results demonstrate that the oil circulation ratio is always detrimental to the performance of the refrigeration system, with the reduction of the coefficient of performance becoming more pronounced as the oil circulation ratio increases. In order to investigate the absorption phenomena that occur during the compressor off-cycle, a model was developed for calculating the rates of refrigerant vapor absorption in a stagnant oil layer in a closed system (PVT cell). The model is based on Ficks law for mass diffusion in the liquid layer and takes into account the departure from the ideal solution behavior in the calculation of the interfacial boundary condition and in the determination of the mixture thermophysical properties. The model was validated against experimental data for pressure decay for mixtures involving R-600a and R-744. Average absolute errors for the pressure predictions lower than 4% were observed for both mixtures.

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