Average in-tube heat transfer coefficients and pressure drops during condensation are reported for condensation of refrigerant R134a/lubricant mixtures in a smooth tube and a micro-fin tube of 9.52mm (three/eighths in.) outer diameter. The lubricants tested were 169-SUS and 369-SUS penta erythritol ester mixed acids. Lubricant concentrations ranged from 0% to 5%. The average saturation temperature in the test tube was approximately 40 deg C (104 deg F). The mass flux of the refrigerant-lubricant mixtures was varied from 85 kg/m²×s (62,700 lb/ft²×h) to 375 kg/m²×s (276,649 lb/ft²×h). Heat transfer coefficients during condensation decreased with the addition of lubricants in all cases. Condensation pressure drops increased with the addition of the 169-SUS ester lubricant in both the smooth tube and the micro-fin tube. The addition of the 369-SUS lubricant did not affect pressure drops in the smooth tube, but it decreased the pressure drops in the micro-fin tube. Pure R134a heat transfer coefficients in the micro-fin tube were 100% to 200% higher than those in the smooth tube, with the higher values occurring at the lower mass fluxes. Pressure drops in the micro-fin tube were 20% to 50% higher than those in the smooth tube. Design equations are presented that aid in predicting the heat transfer coefficients and pressure drops of R134a/lubricant mixtures in the smooth and micro-fin tubes.

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