Heat transfer coefficient during condensation inside a minichannel multiport tube with R32 and R410A as working fluids

Abstract

The use of microchannels and minichannels in a heat exchanger has increased in recent decades. They contribute to increasing efficiency and to reducing refrigerant charge and compactness of heat exchangers. The aim of this study is to experimentally determine the heat transfer coefficient in minichannel two-phase flow processes with the low global warming potential refrigerant R32 and to compare it with the values experimentally obtained for R410A and some of the correlations encountered in the existing literature. There are a few publications studying refrigerant R32 inside multiport minichannel tubes; this has low flammability and has been classified as A2L by ASHRAE. European air-conditioning manufacturers recommend its use instead of R410A as it has lower global warming potential (R32GWP = 675, R410AGWP = 2088). Other environmental improvements may also be considered: R32 is a monocomponent refrigerant, so recycling is easier than R410A; it is also safer according to NFPA 704 classification; and a breathing apparatus is required in the case of an accident with R410A. On the other hand, R410A flammability is lower than R32 because of the addition of R125. An installation for the study of condensation processes has been constructed at the Technical University of Cartagena, Spain. The more relevant results of heat transfer coefficient will be presented in this article. The analyzed data have been measured for R32 and R410A flowing through aluminum square multiport tubes with a hydraulic diameter of 1.16 mm, then compared. The influence of saturation temperature (or pressure), flow velocity, and vapor quality in heat transfer coefficient and frictional pressure gradient has been studied. The values considered for these variables are saturation pressure corresponding to 30°C, 35°C, 40°C, 45°C, and 50°C; flow velocities from 100 to 800 kg/(s·m2); and vapor quality from 0.05 to 0.9.