Local Heat Transfer Coefficients during Condensation of R-22 and R-32/R-125 Mixtures

Abstract

In this paper local and average heat transfer coefficients during condensation of refrigerants R-22 and two mixtures of R-32/R-125 (45%/55% and 25%/75%) are presented. The local heat transfer coefficients were determined using a specially instrumented test section that measured local energy flow and surface temperatures during condensation. The local test section used an 8.0 mm (0.32 in) inner diameter smooth tube that was 1.93 m (6.3 ft) long. Average heat transfer coefficients were determined using the Log-Mean-Temperature-Difference equations. The local method yielded useful information on the variation of heat transfer coefficients with refrigerant quality and mass flux. Typically, heat transfer coefficients were found to double from low to high quality at a particular mass flux, while smaller increases occurred with mass flux. Evaluation of local heat transfer coefficients confirmed that the method yields reasonable results. Specifically, comparison of the average of local heat transfer coefficients with measured average heat transfer coefficients showed excellent agreement. The results also indicated that the R-32 (45%)/ R-125 (55%) mixture had slightly better heat transfer performance than R-22, while the R-32 (25%)/R-125 (75%) mixture had significantly lower heat transfer performance than R-22.