Lubricant, Refrigerant, Viscosity
Since the Montreal and Kyoto protocols mandated the phase-out of refrigerants which deplete the ozone layer and have high global warming potential, respectively, there has been an extensive global initiative to identify suitable environmentally sustainable alternatives. Replacement of CFCs and HCFCs with HFCs has successfully addressed the objectives of the Montreal protocol. However, most HFCs used today are not acceptable for the long term because they have GWPs greater than 1,000. Low GWP refrigerants that are, or will be, considered as replacements for HFCs include R32, hydrocarbons such as R290, carbon dioxide (R744), and hydroflouro olefins such as HFO1234yf and HFO1234ze(E).It has already been determined that in many cases new lubricants will be required for these refrigerants to ensure long term compressor reliability and the best possible system performance. The primary issue that must be addressed is the unacceptable high mutual solubility of the refrigerant and lubricant at high lubricant concentrations. The first consequence of this high solubility is the excessive reduction of viscosity that affects proper lubrication of the compressor and components, sealing of clearances between low and high pressure sides of the compressor. The second is a significant change in the steady state amount of oil in the circulation stream in the system, which can impact the heat transfer performance in both the evaporator and condenser.Another potential minor issue is refrigerant flash evaporation at discharge creating excessive foaming and noise. This paper describes details of a method used to measure the thermophysical properties of refrigerant lubricant mixtures. The general methods for data acquisition and processing, along with creation of Daniel charts were in accord with those developed by Chris Seeton. The results of measurements involving mixtures of traditional POEs with R-32 or R-410A shows that lubricant viscosities in the compressor at various conditions within the normal operating envelope decrease by as much as 25-54%. This illustrates the excessive lubricant viscosity dilution of R-32 relative to R-410A with traditional POs used today with HFCs refrigerants. The solution property measurement technique was also used to develop a class of advanced ester POE lubricants optimized for R-32 to eliminates the viscosity dilution problem. Significant energy savings can be achieved through proper optimization of lubricant/refrigerant solution properties to provide the best balance of lubrication in the compressor while maintaining excellent heat transfer in the refrigeration cycle.