Heat Transfer of R-22 and Alternatives in a Plate-Type Evaporator

This paper deals with the investigation of substitutes for R-22 and their heat transfer behaviour in a plate-type evaporator with a falling water film as heat source. Alternatives investigated were R-134a and two new refrigerant mixtures. Measurements on a 3m x 0,5m plate showed a superior performance ofR-134a compared to R-22 and mixtures at equal test conditions and a distinct difference between mixtures and pure fluids at very 1ow water temperatures. Considering the decrease in duty when changing to R134a in an existing plant, it was seen that with decreasing evaporator duty the U-value decreased too. A dependency on conditions prior to start-up was observed, with U-values differing as much as 15%. Reasons discussed are activation of nucleation sites and different flow patterns in the plate. A LMTD u Q Tw,in Tw,out TR,bubb TR,out NOMENCLATURE evaporator area, m logarithmic mean temperature difference, K overall heat transfer coefficient, W/mK duty, W water temperature to the evaporator, K water temperature from the evaporator, K refrigerant bubble point temperature, K refrigerant temperature at the outlet, K Eighth International Refrigeration Conference at Purdue University, West Lafayette, IN, USAJuly 25-28, 2000 161