Condensation of Different Refrigerants on the Outside Surface of Smooth Cylindrical Tubes

The Nusselt model of condensation provides the fundamental theory in predicting the heat transfer during the condensation process. Widely verified, its significance lies in the fact that it has been used as the baseline in evaluating the heat transfer enhancement of the condensation and often used as the basis of validating the test rig for multiphase heat transfer. The aim of this work is to re-examine the correlation for condensation on smooth cylindrical tubes. The heat transfer coefficients during condensation of four different refrigerants R123, R245fa, R134a, and R22 on the outside surface of a smooth cylindrical tube were individually measured at large degrees of subcooling, up to 25 K. The experiments were conducted at a fixed saturation temperature of 36.1 °C. Measurements showed that, for each refrigerant, the condensation heat transfer coefficient decreases with increasing degree of subcooling. At a given degree of subcooling, a higher-pressure refrigerant corresponds to a higher condensation heat transfer coefficient, with the exception that the condensation heat transfer coefficients of R134a and R245fa are nearly the same in spite of much higher pressure of the former. The predictions from the Nusselt theory for condensation heat transfer over cylinder tubes match very well with the measurements, where the predictions are 3–9% lower than the measurements for all refrigerants within the range of degree of subcooling considered in this work. A modified constant in the Nusselt number provides more accurate prediction of condensation on smooth cylindrical tubes.