Process and cause of liquid shock in piston refrigeration compressor

Process and cause of liquid shock in piston refrigeration compressor Process and cause of liquid shock in piston refrigeration compressor 1. Introduction The phenomenon that liquid refrigerant and/or lubricating oil damage the suction valve plate when the gas is sucked into the cylinder of the compressor, and it is produced when the piston is compressed when the piston is close to the top dead center and is not quickly discharged during the exhaust process after entering the cylinder. The phenomenon of momentary high hydraulic pressure is usually called liquid hammer. Liquid shock can cause damage to compression components (such as valve discs, pistons, connecting rods, crankshafts, piston pins, etc.) in a short period of time, and is the fatal killer of reciprocating compressors. Reducing or avoiding liquid entering the cylinder can prevent the occurrence of liquid hammer, so liquid hammer 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 the suction at a higher speed, due to the impact and incompressibility of the liquid, the suction valve sheet 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 deformation and damage of the force-bearing parts. These force-bearing parts include intake and exhaust valve plates, valve plates, valve plate gaskets, pistons (top), piston pins, connecting rods, crankshafts, bearing bushes, etc. 2. Process and phenomenon (1) The suction valve plate 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 plate 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 liquid is tens or even hundreds of times that of gas, so the momentum of liquid flowing is much larger than that of gas, and the impact force generated is much larger. The flow when many droplets in the suction enter the cylinder is a two-phase flow. The impact of the two-phase flow on the suction valve plate is not only high in intensity but also high in frequency, just like a typhoon mixed with pebbles hitting the glass window, its destructiveness is self-evident. The breakage of the suction valve plate is one of the typical characteristics and processes of liquid hammer. (2) The time for the connecting rod to break and the compression stroke is about 0.02 seconds, and the exhaust process will be shorter. The liquid droplets or liquid in the cylinder must be discharged from the exhaust hole in such a short time, and 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 be deformed and lifted.　If the liquid does not evaporate and exit the cylinder in time, the piston will compress the liquid when it is close to the top dead center. Due to the short time, the process of compressing the liquid seems to be an impact, and the metal knocking sound will also be heard in the cylinder head. Compressed liquid is another part or process of the liquid hammer phenomenon. The high pressure generated at the moment of liquid hammer has great destructiveness. In addition to the familiar connecting rod being bent or even broken, other compression forces (valve plate, valve plate gasket, crankshaft, piston, piston pin, etc.) will also be deformed Or damaged, but often overlooked, or confused with excessive exhaust pressure. When repairing the compressor, people will easily find the bent or broken connecting rod and replace it, and forget to check whether other parts are deformed or damaged, thus laying the root cause for future failures.　The connecting rod fracture caused by the liquid hammer is different from the shaft holding and piston biting cylinder, which can be distinguished. First of all, the bending or fracture of the connecting rod caused by the hydraulic hammer occurs in a short time. The pistons and crankshafts at both ends of the connecting rod move freely, and there is generally no shaft holding or cylinder seizure caused by severe wear. Although the valve chip fragments occasionally cause severe scratches on the piston and cylinder surfaces after the intake 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 stubble have squeezing characteristics. Although the connecting rod after the piston bites the cylinder may also be squeezed, but the premise is that the piston must be stuck in the cylinder. The connecting rod breaking after holding the shaft is even more different. The big end of the connecting rod and the crankshaft are severely worn, and the force causing the breaking is shear force, and the stubble is different. *Afterwards, before holding the axle and biting the cylinder, the motor will be overloaded, the motor will heat up, and the thermal protector will act. 3. Reason analysis Obviously, the liquid that can cause compressor liquid hammer is nothing more than the following sources: liquid back, that is, liquid refrigerant or lubricating oil flowing back to the compressor from the evaporator; 2) foam when starting with liquid; 3) compressor There is too much lubricant inside. This article will analyze these reasons one by one. "(1) Liquid back" Generally, liquid back 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 the selection and use of expansion valves. Expansion valve selection is too large, the superheat setting is too small, the temperature sensor installation method is incorrect or the insulation bandage is damaged, 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 to defrost is prone to liquid back. 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 defrosts, and 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 hammer accidents caused by liquid return mostly occur in air-cooled (air-cooled or air-cooled) semi-hermetic compressors and single-unit two-stage compressors, because the cylinders of these compressors are directly connected to the air return pipe. Once liquid returns, It is easy to cause a liquid shock accident. Even if there is no liquid hammer, the returned liquid entering the cylinder will dilute or wash away the lubricating oil on the piston and cylinder wall, and aggravate piston wear.　For the return air (refrigerant vapor) cooling type semi-hermetic and hermetic compressor, the liquid return rarely causes liquid shock. 