Experimental study on the measurement criteria for measuring surface tension in oil/refrigerant mixtures by maximum bubble pressure method

Abstract

Refrigeration oil used in refrigeration compressors is usually compatible with refrigerant. The properties of the mixture are changed when refrigeration oil and refrigerant are mixed in the oil sump of the compressor because the refrigerant dissolves in the oil. Surface tension is one of the properties of the oil/refrigerant mixture that changes during the dissolving process. This change in property has an impact on foaming and the production of oil mist in compressors. In addition, surface tension can be utilized as an indicator of refrigerant concentration in oil. Hence, it is essential to determine the surface tension of the oil/refrigerant mixture. This study employed the maximum bubble pressure method (MBPM) to measure the surface tensions of the refrigeration oil and refrigerant mixtures. The oil was polyvinyl ether (PVE), while the refrigerants were difluoromethane (R32) and R410A, which was a blend of R32 and pentafluoroethane (R125). Since the viscosity of refrigeration oil changed dramatically with the dissolution of refrigerant, the measurement criterion of the MBPM applicable to a wide range of viscosities was clarified using silicone oil with various viscosities. Based on the findings of the surface tension of the PVE/R410A and PVE/R32 mixtures, a predictive formula was presented to estimate the mixture’s surface tension as a function of temperature and refrigerant concentration in oil. Relationships between the refrigerant concentration and normalized surface tension were also proposed.