rolling bearings, refrigerant compressors, lubrication quality, oil-refrigerant mixtures
Rolling bearings in refrigerant compressors are lubricated with a mixture of oil and refrigerant. This has always represented a challenge for the bearing lubrication quality estimation (e.g.: kappa or lambda parameters). Even if the mixture rate is known, the exact effect on the lubricant film thickness remains doubtful due to the unknown piezoviscosity and compressibility of the refrigerant and their variation with pressure and temperature. In this work, existing mixing laws for viscosity and piezo-viscosity are examined and adapted to better represent existing measurements. The results are compared with published Daniel plots showing reasonable agreement. Once this is done a modification to the bearing lubrication quality parameter kappa is proposed to better reflect the effect of the refrigerant on the lubrication quality of compressor rolling bearings. This is a first step in the direction of predicting the bearing life for this challenging application. Considerations of machine efficiency and environmental impact lead to ever new development of refrigerants and refrigeration oils. The refrigeration industry must adopt quickly to these changes. The capability to predict accurately the film thickness in lubricated contacts and having tools available to perform a reliable bearing selection are prerequisites for a robust machine design. The modelling capability of the lubrication quality does not only support the bearing life prediction but will facilitate the assessment of other fundamental bearing performance ratings such as friction and heat generation. The potential of the lubrication quality prediction is outlined in an example of a screw compressor in an air conditioning application.