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Evaporation and condensation heat transfer coefficients were measured for smooth and micro-fin tubes with HFC134a and CFC12. Micro-fin tubes are internally enhanced tubes that are characterised by numerous small fins that spiral down the tube. For example, in this study, the micro-fin tube had 60 fins with a height of 0.2mm and a 17 deg spiral angle. Heat transfer measurements were performed on 3.67m (12 ft) long tubes with inside diameters of 8.0mm (0.31 in.). Test conditions varied from 5 deg C to 15 deg C for evaporation and 30 deg C to 50 deg C for condensation. The refrigerant mass flux was varied from 130 kg/m2.s (95,860 lb/ft2.h) to 400 kg/m2.s (294,000 lb/ft2.h). When HFC134a was compared to CFC12 at similar mass fluxes in smooth tubes, the evaporation and condensation heat transfer coefficients were about 40% and 25% higher, respectively. A more relevant comparison of heat transfer coefficients is at equivalent cooling (or heating) capacities. In this case, the HFC134a heat transfer coefficients were about 10% higher than CFC12 values for both evaporation and condensation. The micro-fin tube produced higher heat transfer coefficients and pressure drops for all conditions when compared to the smooth tube. For example, for HFC134a, heat transfer enhancement factors (defined as the convective heat transfer coefficients for the micro-fin tube divided by the value for the smooth tube measured at similar conditions) varied from .5 to 2.5 during evaporation and from .8 to 2.5 during condensation. Pressure drop penalty factors (defined similarly to enhancement factors) for both refrigerants were usually less than the heat transfer enhancement factors. However, in the case of HFC134a at the lowest temperature and highest mass flux, the penalty factor slightly exceeded the enhancement factor.

KEYWORDS: evaporation, condensation, R134a, R12, refrigerants, pressure drop, heat transfer coefficient, evaporators, tubes, finned tubes, comparing, measuring, heat flow

Citation: ASHRAE Transactions, vol.97, pt. 2, Indianapolis, IN 1991