Heat transfer evaluation of HFC-236ea and HFC-236fa in evaporation and condensation

摘要

This research evaluated the shell-side heat transfer coefficients of HFC-236ea and HFC-236fa, which are environment-safe and considered as prospective substitutes for replacing CFC-114 in Navy shipboard chillers;Work has been performed on two types of low integral-fin tubes (1024-fpm and 1575-fpm tubes) and three types of high performance enhanced tubes (Turbo-CII, Turbo-B, and Turbo-BII tubes). HFC-236fa was tested with these five tubes during pool boiling and condensation, while HFC-236ea was tested mainly with the three high performance enhanced tubes during pool boiling, spray evaporation, and condensation;Measurements were conducted on a single-tube configuration at a saturation temperature of 2°C for pool boiling as well as spray evaporation and at 40°C for condensation. The tubes which have nominal outer diameters of 3/4 inch (19.1 mm) were tested from 15 kW/m2 to 40 kW/m2 for both pool boiling and condensation, and 10 kW/m2 to 30 kW/m2 for spray evaporation;The high performance enhanced tubes were found to effectively enhance heat transfer and produced higher heat transfer coefficients than the integral-fin tubes for both HFC-236ea and HFC-236fa during either pool boiling or condensation;Shell-side heat transfer coefficients of HFC-236fa, HFC-236ea, and CFC-114 were compared with each other. In general, HFC-236fa had higher heat transfer coefficients than CFC-114 and HFC-236ea during both pool boiling and condensation;The comparative heat transfer performance of spray evaporation with pool boiling was made for HFC-236ea by using a Turbo-B tube and a 1575-fpm tube. For the range of liquid refrigerant feed rates evaluated, the superiority of spray evaporation over pool boiling was found to exist only at the heat fluxes below the approximate value of 30 kW/m2.;The effects of compressor oil on heat transfer performance during pool boiling were investigated. The presence of up to 3% oil in HFC-236fa affected the boiling performance by less than 10% from the pure HFC-236fa value for all the tubes tested except the Turbo-BII tube.