Evaporation and Condensation Heat Transfer Coefficients for a HCFC-124/HCFC-22/HFC-152a Blend and CFC-12

Tests were run with both R-12 and R-l34a in a water calorimeter. Refrigerant 12 was run with mineral oil as the lubricant. Evaporating and condensing heat transfer coefficients were calculated over a range of temperatures. To evaluate the impact of the lubricant on the heat transfer coefficients, a oil separator was added. Tests were then run with the oil separator over the same temperature range as the first test series. The refrigeration loop was then charged with refrigerant l34a along with a polyalkylene glycol (PAG) oil. Heat transfer coefficients were calculated for the low oil and normal oil circulation test series and then compared to those calculated during the R-12 tests. Test results show a near consistent trend of improved heat transfer with R-134a as compared to R-12. The presence of oil reduces the condensing heat transfer while slightly improving the evaporation. INTRODUCTION Refrigerant 134a has been identified as an environmentally safe substitute for refrigerant 12. In order to utilize R-134a in an efficient manner, the refrigerant's impact on heat exchanger performance needs to be determined. This is not a straightforward task, since many parameters can effect the heat transfer process. Flow channel geometry, flow velocity, temperature, surface characteristics, the presence of lubricating oils, and many other factors can impact the heat transfer characteristic of a given refrigerant. A comparison of the heat transfer characteristics of R-134a relative to R-12 for a system with refrigerant evaporating and condensing in an annulus was made to quantify the impacts of both the new refrigerant (R-l34a) and a new lubricating oil (a PAG) on a given system. As a precursor to the experimental evaluation of the refrigerant heat transfer, a prediction of the heat transfer impact of R-134a was made using empirical correlations for in-tube evaporation and condensation. A correlation found in ASHRAE Fundamentals (1989) for forced convection evaporation of R-12 & R-22 in copper tubes (Pierre 1957, 1955) was used to predict the impact on the evaporation heat transfer coefficient. where: h evaporation heat transfer coefficient C1 .0009 for exit quality of less than 0.90 n 0.5 for exit quality of less than 0.90 C1 • 0082 for 11 "F superheat at exit n 0.4 for ll°F superheat at exit