Condensation heat transfer and pressure drop characteristics of low GWP R-404A alternative refrigerants in a multiport tube having axial microfins

Abstract

Considering the phase out of the hydrofluorocarbons (HFCs), the search for an alternative refrigerants is of an urgent issue. In this study, four R-404A alternative refrigerants (R-448A, R-449A, R-455A, R-454C) were tested in a microfin multiport tube having 1.30 mm hydraulic diameter. The smooth mutiport tube of 0.8 mm hydraulic diamter was also tested for a comparison purpose. The results showed that, at a low mass flux and low vapour quality, the microfin tube yielded approximately equal heat transfer coefficients to those of the smooth tube. At a high mass flux and high vapour quality, however, the heat transfer coefficient trends were different depending on the refrigerants. For R-404A, which has a negligible temperature glide of 0.31 K (at 45 °C saturation temperature), the heat transfer coefficients of the two tubes were approximately the same. However, for R-454C and R-445A, which have the large temperature glides of 6.59 K and 9.46 K, the heat transfer coefficients of the microfin tube were smaller than those of the smooth tube. Based on the trends of the heat transfer coefficients of this study, three criteria for a possible good correlation are proposed. First, the correlation should predict the heat transfer coefficients of pure (or near azeotropic) refrigerants in a smooth tube adequately. Second, the correlation should predict the heat transfer coefficients of pure (or near-azeotropic) refrigerants in a microfin tube smaller than those in a smooth tube. Third, the underprediction of the microfin data should get weakened as the temperature glide increases. Based on the above criteria, it was found that some micro-channel or conventional channel correlations may be used to predict the condensation heat transfer coefficients in a multiport tube with/without microfins. The pressure drops were generally in the reverse order to the value of vapour density. Furthermore, the pressure drops of the alternative refrigerants were larger than those of R-404A.