Miscibility of Refrigerant Gas and Lubricating Oil

The solubility of refrigerant and lubricating oil is an important characteristic of the refrigerant. According to the degree of mutual solubility of refrigerant and lubricating oil, it can be divided into three categories:

(1) It is insoluble or slightly soluble in lubricating oil. This kind of refrigerant and lubricating oil are almost insoluble in each other. When mixed with lubricating oil, there will be obvious stratification and the oil is easily separated from the refrigerant.

(2) It is completely dissolved in lubricating oil. This kind of refrigerant and lubricating oil will dissolve into a uniform solution without delamination.

(3) Limited dissolution in lubricating oil, this kind of refrigerant is completely miscible with lubricating oil at high temperature, but at low temperature, the dissolution of refrigerant and oil causes stratification.

Refrigerant dissolved in lubricating oil has the following advantages: it will not form an oil film on the surface of the heat exchanger, thereby affecting heat transfer; lowering the freezing point is beneficial to low-temperature systems; lubricating oil can also penetrate into various parts of the compressor along with the refrigerant, Form good lubrication conditions.

The disadvantage is that the refrigerant dissolves in the lubricating oil, which reduces the viscosity of the lubricating oil, which causes the oil film on the lubricating surface to be too thin or not to form an oil film, which affects its lubrication; when the refrigerant and the lubricating oil are mutually soluble, their characteristics will deviate from the characteristics of pure refrigerants. The more lubricating oil dissolved in the refrigerant under the same pressure, the higher the evaporation temperature, and the less dissolved lubricating oil, the lower the evaporation temperature. Therefore, at the same evaporation temperature, the corresponding evaporation pressure is higher than that of pure refrigerants. Low, resulting in a decrease in the cooling capacity of the compressor. If the refrigerant does not dissolve or hardly dissolves the lubricating oil, the advantages and disadvantages are opposite to the above-mentioned situation.

Therefore, it is particularly important to know whether the oil and the refrigerant are miscible and the temperature at which they are miscible. This article uses the MI7000 miscibility tester to test the critical miscibility temperature of the oil and the refrigerant.

testing sample

R32 refrigerant and lubricating oil

Test Conditions

Temperature: -30℃-100℃

Test Results

Phenomenon description: 

The oil and refrigerant with a mass ratio of 5% are mutually dissolved into a uniform and transparent shape at room temperature. As shown in Picture 1, the temperature starts to drop at 3°C intervals. During the cooling process, observe the sample changes in the test chamber on the software. When the temperature drops to a certain level, the sample appears turbid, as shown in Picture 2.

Phenomenon description: 

The oil and refrigerant with a mass ratio of 15% are mutually dissolved into a uniform and transparent shape at room temperature. As shown in Picture 3, the temperature starts to drop at 3°C intervals. During the cooling process, observe the sample changes in the test chamber on the software. When the temperature drops to a certain level, the sample appears delamination, as shown in Picture 4.

Test conclusion

The samples of different proportions are uniform and transparent at room temperature, and turbidity or stratification occurs during the temperature decrease. The temperature at this time is the critical miscibility temperature.