Accessories of the Refrigeration System—Gas-Liquid Separator

2025-04-16

As a whole, in addition to the four major components (compressor, condenser, throttling device, and evaporator), the refrigeration system also has many auxiliary components. Today, I will share with you some basic knowledge about the gas-liquid separator.
As the name implies, the basic function of the gas-liquid separator in the refrigeration system is to separate and store the refrigerant liquid in the return air pipe to prevent liquid slugging in the compressor. Therefore, it can temporarily store the excess refrigerant liquid and also prevent the excess refrigerant from flowing into the compressor crankcase, which may cause the dilution of the oil. Sometimes it is also called a low-pressure liquid receiver.
Since the refrigeration oil will also be separated and accumulated at the bottom of the gas-liquid separator during the process of separating the refrigerant liquid, there will be an oil hole at the outlet pipe and the bottom of the gas-liquid separator to ensure that the refrigeration oil can return to the compressor, thus avoiding oil shortage in the compressor. Gas-liquid separators are mainly divided into vertical type, horizontal type, and those with a heat regeneration device. In some small systems such as refrigerators, some copper pipes are used to make a simple gas-liquid separator. The working principle of the gas-liquid separator is that when the refrigerant with liquid enters the gas-liquid separator, the liquid is separated due to the decrease in the expansion speed or by hitting a baffle, thus separating the liquid.

Accessories of the Refrigeration System—Gas-Liquid Separator1744810900724

I. The design and use of the gas-liquid separator must follow the following principles:

The gas-liquid separator must have sufficient capacity to store the excess liquid refrigerant. Especially for the heat pump system, it is better not to be less than 50% of the charging amount. If possible, it is best to conduct a test for verification, because when using a throttling orifice plate or capillary tube for throttling during heating, about 70% of the liquid refrigerant may return to the gas-liquid separator. Also, a high exhaust pressure and a low suction pressure will allow more liquid refrigerant to enter the gas-liquid separator. Using a thermal expansion valve will result in less, but about 50% of the refrigerant may still flow into the gas-liquid separator. This mainly occurs after the defrosting starts. The external balanced thermal bulb is still hot, so a large amount of refrigerant will flow through the evaporator without evaporating and then enter the gas-liquid separator. When the machine stops, the gas-liquid separator is the coldest component in the system, so the refrigerant will migrate here. Therefore, it is necessary to ensure that the gas-liquid separator has sufficient capacity to store this liquid refrigerant.
Appropriate oil return holes and filters ensure that the refrigeration oil and refrigerant return to the compressor. The size of the oil return hole should try to ensure that no liquid refrigerant flows back to the compressor, but also ensure that the refrigeration oil can return to the compressor as much as possible. If it is the liquid refrigerant stored in the gas-liquid separator during operation, it is recommended to use a diameter of 0.040 in (1.02mm). If the refrigerant migrates to the gas-liquid separator due to the shutdown of the machine, it is recommended to use 0.055 in (1.4mm) (The application engineering manual of Copeland directly gives 0.040 - 0.050 in (1.02 - 1.3 mm), and generally, for a gas-liquid separator, it is 0.0625 - 0.125 (1.6 - 3.2mm)). Of course, if possible, the size can also be optimized through experiments to achieve the best effect. As for the filter, Copeland recommends using a mesh size not less than 30X30 (0.6mm aperture). Here, it is recommended to use 50X60 mesh. Considering the installation level of air conditioners in China, especially the installation of split-type air conditioners, impurities often enter the system, so using a smaller aperture will be more reliable.
The pressure loss of the gas-liquid separator should be as small as possible. The flow rate of the refrigeration oil and refrigerant is controlled by the size of the outlet U-shaped pipe, so its size also determines the pressure loss of the refrigerant, because the refrigerant entering the outlet pipe is at a high speed. Here is a reference value. For R22, R134, R404A, and R410A, at an evaporation temperature of 5°C and a suction temperature of 30°C, the pressure loss is 7kPa. In some company documents, the pressure loss is 1/2F (0.
5C), which should refer to the pressure in the saturated state. However, the converted pressure is different for different refrigerants, and the previously mentioned pressure loss is for several refrigerants. Therefore, these parameters are only for reference.
When designing the gas-liquid separator, the following requirements should be clearly specified in the drawings:
Corresponding piping dimensions and external dimensions
Pressure tests such as airtightness test and strength test
Cleanliness and residual moisture
The cylinder body at the intake pipe should be clearly marked
The cylinder body should be filled with high-purity nitrogen at 0.05MPa for pressure maintenance
Other key technical requirements or material and dimension requirements
Mesh number of the filter
II. Here, we will focus on introducing the calculation method of the size of the oil return hole:
If the oil return hole is too large, the oil return will be better, but the backflow of the liquid refrigerant will also increase, resulting in the dilution of the oil (the lubricating effect of the oil decreases), abnormal wear of the scroll part, and possible failure of the compressor.
If the oil return hole is too small, the backflow of the liquid refrigerant will decrease, but the oil return will also decrease, resulting in insufficient oil supply in the machine. Due to the insufficient oil supply to the scroll part, abnormal wear will occur, leading to the failure of the compressor.
Therefore, the size of the oil return hole should ensure the oil quantity in the compressor and suppress the backflow of the liquid refrigerant to be below the specified level of oil dilution.
It is necessary to design an appropriate aperture.
The area of the pressure equalizing hole (mm²) = the area of the outer diameter of the outlet pipe (mm²) × (0.03～0.033)
The final size of the pressure equalizing hole must be verified and confirmed according to the test after the calculation is completed.
Test standards:
(1) When the compressor stops with the liquid refrigerant level in the gas-liquid separator fixed, the liquid refrigerant will not flow into the compressor.
(2) Conduct an oil return test for confirmation. The above confirmation can be observed by installing a liquid sight glass on the return air pipe between the gas-liquid separator and the compressor.
Calculation example:
It is recommended that for the unit designed for ultra-low temperature, when designing the oil return hole, the design method of multiple oil return holes be adopted (the oil return holes are distributed at appropriate heights, which can improve the reliability of the compressor, and the total area of the oil return holes is the same as that of a single hole).
III. Design of the Pressure Equalizing Hole of the Gas-Liquid Separator
The size of the pressure equalizing hole of the outlet pipe of the gas-liquid separator is calculated as follows.
The area of the pressure equalizing pipe aperture (mm²) = the cross-sectional area of the outer diameter of the outlet pipe (mm²) × (0.03～0.033) (Note) The final calculation of the size of the pressure equalizing hole is still determined according to the experiment.
When the compressor stops with the accumulated amount of liquid refrigerant in the gas-liquid separator fixed, the liquid refrigerant will not flow into the compressor.

Install a liquid sight glass between the gas-liquid separator and the compressor for confirmation.
<Calculation example> Design conditions: Outer diameter of the outlet pipe: φ22.3
The area of the pressure equalizing pipe aperture (mm²) = {1/4×3.14×(22.3²)}×0.03 = 11.71
The diameter of the pressure equalizing hole φ (mm) = 11.71÷(1/4×3.14) = 3.9mm→ Initially adopt a pressure equalizing hole with a diameter of φ4.0, and then confirm it through the experiment.
The gas-liquid separator should be installed as close as possible to the compressor. If there is a four-way valve, install it between the four-way valve and the compressor. If there is a filter, install it between the filter and the compressor.