Do you know the causes, hazards, treatment and prevention methods of compressor liquid strike?
Liquid hammer refers to the situation in which the refrigerant liquid or wet vapor is sucked into the compressor by the compressor due to the failure or insufficient heat absorption and evaporation of the refrigerant, which is called liquid hammer.
1. The main reasons are:
1. Liquid back: Incomplete evaporation of the refrigerant: Liquid back is closely related to the improper selection and use of expansion valves. Excessive selection of the expansion valve, too small superheat setting, incorrect installation of the temperature sensing bulb, damaged adiabatic dressing, and failure of the expansion valve may cause liquid back.
2. Excessive foaming when starting with liquid: The phenomenon that the lubricating oil in the crankcase foams violently when the return air cooling compressor is started is called starting with liquid. 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 bubbling of the lubricating oil. The liquid hammer caused by the starting with liquid only occurs During the startup process.
3. Too much lubricating oil in the compressor: 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.
4. Too much additional refrigerant: Overcharging of refrigerant will not only cause the compressor to return air with liquid, but also may cause the refrigerant flow rate in the compressor return pipe to be too low, and the lubricating oil in the evaporator cannot return to the compressor, causing compression The machine is out of oil and burns out.
5. Evaporator failure: it enters the compressor return pipe without evaporation in the evaporator, which is easy to cause compressor liquid shock. If the evaporator fails to cause serious frosting on the coil, incomplete refrigerant evaporation is also the main reason for the compressor to return air and liquid.
6. The volume design and selection of the gas-liquid separator is unreasonable: adding a gas-liquid separator to the system is the most effective way to prevent the compressor from lacking oil and liquid hammer. The gas-liquid separator can temporarily store the excess refrigerant in the gas-liquid separation. In the heat pump system, the refrigerant liquid accumulated in the condenser can be transferred to the gas-liquid separator when the hot gas defrosting cooling and heating switch work, which greatly reduces the risk of liquid shock.
7. Refrigerant migration: Refrigerant migration means that when the compressor stops running, the refrigerant in the evaporator enters the compressor in gas form through the return line and is absorbed by the lubricating oil, or condensed in the compressor with the lubricating oil. The process or phenomenon of mixing.
2. What harm will the liquid strike cause?
For piston machines:
Make the lubricating oil produce a lot of bubbles, destroy the oil film on the lubricated surface, and make the oil pressure unstable; make the moving parts run without good lubrication, resulting in the pulling of the shaft; the cylinder liner is cooled and contracted rapidly, and the piston is hugged; Damage to the cylinder liner, piston, connecting rod, piston pin, etc.
For screw machine:
Liquid hammer can cause vibration, increase noise, and damage the rotor and bearings (excessive force); severe liquid hammer can also damage equipment and cause accidents.
For scroll compressors:
The liquid hammer produces a great impulse to the scroll, which may break the scroll. The lubricating oil containing a large amount of liquid refrigerant has a low viscosity and cannot form a sufficient oil film on the friction surface, resulting in rapid wear of the moving parts in the compressor; in addition, the lubricating oil The refrigerant will boil when heated during transportation, which will affect the normal transportation of lubricating oil.
Three. How to deal with the change in liquid strike:In the event of liquid shock, immediately close the compressor's suction shut-off valve and close the throttle valve to stop the liquid supply. If the suction temperature continues to decrease, keep turning down or even close the suction valve. The frictional heat between the crankshaft and the bearing bush is used to evaporate the refrigerant in the crankcase until the refrigerant in the crankcase is completely evaporated. After that, slightly open the suction shut-off valve and slowly increase the load. When a liquid strike occurs, pay attention to 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, and refuel and start the machine.
Fourth, how to prevent the compressor from liquid hammer::1. Reduce refrigerant charge: The best way to protect the compressor from malfunctions caused by liquid refrigerant is to limit the refrigerant charge within the compressor's allowable range. If it is not possible to achieve this, the filling volume should be reduced as much as possible under the condition of ensuring the refrigeration effect. It is necessary to be alert to the bubbles in the sight glass caused by the liquid pipe diameter being too small and the pressure head too low, and charging reasonably Refrigerant injection is very important.
2. Crankcase heater: The function of the crankcase heater is to keep the temperature of the refrigerated oil in the crankcase higher than the temperature of the compressor suction inlet in the system. The crankcase heater is generally continuously heated during use. The crankcase heater is very effective to prevent migration, but the crankcase heater cannot protect the compressor from damage caused by the liquid reflux, and the heater must be turned on when the compressor is turned on. It has been warmed up for a long time before, and the operability for practical applications is not strong.
3. Return air pipe superheating heater: In cold seasons, there is still the possibility of return air with liquid when the compressor is running. In this way, an electric heater or hot air can be added to the compressor return pipe section to increase the compressor return air superheat. The method is simple and effective, and an overheating detection device can be added to make the control more precise, thereby protecting the compressor and preventing liquid hammer.
4. Suction pipe gas-liquid separator: 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. Because different refrigeration systems have different overall charging requirements and different refrigerant control methods, whether or not 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 throttling element and increasing the heat exchange area of the evaporator in the design stage of the refrigeration system can effectively increase the return air superheat and reduce the occurrence of compressor liquid hammer during system operation. When charging refrigerant, avoid liquid charging from the low pressure side to prevent the compressor from starting with liquid.
