An Experimental Comparison of Evaporation and Condensation Heat Transfer Coefficients for HFC-134a and CFC-12

Evaporation of HFC134a inside smooth, horizontal tubes is studied. Te,;ts are made with pure refngerant and with oil-refrigerant mixtures. Heat flux has varied from 2 kW/n? to 10 kW/m . The inner diameter of the tubes are 12 mm. T1.1o evaporators are used, 4 and 10 m long. Oil content is varied from 0 to 2.5 mass percentage (synthetic oil, EXP-0275). Oil free HFC134a is found to have higher heat transfer coefficient than HCFC22 at the same heat flux. as well as mass flux. The effect of oil in the refrigerant depends on the heat flux. At 2 and 4 kW/mz. the heat transfer coefficient has a maximum value for an oil content of around 0. 5 mass percentage. No increase at all is registered for a heat flux of 6 kW/m . Heat transfer coefficients for pure refrigerant are also compared to existing correlations. Pierre's correlation predict values with a reasonable accuracy. The by Jung modified Chen-relation, however. overestimate the heat transfer coefficient. Discrepancies are probably malnly due to errors in thermodynamic properties. INTRODUCTION When it became known that chlorine in refrigerants participate in the depletion of the ozone layer, a search for alternatives started. One refrigerant which has a high ozone depletion factor is CFC12. A possible substitute for this media is HFC134a. It is necessary that as much as possible is known of the behavior of HFC134a when introducing it to the market One particular area of interest is the performance in the evaporator. This paper presents preliminar-y results frorn heat transfer coefficient measurements for two phase flow boiling in horizontal tubes_ The tests are carried out f9r pure refrigerant as well as for a m1xture of refrigerant and oil. Applied power varies from 2 kW/m to 10 kW/m . Inlet quality is around 0.25 in the long evaporator, and 0.30 in the short. Evaporation temperature has been varied from -20°C to 5°C in the oil free tests In t"sts with oil-refrigerant mixture, the evaporation pressure has been kept as constant as possible and close to 2.3 bar, which corresponds to an evaporation temperature of -6°C for pure refrigerant_ EXPERIMENTAL APPARATUS The tested evaporator consists of ten smooth copper tubes llnked together (inner diameter; 12 mm, length: 1 m). There ar" four sight glasses (same inner diameter and 10 em long) which enable a visual study of the flow at given sections of the evaporator. The tubes are connected to each other and form a horizontal "U". The sight glasses are placed after tube 1, 4, 6 and 9. A series of tests are carried out with a shorter evaporator, 4 m. The evaporator tubes are heated electrically and power is applied uniformly along the tubes. Mass flow of refrigerant is controlled by a manual expansion valve. It is adjust"d so that all superheating takes