Conference Agenda

For vapor compression systems, the standard method for measuring the oil circulation ratio (OCR) is via sampling, as described in the ASHRAE standard 41.4. Sampling is tedious, alters the steady state of the system and can depend on several factors such as refrigerant-oil miscibility and sampling cylinder orientation. Many compressor manufacturers conduct compressor performance tests which rely on hot gas cycles that do not have a liquid line for sampling. They mount oil separators at the compressor discharge to separate the oil flow from the refrigerant flow and use the mass flow rates to estimate real-time OCR. In a real case, some oil will always escape with the separated refrigerant, and some refrigerant, dissolved in oil, will always escape with the separated oil, significantly reducing the accuracy of the procedure. The present study investigates OCR measurements using an oil separator-based approach for a full vapor compression cycle working with R134a and PAG ISO 46 oil. A full cycle allows sampling to also be performed in parallel for validation. Solubility corrections were performed to account for the refrigerant dissolved in the oil returning to the compressor suction. It was found that the OCR values from the oil separator-based approach, upon solubility correction, fell within 6% of the sampling results. Discissions were also made on the usefulness of the oil separator efficiency values at the oil and vapor outlet ports for the oil separator-based approach.

The majority of current refrigeration and air conditioning systems utilize high GWP refrigerants such as HFC's which have been deemed a threat to the environment. These growing environmental concerns are driving stringent regulations to phase out these high GWP refrigerants and move towards adoption of lower GWP refrigerants. Koura is developing solutions for high-performing, economical alternatives to these high GWP refrigerants to provide more sustainable, lower global warming potential (GWP) products. This transition to new refrigerants and new chemistries requires understanding of the chemical interactions as well as the material compatibility between these low GWP refrigerants, lubricating oils, materials of construction and polymers used in direct contact with both refrigerant and lubricant.

The purpose of this study is to provide testing and acceptance evaluation for the compatibility of air conditioning system materials to use Koura’s materials. In this paper, ASHRAE Guideline 38 methodology materials compatibility testing of selected metals catalysts are evaluated by using various commercially available (POE and PAG) lubricants over a temperature range of 80 °C to 175 °C. Results will be reported and compared with those obtained using traditional HFC refrigerants as reference. Guidance will be offered as to what materials are better suited in applications, and which materials engineers may want to avoid.

The oil circulation ratio (OCR) significantly impacts automotive air conditioning system performance, both at system and component levels. It represents the mass percentage of oil in a representative oil-refrigerant mixture sample drawn from the system at steady state. With industries increasingly favoring low-OCR compressors, accurate and repeatable OCR measurement becomes essential. Various OCR measurement techniques rely on calibration per ASHRAE standard 41.4, which outlines sampling using an evacuated cylinder connected to the system liquid line. However, factors like cylinder orientation and valve opening speed can influence OCR results. During sampling, the initial flow contains liquid refrigerant and oil. As the sampling valve opens, refrigerant undergoes flashing, vapor recompression, and re-condensation, filling the cylinder until pressure equalization occurs. This two-phase, two-component, phase changing flow is complex, and for the present study, flow visualization using a high-speed camera and analytical tools are used to study the flashing flow entering an evacuated sampling cylinder. Insights gained shed light on how sampling cylinder orientation and valve speed can impact OCR measurements.

Refrigeration oil is used in refrigerant compressors for lubrication, sealing, and cooling. The refrigeration oil has generally good solubility with refrigerants for oil return. In cases where the refrigerant dissolves excessively in the refrigeration oil, the mixture of oil and refrigerant foams easily due to the stirring effect of moving parts such as the rotating shaft and rotor in the compressor. If refrigeration oil flows out from the compressor with refrigerant into a refrigeration cycle by violent foaming, some negative effects may occur. Two such examples are the reduction of heat transfer efficiency at heat exchangers, and the risk of compressor failure as a result of low refrigeration oil level in the compressor. To avoid these problems, the foaming characteristics of the refrigeration oil and refrigerant mixture are necessary to clarify. In this study, the foaming characteristics were examined in a cylindrical vessel with various actions causing the foaming under a wide range of refrigerant concentrations in the oil. R410A and R32 were the refrigerants used, and POE-1 and POE-2 (polyol ester oil) were used as the refrigeration oils. It was found that the stirring effect was the primary factor of the foaming, as compared to gas blowing and heat input into the oil. The foaming characteristics from the stirring were evaluated by two viewpoints: the foaming height and the collapsing speed of the foam. The results showed that the highest foaming height was obtained in the refrigerant concentration range of 33–40%. In addition, it was found that the foaming generation can be related to the pressure ratio of the pressure of oil/refrigerant mixture to that of saturated refrigerant, even when different types of oil or refrigerant are used.

Recent automotive air conditioners need many functions, such as multiplying the air conditioning spaces inside the car and cooling batteries of electric vehicles and the automotive air conditioning. System, therefore, is becoming increasingly complex. A certain amount of refrigeration oil is discharged from a compressor and circulates in the system with refrigerant, and the oil behavior in the system also becomes complicated. In a system having two evaporators, there is no problem of an oil return when both evaporators are in operation. However, when one of the evaporators stops, part of oil flows into a branch pipe in a suction line connected to the unoperated evaporator and the oil is accumulated in it. It results in the reduction of oil return to the compressor. Consequently, it may cause the compressor failure. Generally, the branch of pipe from unoperated evaporator is connected to the top of suction line with a vertical pipe to prevent the oil flowing to the unoperated evaporator, but the oil disperses as droplets at the branch by breakup of the oil film and flows into the unoperated evaporator. To establish a design criterion of the pipes in the suction line, the oil behavior at the suction line with branch is necessary to be clarified. In this study, visualization of oil droplet dispersed at the confluence was carried out and the droplet size of dispersal oil and the amount of accumulated oil in the branch pipe were measured with changing the compressor rotational speed from 4000 rpm to 8000 rpm. A visualization section is installed in the suction line which has an inverted U-shape at the branch, and oil behavior was captured by a high-speed camera under each condition. Annular oil film in the suction line and the breakup of the oil film at the branch were observed in the visualization section. The amount of oil flows beyond the inverted U-tube at the branch in the visualization section in the suction pipe increases with increasing the compressor rotational speed from 4000 rpm to 7000 rpm. However, the amount of oil accumulation decreased at higher rotational speed then 7000 rpm. It was found that the oil droplet size distribution differed significantly between 7000 rpm and higher compressor rotational speeds. Collision of large oil droplets on the wall of vertical pipe, absorption of scattered oil droplets by the oil film on the wall were observed, and this peculiar oil behavior above 7000 rpm resulted in a decrease in the amount of oil accumulation.