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In-tube heat transfer coefficients and pressure drops during evaporation are reported for mixtures of refrigerant R134a and a penta erythritol ester mixed-acid lubricant. The ester lubricant was tested at viscosities of 169 SUS and 369 SUS over a lubricant concentration range of 0% to 5% in both a smooth tube and a micro-fin tube. The average saturation temperature used was 1 deg C (33.8 deg F). Measurements were taken for the refrigerant-lubricant mixture over a mass flux range of 85 kg/m2.s (62,700 lb/ft2×h) to 375 kg/m2×s (276,640 lb/ft2×h) in test tubes with an outer diameter of 9.52mm (three/eighths in.). Heat transfer coefficients during evaporation increased at low concentrations of the 169-SUS ester lubricant and then dropped off at high lubricant concentrations in both the smooth tube and the micro-fin tube. The higher viscosity 369-SUS lubricant decreased the heat transfer coefficients in both tubes over the range of lubricant concentrations tested. Pressure drops during evaporation increased in both the smooth tube and the micro-fin tube with the addition of ester lubricant viscosity. The heat transfer coefficients for the micro-fin tube were 100% to 50% higher than those for the smooth tube, with the higher values occurring at low mass fluxes. Pressure drops in the micro-fin tube were 10 to 20% higher than those in the smooth tube.

KEYWORDS: heat flow, heat transfer coefficient, pressure drop, refrigerants, lubricants, R134a, mixtures, smooth tube heat exchangers, finned tube heat exchangers, micro, evaporation, testing, viscosity, content, saturation temperature, comparing, measuring, testing, calculating, performance