Revisiting the Compressibility of Oil/Refrigerant Lubricants

A fixture was fabricated for the purpose of restraining the expansion of an existing metal bellows piezometer so that a refrigerant and oil mixture can be admitted under pressure. Measurements on a polyol ester (POE) with 9.2 wt.% of R134a show that the addition of refrigerant slightly increases compressibility. The previously reported reduction in compressibility (increase in bulk modulus) by Tuomas and Isaksson (2006, “Compressibility of Oil/Refrigerant Lubricants in Elasto-Hydrodynamic Contacts,” ASME J. Tribol., 128(1), pp. 218–220) of an ISO 68 POE when mixed with R134a cannot be supported by precise measurements of the volume compression. The increased compressibility found by Comuñas and co-workers (2002, “High-Pressure Volumetric Behavior of x 1, 1, 1, 2-Tetrafluoroethane + (1 − x) 2, 5, 8, 11, 14-Pentaoxapentadecane (TEGDME) Mixtures,” J. Chem. Eng. Data, 47(2), pp. 233–238) is the correct trend. The Tait equation of state (EoS) has been fitted to the data for both the neat POE and its 9.2% by weight mixture with refrigerant. The usual problem was encountered for the mixture with the Tait EoS at low pressure where the compressibility becomes greater than predicted due to proximity to the vapor dome. The measured relative volumes of the mixture can be used to collapse the viscosity to a master curve when plotted against the Ashurst–Hoover thermodynamic scaling parameter. The thermodynamic scaling interaction parameter is approximately the same as for the neat oil.