Liquid shock refers to the damage caused to the suction valve plate when liquid refrigerant or lubricating oil is sucked into the compressor cylinder along with the gas, and the liquid material fails to be discharged quickly during the exhaust process after entering the cylinder. When the piston is close to It is compressed at the top dead center, resulting in instantaneous high pressure. Liquid shock can cause damage to important components of the compressor such as exhaust valve plate, valve plate, valve plate gasket, piston (especially the top), piston pin, connecting rod, crankshaft, bearing bush, etc. in a very short period of time. Especially fatal for compressors.
The density of liquid is much greater than that of gas, and its momentum and impact force when flowing are also much greater than that of gas. When a large number of droplets are included in the suction air and enter the cylinder, this two-phase flow will produce high-intensity and high-frequency impacts on the suction valve piece, just like a typhoon mixed with pebbles hitting a glass window, and its destructive effect is obvious. The suction valve plate fracture is a significant feature and process of liquid slugging.
When the liquid fails to evaporate and be discharged from the cylinder in time, the piston will compress the liquid when it is close to the top dead center. Since the compression time is very short, this process is similar to an impact, and a metal knocking sound will also be heard in the cylinder head. This act of compressing a liquid is another part or process of liquid slugging. The connecting rod fracture caused by liquid impact is different from the fracture caused by shaft holding or piston biting cylinder. The former occurs in a short time. The pistons and crankshaft at both ends of the connecting rod can still move freely, and the fractured part will have extrusion characteristics. .
The liquid that causes liquid shock in the compressor mainly comes from the following points:
Liquid backflow: This refers to the phenomenon of liquid refrigerant or lubricating oil returning from the evaporator to the compressor through the suction line. In a refrigeration system using an expansion valve, liquid backflow may occur if the expansion valve is too large, the superheat setting is too small, the temperature sensing package is installed incorrectly or the insulation wrapping is damaged, or the expansion valve fails. For small refrigeration systems using capillary tubes, adding too much liquid will also cause liquid backflow. Systems that utilize hot gas defrost are prone to liquid backflow. Regardless of whether a four-way valve is used for heat pump operation or a hot gas bypass valve is used for refrigeration operation, a large amount of liquid formed in the evaporator after hot gas defrost may flow back when the subsequent refrigeration operation starts. compressor. In addition, severe frosting on the evaporator or failure of the fan causes the heat transfer effect to decrease, and the unevaporated liquid will cause liquid backflow. Frequent temperature fluctuations in cold storage can also cause expansion valve response failure, causing liquid backflow.
Start-up with liquid: This refers to the phenomenon that when a return air-cooled compressor is started, the lubricating oil in the crankcase foams violently when the pressure suddenly drops due to a large amount of dissolved refrigerant. Refrigerant migration refers to the process in which the refrigerant in the evaporator enters the compressor in the form of gas through the return pipe and is absorbed by the lubricating oil after the compressor is shut down, or is condensed in the compressor and mixed with the lubricating oil. Refrigerant migration not only easily causes liquid slugging, but also dilutes the lubricating oil, causing severe wear.
Too much lubricating oil: When the lubricating oil exceeds the safe range indicated by the oil sight glass, the high-speed rotating crankshaft and the big end of the connecting rod may frequently hit the oil surface, causing the lubricating oil to splash into the air intake, thereby causing liquid shock.
Liquid slugging is not only one of the common faults of compressors, but also a signal that there are problems in system design, installation or maintenance. If the root cause of the problem is not addressed and the damaged compressor is simply repaired or replaced, the liquid slugging problem will recur.
