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The overa// performance of a smooth tube and two augmented tubes is compared by using an enhancement performance ratio, defined as the ratio of heat transfer enhancement to pressure drop increase. The augmented tubes are compared to the smooth tube with pure R-22 and with mixtures of R-22 plus 150- or 300-SUS naphthenic mineral off. Additionally, the performance of all three tubes with refrigerant-off mixtures is compared to performance of the same tube with pure refrigerant. Various off concentrations up to 5% by weight were tested and mass flux was varied from 92~000 to 294,000 Ib/h .~ (125 to 400 kg/m2 .s). Nominal evaporation conditions were 37°F(3°C) with inlet and outlet quafities of 15% and 85%, respectively. The condensation conditions were 105°F (41°C) with inlet and outlet quafities of 85% and 15%, respectively.

The enhancement performance ratio of the micro-fin tube is consistently higher than that of the low-fin tube with either pure refrigerant or refrigerant-oil mixtures. With pure refrigerant, the enhancement performance ratio of the augmented tubes is generally greater than unity, indicating improved performance relative to a smooth tube. With refrigerant-oil mixtures, however, the enhancement performance ratio of the augmented tubes relative to the smooth tube tends to decrease; the overall advantage of the augmented tubes is not as definitive with refrigerant-off mixtures.

Citation: ASHRAE Transactions, vol. 95, pt. 2, Vancouver, BC 1989