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Analysis of the causes of liquid compressor blowdown

Analysis of the causes of liquid compressor blowdown 1. Introduction to refrigeration compressor The phenomenon that the liquid refrigerant and/or lubricating oil of the refrigeration system damages the suction valve piece when the gas is sucked into the compressor cylinder, and is not quickly discharged during the exhaust process after entering the cylinder, and is caused by the compression when the piston approaches top dead center The phenomenon of instantaneous high hydraulic pressure is often referred to as liquid blow. Liquid shock can cause damage to compression force parts (such as valve discs, pistons, connecting rods, crankshafts, piston pins, etc.) in a short time, and is a fatal killer of reciprocating compressors. Reducing or avoiding liquid entering the cylinder can prevent the occurrence of liquid blow, so liquid blow can be completely avoided. 　　 Generally, the liquid hammer phenomenon can be divided into two parts or processes. First, when more liquid refrigerant, lubricating oil, or a mixture of the two enters the compressor cylinder with suction at a higher speed, due to the impact and incompressibility of the liquid, the intake valve plate will be excessively bent or broken; second, When the liquid in the cylinder that has not been evaporated and discharged in time is compressed by the piston, the huge pressure that occurs in an instant causes the deformation and damage of the force-bearing parts. These load-bearing parts include intake and exhaust valve plates, valve plates, valve plate gaskets, pistons (top), piston pins, connecting rods, crankshafts, bearing bushes, etc. 2. Refrigeration compressor liquid blowout process and phenomenon (1) The suction valve is broken. The compressor is a machine that compresses gas. Generally, the piston compresses gas 1450 times per minute (half-sealed compressor) or 2900 times (full-sealed compressor), that is, the time to complete an intake or exhaust process is 0.02 seconds or even shorter. The size of the suction and exhaust aperture on the valve plate and the elasticity and strength of the suction and exhaust valve are designed according to the gas flow. From the perspective of the force of the valve disc, the impact force generated when the gas flows is relatively uniform. 　　 The density of the liquid is tens or even hundreds of times that of the gas, so the momentum when the liquid flows is much larger than that of the gas, and the impact force generated is also much larger. The flow when more droplets are mixed into the cylinder in the suction is a two-phase flow. The impact of the two-phase flow on the suction valve sheet is not only strong and high in frequency, just like a typhoon striking a glass window with pebbles, its destructiveness is self-evident. The cracking of the suction valve is one of the typical characteristics and processes of liquid blow. 　　 (2) Broken connecting rod of refrigeration compressor The compression stroke time is about 0.02 seconds, and the exhaust process will be shorter. The droplets or liquid in the cylinder must be discharged from the exhaust hole in such a short time, the speed and momentum are great. The situation of the exhaust valve plate is the same as that of the intake valve plate, the difference is that the exhaust valve plate is supported by the limit plate and the spring plate, and it is not easy to break. When the impact is severe, the limit plate will also deform and lift. 　　If the liquid does not evaporate and exit the cylinder in time, the piston will compress the liquid when it reaches the top dead center. Due to the short time, this process of compressing the liquid seems to hit, and the metal knocking sound will also be heard in the cylinder head. Compressed liquid is another part or process of liquid hammering. The high pressure generated by the instantaneous liquid impact has a great ring-breaking property. In addition to the bending or even breakage of the connecting rods that are familiar to people at the beginning, other compression components (valve plates, valve plate gaskets, crankshafts, pistons, piston pins, etc.) will also be deformed Or damaged, but often overlooked, or confused with excessive steam pressure. When repairing the compressor, people will easily find the bent or broken connecting rods and replace them, and forget to check whether other parts are deformed or damaged, thus laying the root cause for future failures. 　　 The breakage of the connecting rod caused by the liquid impact is different from the holding shaft and the piston biting cylinder, which can be distinguished. First of all, the bending or breaking of the connecting rod caused by hydraulic shock occurs within a short period of time. The pistons and crankshafts at both ends of the connecting rod move freely, and generally there will be no shaft or bite cylinder caused by severe wear. Although the valve chip fragments can occasionally cause severe scratches on the piston and cylinder surfaces after the suction valve chip is broken, the surface scratches are very different from the wear caused by lubrication failure. Secondly, the fracture of the connecting rod caused by liquid hammer is caused by pressure, and the connecting rod and the broken stubble have squeezing characteristics. Although the connecting rod fracture after the piston bites the cylinder may also be squeezed, but the premise is that the piston must be stuck in the cylinder. The breakage of the connecting rod after holding the shaft is even more different. The big end of the connecting rod and the crankshaft are severely worn. The force that causes the breakage is a shearing force, and the broken stubble is also different. Finally, before holding the shaft and biting the cylinder, the motor will be overloaded, the motor will generate severe heat, and the thermal protector will operate. 　　