Principles that must be followed in the design and use of gas-liquid separators

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In addition to the four major components (compressor, condenser, throttling device, and evaporator), the refrigeration system as a whole also has many accessory components—gas-liquid separators, etc. Regarding some relevant knowledge about gas-liquid separators, today we will learn about its basic common sense. The gas-liquid separator, as the name suggests, has the basic function in the refrigeration system to separate and preserve the refrigerant liquid in the return pipe to prevent compressor liquid shock. Therefore, it can temporarily store excess refrigerant liquid and also prevent excess refrigerant from flowing to the compressor crankcase and causing oil dilution. Sometimes called a low pressure reservoir. Since during the process of separating the refrigerant liquid, the refrigeration oil will also be separated and accumulated at the bottom of the gas-liquid separator, there will be an oil hole in the outlet pipe and the bottom of the gas-liquid separator to ensure that the refrigeration oil can return to the compressor, thus avoiding the compressor's oil shortage. The basic structure of the gas-liquid separator is as shown below: The main function of the gas-liquid separator in the refrigeration system is to accommodate the refrigerant in the liquid return part of the system, prevent liquid shock to the compressor, and prevent excessive refrigerant from diluting the compressor oil. Structure, advantages and disadvantages of gas-liquid separator 1. Gravity settlement: principle: The structure is very simple and the principle is also very simple. It uses the difference in weight of liquid and gas to achieve separation. advantage: Simple design ； Equipment is easy to make ； Low resistance. shortcoming: Separation efficiency is very poor ； The gas flow rate is required to be very slow, so the corresponding equipment volume is very large. , Baffle separation (baffle separation) principle: The densities of gas and liquid are different, and the liquid has large inertia, so it will directly collide with the baffle to achieve separation. advantage: (Relative to gravity separation) The separation efficiency is higher; the volume is smaller; and the work is stable. shortcoming: The separation load range is narrow, and after the gas flow rate exceeds a certain range, the separation efficiency decreases. 　 reason: 1. If the gas flow rate is too slow, the inertia of the liquid is too small, and collision and separation cannot occur. 2. If the gas flow rate is too fast, the liquid that has hit the wall will be taken away again. 3. The liquid is easily fragmented and produces finer droplets when it collides with the baffle. The greater the gas flow rate, the greater the collision force, the more fine droplets produced, and the worse the separation effect. 3. Principle of centrifugal separation: Gas and liquid have different densities. When the liquid and gas mix and rotate together, the centrifugal force on the liquid is greater than that of the gas, so it collides with the cylinder wall and adheres to it, and then separates due to the action of gravity. advantage: Higher separation efficiency ； Smaller size ； The job is stable. shortcoming: Similar to baffle separation, the separation load range is narrow. After the gas flow rate exceeds a certain range, the separation efficiency decreases. reason: 1. If the gas flow rate is too slow, the inertia of the liquid is too small, and collision and separation cannot occur. 2. If the gas flow rate is too fast, the liquid that has hit the wall will be taken away again. 4. Principle of packing separation: The principles of packing separation and baffle separation are similar. They also rely on the inertia of the liquid to collide with the packing to achieve separation. However, compared with baffle separation, packing separation has a much larger blocking collection wall area, and the baffles are repeated many times, so the liquid can easily hit the wall, so the separation efficiency is higher. advantage: The separation efficiency is higher than baffle separation or centrifugal separation ； The structure is simple, just make a stuffing rack. shortcoming: The separation load range is narrower. After the gas flow rate exceeds a certain range, the separation efficiency drops sharply. ； Easy to block. 　 reason: 1. When the gas-liquid ratio is constant, the greater the flow rate of the gas-liquid mixture, the greater the separation load per unit time, and the shorter the time the mixture stays in the separator. 2. While the gas is deflecting, it also pushes the liquid that has hit the wall to flow upward. If the gas flow rate is too large and the flow speed is too fast, the liquid will not flow smoothly. As the liquid accumulates more and more, the gas flow area will become smaller and smaller. Under these dual effects, the liquid that has hit the wall can easily be taken away by the gas again. 