Properties of oil and refrigerant mixtures : lubrication of ball bearings in refrigeration compressors

Sammanfattning: A refrigeration compressor contains highly loaded mechanical contacts that have to be lubricated. The lubricant used in refrigeration compressors consists of an oil and refrigerant mixture, and the refrigerant concentration can be up to 40 wt%. However, the oil in the refrigeration system is a contaminant that lowers the efficiency of the refrigeration system. Therefore, it is important that the oil and the refrigerant are miscible with each other so that oil following the refrigerant out in the system is returned to the compressor and not accumulated in the system. HFC refrigerants are not miscible with the mineral oils used in CFC refrigeration systems. Hence, new synthetic ester lubricants were developed for this application. It was soon evident, though, that the lack of chlorine and that the new ester oils did not have the same good lubrication properties as mineral oil and CFC refrigerants. The objective of this thesis is to increase the understanding of lubricating rolling element bearings in refrigeration compressors. The thesis will also give recommendations for lubrication of bearing in refrigeration compressors based on the results. The thesis is based on an experimental study of parameters that are important for life of rolling element bearings. The thesis consists of five papers that describe a test apparatus designed to measure and monitor the lubrication status during operation with capacitance. The effect of two phosphate additives in refrigeration oil is also examined and properties such as viscosity, pressure-viscosity coefficient, compressibility and shear strength coefficient of oil and refrigerant mixtures are measured. The general conclusion of the thesis is that the refrigerant affects all of the measured properties negatively on the subject of lubrication and life of the rolling element bearing. The usage of oil-free compressors solves many of the problems that lubricants consisting of a mixture of oil and refrigerant cause. If the oil has to be in the system it is of great importance that the lubrication issues are taken care of early in the design process. The design parameters for temperature, viscosity, pressure- viscosity coefficient, contamination etc. must be followed when the compressor is in use.