low GWP, lubricant, R-32, polyol ester, air conditioning
Lubricants are important components of almost all air conditioning and refrigeration systems. Their primary function is to lubricate the compressor, provide sealing of clearances between low and high pressure sides of the compressor and remove frictional heat. But the lubricant is in contact with refrigerant at all times and plays a thermo-fluidic role in the air conditioning system that can impact both system capacity and coefficient of performance (COP). Lubricants can influence capacity by altering the refrigerant-side heat transfer coefficients, and increasing pressure drop required to maintain set point temperatures. Lubricants can also affect the isentropic efficiency of the compressor. The transition to lower global warming potential (GWP) alternative refrigerants is critical to the realization of environmentally sustainable and more energy efficient refrigeration technologies. Leading candidates to replace R-22 and R-410A in air conditioning and heat pump applications include R-32 (difluoromethane) and a plethora of HFC/hydrofluoro-olefin blends with GWPs in the range of 400-650. Considerable data has been generated comparing R-410A with various low GWP alternative refrigerants in full system tests. Most notable is the work sponsored by AHRI under the Alternative Refrigerant Evaluation Program (AREP). But these studies have either been refrigerant “drop in” tests to commercial R-410A systems or “soft optimized” tests, where minor component modifications were made to better adapt a system to the properties of the new refrigerants. In all cases, the lubricants used for these studies were the commercial polyol ester (POE) lubricants used with R-410A. But commercial POE lubricants used today are much less compatible with R-32 and HFC/HFO blends. There is concern that issues may arise with long term reliability of compressors due to inadequate lubrication, poor oil return to the compressor and undesirable lubricant hold up in the system; problems that would not be observed in the short term capacity and energy efficiency tests. But regardless, there is also interest in understanding if properly optimized lubricants can improve the overall performance of low GWP-based systems. This paper presents the results of a study of the solution phase behavior and lubricating performance of several commercial and new developmental POE lubricants with low GWP R-410A replacement refrigerants. The results suggest that POE lubricants used today with R-410A may not be acceptable for use with R-32 or related HFC/HFO blends. An undesirable miscibility "gap" is observed in mixtures of traditional POEs with R-32 in the concentration range of 10-40 wt% lubricant in refrigerant. In addition, the viscosity dilution of refrigerant/lubricant mixtures at high lubricant concentrations (typical of those observed in the compressor sump) is as much as 50% more pronounced with R-32 than R-410A. Studies conducted with a new class of advanced polyol esters show that it is possible to design synthetic lubricants optimized for R-32, combining good refrigerant miscibility with limited viscosity dilution.