5. Principle of screen separation: The particle sizes of gas and liquid are different. When the liquid and gas are mixed and flow through the wire mesh, it is like sieving. The gas passes but the liquid is intercepted, thereby achieving separation. Simply put, the liquid particles are too large to pass through the screen. advantage: The separation efficiency is higher than packing separation. shortcoming: The separation load range is narrower. After exceeding the gas limit flow rate or liquid-gas ratio, the separation efficiency drops sharply; it is easy to block; the mesh size and material selection of the wire mesh are very important. 6. Micropore filtration separation principle: The particle sizes of gas and liquid are different. The liquid and gas are mixed and flow together through microporous filtration. Just like sieving, the gas passes but the liquid is intercepted, thereby achieving separation. The screening function of the micropore filter separator is screening in the true sense. The pore diameter is generally below 50 microns, and liquid particles larger than the pore diameter cannot pass through. Moreover, the blocking and collection surface area of ​​the microporous filter separator is extremely large per unit volume, and the number of baffles and screening times per unit volume is greater than that of wire mesh filtration. advantage: Very high separation efficiency ； The volume is smaller than the wire mesh separator. shortcoming: 1. High cost. 2. If the liquid-gas ratio is too large, liquid resistance will easily occur and the resistance will rise sharply (other coarse filtration devices need to be installed at the front end). 3. The resistance itself is large. 4. Easier to block. The design and use of gas-liquid separators must follow the following principles: 1. The gas-liquid separator must have sufficient capacity to store excess liquid refrigerant. Especially for heat pump systems, it is best not to be less than 50% of the charge. If possible, it is best to conduct a test to verify it, because when throttling the flow during heating with a throttling orifice plate or capillary tube, 70% of the liquid refrigerant may return to the gas-liquid separator. There are also high exhaust pressure and low suction pressure that will allow more liquid refrigerant to enter the gas-liquid separator. Using a thermal expansion valve will reduce the amount, but 50% may flow to the gas-liquid separator. This is mainly because after the defrost starts, the external balance temperature sensing bag is still hot, so the refrigerant will flow through the evaporator in large amounts without evaporating and entering the gas-liquid separator. When shut down, the gas-liquid separator is the coldest component in the system, so the refrigerant will migrate here, so it is necessary to ensure that the gas separator has sufficient capacity to store the liquid refrigerant. 2. 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 it should 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 it is because the refrigerant migrates to the gas-liquid separator during shutdown, it is recommended to use 0.055 in (1.4mm) (Copeland's application engineering manual directly gives 0.040-0.050 in (1.02-1.3 mm), and the general gas-liquid separator is 0.0625-0.125 (1.6-3.2mm)). Of course, if conditions permit, it is possible to optimize this size through experiments to achieve the best results. There is also a filter. Copeland recommends using no less than 30X30 mesh (0.6mm aperture), and here it is recommended to use 50X60 mesh. This seems a bit contradictory, but considering the level of air conditioning installation in China, especially split-type installation, impurities often enter the system, so it is safer to use a smaller aperture. 3. The pressure loss of the gas-liquid separator should be as small as possible. The flow rate of 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 high-speed. Here is a reference value. For R22, R134, R404A, and R410A, the pressure loss is 7kPa at the evaporation temperature of 5°C and the suction temperature of 30°C. According to some company data, the pressure loss is 1/2F (0.5C). This should refer to the pressure in the saturated state. However, different refrigerants are converted into different pressures. The pressure loss mentioned above is for several refrigerants, so these parameters are only for reference. When designing the gas-liquid separator, the following requirements should be clearly stated in the drawings:: 1. Corresponding piping size and outer dimensions 2. Pressure tests such as air tightness test and strength test 3. Cleanliness and residual moisture 4. The cylinder at the air inlet pipe should be clearly marked 5. The cylinder should be filled with 0.05MPa high-purity nitrogen to maintain pressure 6. Other key technical requirements or material and size requirements 7. Filter mesh number, etc.