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 heated during transportation, which affects the normal delivery of lubricating oil. The further away from the oil pump, the more obvious and serious the problem. If the bearings at the motor end are severely worn, the crankshaft may sink to one side, which may easily cause the stator to sweep and the motor to burn out. 　Obviously, liquid return will not only cause liquid shock, but also dilute the lubricant and cause wear. The load and current of the motor will greatly increase when worn, and it will cause the motor to malfunction over time.　 For the refrigeration system where liquid return is difficult to avoid, installing a gas-liquid separator and adopting evacuation shutdown control can effectively prevent or reduce the harm of liquid return. 2) The phenomenon that the lubricating oil in the crankcase foams violently when the compressor starts with liquid return air cooling is called start with liquid. The blistering phenomenon when starting with liquid can be clearly observed on the oil sight glass. The root cause of 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 lubricating oil to bubble. This phenomenon is very similar to the foaming phenomenon of cola when people suddenly open a cola bottle in daily life. The duration of bubbling is related to the amount of refrigerant, usually several minutes or ten minutes. A lot of foam floated on the oil surface, even filling the crankcase. Once sucked into the cylinder through the intake duct, the foam will be reduced to liquid (a mixture of lubricating oil and refrigerant), which can easily cause liquid hammer. Obviously, the liquid shock caused by starting with liquid only occurs during the starting process.　Different from the liquid return, the refrigerant that causes the start with liquid enters the crankcase in the way of refrigerant migration. Refrigerant migration refers to the process or phenomenon in which the refrigerant in the evaporator enters the compressor in gas form through the return line and is absorbed by the lubricating oil when the compressor stops running, or is mixed with lubricating oil after being condensed in the compressor. After the compressor is stopped, the temperature will decrease and the pressure will increase. Because the partial pressure of refrigerant vapor in the lubricating oil is low, it will absorb refrigerant vapor on the oil surface, causing the crankcase air pressure to be lower than the evaporator air pressure. The lower the oil temperature and the lower the vapor pressure, the greater the absorption power 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 easy to be condensed and enter the lubricating oil.　　Refrigerant migration is a very slow process. The longer the compressor is down, the more refrigerant will migrate into the lubricant. As long as there is liquid refrigerant in the evaporator, this process will proceed. Since the lubricating oil that dissolves the refrigerant is heavy, it will sink to the bottom of the crankcase, and the lubricating oil that floats on it can also absorb more refrigerant.　In addition to easily causing liquid shock, refrigerant migration will also dilute the lubricating oil. After the very thin lubricating oil is pumped to the friction surfaces, it may wash away the original oil film and cause severe wear (this phenomenon is often called refrigerant erosion). Transitional wear will increase the fit clearance, causing oil leakage, which will affect the lubrication of distant parts, and will cause the oil pressure protector to act in severe cases.　Due to structural reasons, the crankcase pressure decrease is much slower 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 startup.　Theoretically, the installation of a crankcase heater (electric heater) on the compressor can effectively prevent the migration of refrigerant. After a short period of shutdown (such as 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 shutdown (such as a winter), heating the lubricating oil for several or ten hours before starting up can evaporate most of the refrigerant in the lubricating oil, which can greatly reduce the possibility of liquid shock during startup with liquid It can also reduce the harm caused by refrigerant erosion. However, in practical applications, it is difficult to maintain the heater's power supply after shutting down or to supply power to the heater ten hours before starting up. Therefore, the actual effect of the crankcase heater will be greatly reduced.　　 For larger systems, let the compressor drain the liquid refrigerant from the evaporator before shutting down (called pump-down shutdown), which 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, the migration of refrigerant can be prevented and the degree of foaming of the lubricating oil can be reduced. By improving the oil return path in the return air-cooled compressor, a checkpoint (return pump, etc.) is added to the migration path of the motor cavity and the crankcase. After stopping, the path can be cut off, and the refrigerant cannot enter the crankshaft cavity; reduce the intake air The passage section of the channel and the crankcase can slow down the speed of the crankcase pressure drop when starting up, thereby controlling the degree of foaming and the amount of foam entering the cylinder. 　(3) Too much lubricating oil　　 Semi-hermetic compressors usually have oil sight glasses 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 the big end of the connecting rod may frequently hit the oil surface, causing a large amount of lubricating oil to splash. Once the splashed lubricating oil enters the intake duct and is brought into the cylinder, it may cause liquid shock.　 Large-scale refrigeration system installation and commissioning, often need to properly add lubricants. However, for systems with poor oil return, it is dangerous to carefully find the root cause of oil return and blindly replenish the lubricant. Even if the oil level is not high temporarily, pay attention to the danger that may be caused when the lubricating oil suddenly returns in large amounts (for example, after defrosting). Hydraulic shock caused by lubricating oil is 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, it is not difficult to find the root cause of the liquid strike. Without preventing the liquid hammer from the root cause, simply repairing the malfunctioning compressor or replacing it with a new compressor can only make the liquid hammer happen again.