An investigation of the effect of entrained oil on the heat transfer rate of a refrigerant evaporator

The investigation is concerned with the effect of entrained oil on the heat transfer characteristics of a direct expansion evaporator coil. A modified refrigeration system, in which the oil content of the refrigerant could be varied, was used in conjunction with an air conditioning system, which supplied the evaporator load. The heat transferred to the refrigerant was measured for different concentrations of oil in the refrigerant.

The load to the evaporator was supplied by 500 cfm of air maintained at 80°F dry bulb and 61°F wet bulb by a washer and an electric heater. The heat transferred to the refrigerant was determined by measuring the temperature of the liquid refrigerant entering the expansion valve and the suction temperature and pressure of the superheated vapor at exit from the evaporator. A sample of the liquid refrigerant flowing to the evaporator during the test was trapped in a sample collector and channeled to an expansion chamber, where the oil and refrigerant were separated and the oil concentration was determined.

The results of the investigation showed that oil tends to reduce heat transfer across the evaporator tube wall. At 25°F suction temperature, the reduction in the heat transferred was 5% with an oil concentration of 0.84% by volume compared to the refrigerant with an oil concentration of 0.063%. At 30°F suction temperature the reduction in the heat transferred was 3% with an oil concentration of 1.52% by volume. The range of suction temperatures for the investigation was from 24°F to 31.9°F, The maximum average oil concentration obtained during the investigation was approximately 1.75% by volume of oil in the refrigerant.