Evaporation and condensation of refrigerant-oil mixtures were studied both inside a smooth tube and a micro-fin tube. The refrigerant was R-22 and the oil was a naphthenic base mineral oil with a viscosity of 150 SUS. The oil and refrigerant were completely miscible for the entire range of test conditions The straight, horizontal test section, 0.375 in (9.52 mm) od with length of 12 ft (3.67 m), was heated or cooled by water flowing in a surrounding annulus. The oil mass concentration was varied from 0% to 5% and mass fluxes were tested over the range of 92,000 to 294,000 lb/h-ft² (125 to 400 kg/m²-s). Evaporation was at 37 F (3°C) with inlet and outlet qualities &approximately 15% and 85%, respectively. The condensation conditions were 105 F (41°C) with nominal inlet and exit qualities of 85% and 15%, respectively.

The micro-fin tube increased the average heat transfer coefficients for both evaporation and condensation by as much-as a factor of 2.5 for refrigerant-oil mixtures as well as for pure refrigerant. For both smooth and micro-fin tubes, small quantities ofoil were found to enhance evaporation heat transfer in comparison with pure refrigerant evaporation. Above a threshold concentration, the addition of increasing quantities of oil caused the enhancement to decrease, but for the micro-fin tube, the enhancement remained greater than the increase in surface area The addition of any oil caused the condensation heat transfer to decrease for both tubes. The micro-fin tube’s heat transfer enhancement factor for condensation showed little dependence on the oil concentration.

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