In Tube Evaporation Heat Transfer of Refrigerant Mixtures

Heat transfer coefficient data was taken during in-tube evaporation of five differenct refrigerant mixtures. These measurements were performed in a 318" O.D. (9.7 mm) smooth copper tube at refrigerant temperatures of -zoe and 4°C. Four different mass fluxes ranging from 125 to 375 kg/s-m2 were tested. On a mass basis, the five different mixtures tested were: R-125 (40%) I R-32 (60%), R-32 (30%) I R-125 (10%) I R-134a (60%), R-134a (90%) I R-32 (10%), R-125 (44%) I R-143a (52%) I R-134a (4%), and R-134a (75%) I R-32 (25%). The evaporative heat transfer coefficient for the mixtures were also compared to data for R-22. The mixture R-32(60%) I R-125(40%) exhibited the highest heat transfer coefficients out of all the refrigerants tested at both 4°C and -zoe when compared on both an equal mass flux and an equal cooling capacity basis. The R-321 R-125 mixture also had a lower pressure drop than R-22. The mixture R-134a(90%) I R-32(10%) had the lowest heat transfer coefficients at both 4°C and _zoe when compared on both an equal mass flux and an equal cooling capacity basis. In addition, this same mixture had the highest pressure drop. Except for the lowest performing refrigerant, all of the refrigerants performed either similar or better than R-22. An evaluation of temperature effects showed that the refrigerants had larger heat transfer coefficients at 4°e than at -2°C. INTRODUCTION Non-azeotropic refrigerant mixtures are being considered as potential replacements for R-22. In this study five different refrigerant mixtures were tested for evaporation heat transfer coefficients and compared against a baseline ofR-22 data. These refrigerants were tested in an in-tube heat transfer test facility, which has been used in the past for evaporation and condensation tests ofrefrigerants R-12, R-113, R-22, and R-134a [Eckels and Pate, 1991, Doerr, et al., 1994]. Several problems are unique to testing non-azeotropic blends, such as a temperature glide which makes measurement of the inlet and outlet temperatures, where the quality can vary as much as 0 to 100%, more difficult than it does for a pure refrigerant. Heat transfer coefficient data was taken during in-tube evaporation of refrigerant mixtures in a 318" O.D. (9.7 mm) smooth copper tube. The data were taken at temperatures of -2.0°C and 4.0°C. Five non-azeotropic refrigerant mixtures ·were tested and compared against baseline data for R-22. Four different mass fluxes ranging from 125 to 375 kgls-m2 were tested. On a mass basis, the mixtures tested were: Mixture 1 R-125 (40%) I R-32 (60%) Mixture 2 R-134a (90%) I R-32 (10%) Mixture 3 R-134a (75%) I R-32 (25%) Mixture 4 R-32 (30%) I R-125 (10%) I R-134a (60%) Mixture 5 R-125 (44%) I R-143a (52%) I R-134a (4%) TEST FACILITY The test facility consists offour main parts: a refrigerant loop, a water loop, a data acquisition system, and a dual test section. For these tests only one· side of the dual test section was used. The following sections provides detailed descriptions of the four main parts of the test facility. A schematic of the test rig is shown in Figure 1. Test Section The test section consists of a horizontal test tube and a surrounding annulus. The inner tube is a 318" O.D. (9.7 mm) copper tube which is 3.67 m long. The annulus which surrounds the tube is also 3.67 m long and is constructed of a copper tube with a 17.2 mm inside diameter. The test tube is centered in the annulus by a series of spacers. The spacers are constructed of three stainless steel rods spaced 120 degrees about the annulus. The spacers are held in place by a series of industrial PG Teflon glands.