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Causes, treatment and prevention of liquid shock in refrigeration compressor

Causes, treatment and prevention of liquid shock in refrigeration compressor Liquid shock refers to the situation where the refrigerant fails to absorb heat or evaporates sufficiently, and the refrigerant liquid or wet steam is sucked into the compressor by the compressor. It is called liquid shock. 1. The main causes are: 1. Liquid return: Incomplete refrigerant evaporation: liquid return is closely related to the selection and use of expansion valves. Expansion valve selection is too large, the superheat setting is too small, the installation method of the temperature sensor is incorrect, the insulation bandage is damaged, and the expansion valve failure may cause liquid return. 2.Excessive foam when starting with liquid: When the air-cooled compressor starts, the phenomenon that the lubricating oil in the crankcase violently foams is called starting with liquid. The basic reason for starting with liquid is that the refrigerant dissolved in the lubricating oil and sinking under the lubricating oil abruptly boils when the pressure drops suddenly, and causes the foaming phenomenon of the lubricating oil. During the startup process. 3. There is too much lubricating oil in the compressor: the oil level is too high, and the crankshaft and connecting rod rotating at high speed may frequently hit the oil surface, causing a large amount of lubricating oil splash. Once the splashed lubricating oil enters the intake port and is brought into the cylinder, it may cause liquid hammer. 4. Excessive addition of refrigerant: Too much refrigerant charging will not only cause the compressor to return liquid with liquid, but also may cause the compressor to return to the refrigerant pipe flow rate is too low, the lubricating oil in the evaporator cannot return to the compressor, causing compression The machine lacks oil and burns out. 5. Failure of the evaporator: It enters the air return pipe of the compressor without evaporating in the evaporator, which is easy to cause liquid shock of the compressor. If there is an evaporator failure that causes severe frosting of the coil, incomplete refrigerant evaporation is also the main reason for the compressor return air. 6. The volume design of the gas-liquid separator is unreasonable: adding a gas-liquid separator to the system is the most effective way to prevent compressor oil shortage and liquid blow. The gas-liquid separator can temporarily store excess refrigerant in the gas-liquid separation. In the heat pump system, the refrigerant liquid accumulated in the condenser can be subjected to a gas-liquid separator when the hot gas defrost refrigeration and heating switching work is performed, which greatly reduces the risk of liquid strike. 7.Refrigerant migration: refrigerant migration means that the refrigerant in the evaporator enters the compressor in the form of gas through the return air line and is absorbed by the lubricating oil when the compressor stops running, or condenses with the lubricating oil in the compressor The process or phenomenon of mixing. 8.Second, what harm will the liquid blow cause: 9.For piston machines: 10.It causes a large number of bubbles in the lubricating oil, destroys the oil film on the lubricating surface, and makes the oil pressure unstable; makes the moving parts run without good lubrication, causing the hair to hug the shaft; causes the cylinder liner to cool and shrink sharply, holding the piston; when serious Damaged cylinder liner, piston, connecting rod, piston pin, etc. 11.For screw machine: 12.Liquid shock will cause vibration, increase noise, and damage the rotor and bearings (excessive force); severe liquid shock will also damage the equipment and cause accidents. 13.For scroll compressors: 14.The liquid impact has a great impact on the scroll, which may break the scroll. The lubricating oil containing a large amount of liquid refrigerant has a low viscosity and cannot form enough oil film on the friction surface, resulting in rapid wear of the moving parts inside the compressor. The refrigerant will boil when it meets heat during transportation, which will affect the normal transportation of lubricating oil. 15.3. How to deal with liquid strike: 16.When liquid shock occurs, the suction cut-off valve of the compressor should be closed immediately, and the throttle valve should be closed to stop the liquid supply. If the suction temperature continues to decrease, continue to close or even close the suction valve. The frictional heat with the bearing shell when the crankshaft rotates is used to evaporate the refrigerant in the crankcase until the refrigerant in the crankcase is completely evaporated. After that, open the suction cut-off valve slightly and increase the load slowly. When liquid shock occurs, observe and adjust the oil pressure. If there is no oil pressure or the oil pressure is too low, stop the machine immediately, release the lubricating oil and refrigerant in the crankcase, refill the oil, and restart the operation. 17.Fourth, how to prevent compressor liquid blow: 1. Reducing the refrigerant charge: The best way to protect the compressor from failures caused by liquid refrigerant is to limit the refrigerant charge to within the compressor's allowable range. If this is not possible, the charging volume should be reduced as much as possible under the condition of ensuring the cooling effect. The bubbles in the sight glass due to the small diameter of the liquid tube and the low pressure head should be alert and reasonably charged. Refrigerant injection is essential. 2. Crankcase heater: The function of the crankcase heater is to keep the temperature of the refrigeration oil in the crankcase higher than the temperature of the compressor suction inlet in the system. The crankcase heater is generally continuously heated in use. The crankcase heater is very effective to prevent migration, but the crankcase heater cannot protect the compressor from damage caused by liquid backflow, and the heater must be turned on when the compressor is turned on. It was warmed up for a long time before and was not operable for practical applications. 3. Superheater for return air pipe: in the cold season, there is still the possibility of return air with liquid when the compressor is running, so that an electric heater or hot gas can be added to the return air pipe section of the compressor to help increase the superheat of the return air of the compressor The method is simple and effective, and an overheat detection device can be added to make the control more accurate, thereby protecting the compressor and preventing liquid shock. 4. Gas-liquid separator of the suction pipe: Install a gas-liquid separator on the suction pipe to temporarily store the overflowing liquid refrigerant in the system and return the liquid refrigerant to the compressor at a rate that the compressor can withstand. Due to the different charge requirements of different refrigeration systems and the different refrigerant control methods, whether a gas-liquid separator is required and how large a gas-liquid separator is required depends largely on the requirements of the specific system. In addition, controlling the opening of the throttle element and increasing the heat exchange area of ​​the evaporator during the design stage of the refrigeration system can effectively improve the superheat of the return air and reduce the occurrence of compressor liquid blow during the operation of the system. When charging refrigerant, avoid liquid charging from the low pressure side to prevent the compressor from starting with liquid. Liquid shock refers to the situation where the refrigerant fails to absorb heat or evaporates sufficiently, and the refrigerant liquid or wet steam is sucked into the compressor by the compressor. It is called liquid shock.

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