3. Reason analysis of liquid compressor blowout Obviously, the liquid that can cause the compressor liquid blow is nothing more than the following sources: 1) liquid return, that is, liquid refrigerant or lubricating oil that flows back from the evaporator to the compressor; 2) foam when starting with liquid; 3) There is too much lubricant in the compressor. This article will analyze these reasons one by one. (1) Liquid return Generally, liquid return refers to the phenomenon or process in which the liquid refrigerant in the evaporator returns to the compressor through the suction line when the compressor is running. For 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. A system that uses hot gas to defrost is prone to liquid return. Regardless of 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 is formed in the evaporator after the hot gas is defrosted. These liquids may both return to the compressor when the subsequent cooling operation starts. 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 (air-cooled or 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. It is easy to cause liquid hammer accidents. 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 (refrigerant vapor) cooling, 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. (2) Refrigeration compressor with liquid start When the air-cooled compressor is started, the phenomenon that the lubricating oil in the crankcase blisters violently is called liquid start. The blistering phenomenon when starting with liquid can be clearly observed on the oil sight glass. The basic reason for starting with liquid is that a large amount of refrigerant dissolved in the lubricating oil and sinking under the lubricating oil suddenly boils when the pressure drops suddenly, and causes the foaming phenomenon of the lubricating oil. This phenomenon is very similar to the bubbling phenomenon of cola when people suddenly open cola bottles in daily life. The duration of blistering depends on the amount of refrigerant, usually a few minutes or a dozen minutes. 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. 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, in cold weather or at night, its temperature is often lower than that of the indoor evaporator, and the pressure in the crankcase is also lower. After the refrigerant migrates to the compressor, it is also easily condensed and enters the lubricating oil. Refrigerant migration is a very slow process. The longer the compressor is stopped, the more refrigerant will migrate into the lubricant. As long as there is liquid refrigerant in the evaporator, this process will take place. Since the lubricating oil that dissolves the refrigerant is heavy, it will sink to the bottom of the crankcase, and the lubricating oil floating on it can also absorb more refrigerant. 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 serious wear (this phenomenon is often referred to as refrigerant flushing). 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. Due to structural reasons, the crankcase pressure decreases much more slowly when the air-cooled compressor is started, the bubbling phenomenon is not very violent, and the foam is difficult to enter the cylinder. Therefore, the air-cooled compressor does not have the problem of liquid start-up. In theory, installing a crankcase heater (electric heater) on the compressor can effectively prevent refrigerant migration. After a short period of shutdown (for example, at night), 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. However, in practical applications, it is difficult to maintain the heater power supply after shutdown or to supply power to the heater more than ten hours before starting. Therefore, the actual effect of the crankcase heater will be greatly reduced. For larger systems, allowing the compressor to drain the liquid refrigerant in the evaporator before it is shut down (referred to as pump-down 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. Of course, by improving the structure of the compressor, you can prevent the migration of refrigerant and slow down the degree of foaming of the lubricating oil. By improving the oil return path in the return air-cooled compressor, a checkpoint (oil return pump, etc.) is added to the migration path of the motor cavity and the crankcase. After shutdown, the path can be cut off, and the refrigerant cannot enter the crankshaft cavity; reduce the intake air The cross section of the channel and the crankcase can slow down the speed of crankcase pressure drop when starting up, thereby controlling the degree of foaming and the amount of foam entering the cylinder. (3) Too much frozen lubricant 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. 4. Refrigeration compressor liquid blow reason summary 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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