Measurements of Oil Circulation Rate using Flow-through and Evacuated Type Sampling Cylinders for an Automotive Air Conditioning System

In automotive air conditioning systems, the oil circulation rate (OCR) is known to affect performance at both the component and system levels. The OCR is the ratio of the mass of the oil in a representative sample of oil-refrigerant mixture from the system to the total mass of the sample taken during steady state operation. With the general industry trend towards low-OCR compressors, the OCR values of interest are getting smaller, and it is becoming increasingly important to acquire an accurate knowledge of OCR for proper system optimization. While there are different OCR measurement techniques available, they all require accurate calibration which is done using the ASHRAE Standard 41.4. The standard describes a sampling technique using an evacuated sampling cylinder with a dead end to draw a sample of oil-refrigerant mixture from the system liquid line. The standard, however, doesn’t specify parameters such as sampling cylinder orientation, valve opening speed, etc. which can potentially affect the measured OCR. For the present study, the OCR is measured in an automotive air conditioning facility that uses R-134a as refrigerant and PAG ISO 46 oil as the lubricant for the electric scroll compressor used. OCR results obtained from sampling with both flow-through and evacuated type sampling cylinders are compared at different compressor speeds, and the discrepancies between the results are discussed. For the sampling technique described by the ASHRAE Standard, the OCR is measured with the evacuated sampling cylinder at different orientations and valve opening speeds, and obtained results revealed significant discrepancy between the OCR values. High-speed camera flow visualizations at the evacuated sampling cylinder entrance were also taken, and the observed flows supported the trend in the OCR results.