Studies on the Oil Retention and Performance of Oil Separator in Multi Heat Pump System

Abstract

This paper presents analytical and experimental studies on the oil retention and performance of oil separator in multi heat pump system. To suggest the proper charge amount of oil in a compressor of heat pump system, the prediction of oil retention amount in each component of heat pump system is necessary. This study proposed analytical models for predicting the oil retention amount in gas, liquid and two phase regions of refrigerant. Based on the proposed models, oil retention amount in each component of multi heat pump system was predicted. From the simulation results, it was confirmed that the oil retention amount decreases 26.1% and 67.3% as the mass flux of refrigerant rises from 100 kgm-2s-1 to 250 kgm-2s-1 and oil circulation ratio diminishes from 0.9% to 0.1%, respectively. Among 6 components which include condenser, horizontal and vertical liquid lines of refrigerant, evaporators, horizontal and vertical gas lines of refrigerant, large amount of oil is retained in the gas lines of refrigerant. Because most of discharged oil is in the gas lines of refrigerant, prediction of oil retention amount in gas lines of refrigerant is important. To verify the model for predicting the oil retention amount in gas lines of refrigerant, experiment was conducted to measure the oil retention amount in horizontal and vertical gas lines of refrigerant. The oil retention amount and pressure drop were measured with respect to the mass flux of refrigerant, oil circulation ratio and pipe diameter. Flow visualization was also conducted for horizontal line. The oil retention amount tends to decrease as mass flux becomes higher or oil circulation ratio diminishes. As pipe diameter increases, the oil retention amount goes up. Generally, the oil retention amount in vertical line was higher than that in horizontal line due to the effect of gravity force. As mass flux of refrigerant rises, however, the effect of gravity force becomes negligible, so the oil retention amounts in horizontal and vertical lines were similar. Based on the experimental results, the model of oil retention amount in gas lines of refrigerant was verified. The mean absolute percentage error of predicted and experimental results was 15.0%. From the analytical and experimental studies on the oil retention in each components of heat pump system, it was confirmed that the oil circulation ratio highly affects the oil retention amount and pressure drop. Reduction of oil circulation ratio is required to enhance the system performance and reduce the initial charge amount of oil in compressor. In order to reduce the oil circulation ratio, high performance oil separator is necessary. This study measured the separation efficiency of oil separator and pressure drop at the oil separator. Separation efficiency and pressure drop were measured with respect to body diameter, body height and inlet configuration. Based on the experimental results, empirical models for predicting the separation efficiency and pressure drop were suggested. Different empirical models were proposed based on the inlet configuration. In case of inlet configuration of type 1, the mean absolute percentage errors of the models for efficiency and pressure drop were 0.4% and 7.8%. In case of type 2, those were 0.3% and 7.6%.