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The refrigeration master analyzes the phenomenon and cause of compressor liquid blow, you are worth reading!

The refrigeration master analyzes the phenomenon and cause of compressor liquid blow, you are worth reading! The phenomenon that the refrigerant or lubricating oil damages the intake valve piece when the gas is sucked into the compressor cylinder, and does not quickly discharge during the exhaust process after entering the cylinder. It is called liquid strike. Liquid shock can cause compression force parts such as: exhaust valve disc, valve plate, valve plate gasket, piston (top), piston pin, connecting rod, crankshaft, bearing bush, etc. in a short time. Refrigeration compressor liquid blow usually has the following phenomena: The suction valve is broken; Broken connecting rod of refrigeration compressor; The connecting rod is different from the holding shaft; Piston bite cylinder; The motor will be overloaded; The motor generates severe heat and the thermal protector will operate. 1. Analysis of the cause of liquid blow The liquids that can cause compressor liquid blow are nothing more than the following sources:  Liquid return (liquid refrigerant or lubricating oil);  Foam when starting with liquid;  Too much lubricant in the compressor. This article will analyze these reasons one by one. Liquid return: refers to the phenomenon or process where the liquid refrigerant in the evaporator returns to the compressor through the suction line when the compressor is running. In refrigeration systems using expansion valves, liquid return is closely related to expansion valve selection and improper use. 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. For small refrigeration systems that use capillaries, excessive liquid addition can cause liquid return. The system using hot gas defrosting is prone to liquid return. No matter whether the four-way valve is used for the heat pump operation or the cooling operation when the hot gas bypass valve is used, a large amount of liquid will form in the evaporator after the hot gas defrosts. It is possible to return to the compressor at the beginning of operation. In addition, when the frost of the evaporator is severe or the fan fails, the heat transfer becomes worse, and the unevaporated liquid will cause liquid return. Frequent fluctuations in the temperature of the cold storage will also cause the expansion valve to fail to respond and cause liquid return. Liquid strike accidents caused by liquid return mostly occur in air-cooled semi-hermetic compressors and single-stage two-stage compressors, because the cylinders of these compressors are directly connected to the air return pipe. . Even if no liquid impact is caused, the return fluid entering the cylinder will dilute or wash away the lubricating oil on the piston and the cylinder wall, exacerbating piston wear. For semi-hermetic and fully hermetic compressors with return air cooling, the liquid return rarely causes liquid blows. But it will dilute the lubricant in the crankcase. The lubricating oil containing a large amount of liquid refrigerant has a low viscosity and cannot form an adequate oil film on the friction surface, resulting in rapid wear of moving parts. In addition, the refrigerant in the lubricating oil will boil when it encounters heat during transportation, which will affect the normal transportation of the lubricating oil. The further away from the oil pump, the more obvious and serious the problem. If the bearing on the motor end is seriously worn, the crankshaft may settle to one side, which may easily cause the stator to sweep the hall and the motor to burn. Obviously, the liquid return will not only cause liquid shock, but also dilute the lubricant and cause wear. The load and current of the motor will increase greatly when it is worn out, and it will cause the motor to fail over time. For refrigeration systems where liquid return is difficult to avoid, the installation of a gas-liquid separator and the use of evacuation shutdown control can effectively prevent or reduce the harm of liquid return. Start with liquid: When the return air cooling compressor starts, the phenomenon that the lubricating oil in the crankcase violently blisters is called. The blistering phenomenon when starting with liquid can be clearly observed on the oil sight glass. The basic cause of starting with liquid is the dissolved refrigerant in the lubricating oil and the large amount of refrigerant that sinks under the lubricating oil. It suddenly boils when the pressure drops suddenly. , And cause blistering of lubricating oil. A lot of foam floated on the oil surface, even filling the crankcase. Once sucked into the cylinder through the intake port, the foam will be reduced to a liquid (a mixture of lubricating oil and refrigerant), which can easily cause liquid shock. Obviously, the liquid shock caused by starting with liquid only occurs during the starting process. Unlike the liquid return, the refrigerant that causes the start with liquid enters the crankcase in a "refrigerant migration" manner. Refrigerant migration: refers to the process or phenomenon in which 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, or is condensed in the compressor and mixed with the lubricating oil. Refrigerant migration is a very slow process. When the compressor is stopped, the temperature will decrease and the pressure will increase. Due to the low partial pressure of refrigerant vapor in the lubricating oil, the refrigerant vapor on the oil surface will be absorbed, resulting in the phenomenon that the crankcase air pressure is lower than the evaporator air pressure. The lower the oil temperature and the lower the steam pressure, the greater the absorption of refrigerant vapor. The steam in the evaporator will slowly "migrate" to the crankcase. In addition, if the compressor is outdoors, when the weather is cold or at night, its temperature is often lower than that of the indoor evaporator, and the pressure in the crankcase is also low. After the refrigerant migrates to the compressor, it is also easily condensed and enters the lubricant. In addition to being liable to cause liquid strikes, refrigerant migration can also dilute the lubricant. After the very thin lubricating oil is pumped to the friction surfaces by the oil, the original oil film may be washed away, causing severe wear and tear (Refrigeration Encyclopedia public number in this article). Transient wear will make the fit gap larger, causing oil leakage, which will affect the lubrication of the remote parts, and in serious cases will cause the action of the hydraulic protector. The crankcase heater installed in the compressor can effectively prevent the migration of refrigerant. After a short period of shutdown, keeping the crankcase heater energized can make the lubricating oil temperature slightly higher than other parts of the system, and refrigerant migration will not occur. After a long period of non-use (such as a winter), heat the lubricating oil for several or more than ten hours before starting it, which can evaporate most of the refrigerant in the lubricating oil, which can greatly reduce the possibility of liquid shock when starting with liquid It can also reduce the damage caused by refrigerant scouring. For larger systems, allowing the compressor to drain the liquid refrigerant in the evaporator before shutdown can fundamentally avoid refrigerant migration. The installation of a gas-liquid separator on the return air pipeline can increase the resistance of refrigerant migration and reduce the amount of migration. More lubricating oil: Semi-hermetic compressors usually have oil sight glass to observe the oil level. The oil level is higher than the oil sight glass, indicating that there is too much oil. If the oil level is too high, the high-speed rotating crankshaft and connecting rod head may frequently hit the oil surface, causing a large amount of lubricant splashing. Once the splashed lubricating oil enters the intake port and is brought into the cylinder, it may cause liquid hammer. When installing and commissioning large-scale refrigeration systems, it is often necessary to add lubricants appropriately. However, for systems with poor oil return, it is dangerous to carefully find the root cause of oil return and blindly add lubricating oil. Even if the oil level is not high temporarily, pay attention to the danger that may occur when the lubricating oil suddenly returns in large amounts (such as after defrosting). Liquid strikes caused by lubricants are not uncommon. Liquid shock is a common failure of compressors. The occurrence of liquid blows indicates that there must be a problem in the system or maintenance and needs to be corrected. Carefully observe the design, construction and maintenance of the analysis system, and it is not difficult to find the root cause of the liquid blow. Instead of preventing liquid hammer from the root cause, simply repairing the failed compressor or replacing it with a new compressor can only cause the liquid hammer to occur again.

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