The phenomenon of liquid refrigerant and/or lubricating oil damaging the intake valve plates when they are sucked into the compressor cylinder with gas, and the phenomenon of high instantaneous hydraulic pressure generated by compression when they are not quickly discharged during the exhaust process when the piston is near the top dead center, is usually called liquid hammer. Liquid hammer can damage the stressed parts of the compressor (such as valve plates, pistons, connecting rods, crankshafts, piston pins, etc.) in a very short time, and is a fatal killer for reciprocating compressors. Reducing or avoiding liquid entering the cylinder can prevent liquid hammer, so liquid hammer is completely avoidable.

Generally, liquid hammer can be divided into two parts or processes. First, when a large amount of liquid refrigerant, lubricating oil, or a mixture of both enters the compressor cylinder at a high speed with suction, due to the impact and incompressibility of the liquid, it will cause excessive bending or breakage of the intake valve plates; 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 appears instantaneously will cause deformation and damage to the stressed parts. These stressed parts include intake and exhaust valve plates, valve plates, valve plate gaskets, pistons (top), piston pins, connecting rods, crankshafts, and bearings.

Refrigeration Compressor Liquid Hammer Process and Phenomenon

(1) Intake valve plate breakage The compressor is a machine that compresses gas. Usually, the piston compresses gas 1450 times (semi-hermetic compressor) or 2900 times (fully hermetic compressor) per minute, that is, the time to complete one intake or exhaust process is 0.02 seconds or even shorter. The size of the intake and exhaust ports on the valve plate and the elasticity and strength of the intake and exhaust valve plates are all designed according to the gas flow. From the perspective of valve plate stress, the impact force generated by gas flow is relatively uniform.

The density of liquid is tens or even hundreds of times that of gas, so the momentum of liquid flow is much larger than that of gas, and the impact force is also much larger. The flow of a larger amount of liquid droplets entering the cylinder during suction belongs to two-phase flow. The impact generated by two-phase flow on the intake valve plate is not only strong but also high in frequency, just like a typhoon with pebbles hitting a glass window, the destructiveness is self-evident. Intake valve plate breakage is one of the typical characteristics and processes of liquid hammer.

(2) Connecting rod breakage of refrigeration compressor The time of the compression stroke 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 port in such a short time, with great speed and momentum. The situation of the exhaust valve plate is the same as that of the intake valve plate, the difference being that the exhaust valve plate has a limit plate and a spring plate support, which is not easy to break. When the impact is severe, the limit plate will also deform and warp.

If the liquid is not evaporated and discharged from the cylinder in time, the liquid will be compressed when the piston is close to the top dead center. Because the time is very short, this process of compressing the liquid is like an impact, and a metallic knocking sound will also be heard in the cylinder head. Compressing the liquid is another part or process of the liquid hammer phenomenon.

The high pressure generated by the instantaneous liquid hammer has great destructive power. In addition to the familiar bending or even breaking of the connecting rod, other compressed stressed parts (valve plate, valve plate gasket, crankshaft, piston, piston pin, etc.) will also be deformed or damaged, but are often overlooked, or confused with excessive exhaust pressure. When overhauling the compressor, people will easily find the bent or broken connecting rod and replace it, but forget to check whether other parts are deformed or damaged, thus laying the groundwork for future failures.

The connecting rod breakage caused by liquid hammer is different from bearing seizure and piston seizure, and can be distinguished. First, the bending or breakage of the connecting rod caused by liquid hammer occurs in a short time, and the piston and crankshaft at both ends of the connecting rod move freely, and generally there will be no serious wear and tear caused by bearing seizure or piston seizure. Although the fragments of the broken intake valve plate will occasionally cause serious scratches on the piston and cylinder surface, the surface scratches are very different from the wear caused by lubrication failure. Second, the breakage of the connecting rod caused by liquid hammer is caused by pressure, and the connecting rod and broken end have extrusion characteristics. Although the breakage of the connecting rod after piston seizure may also have extrusion, the premise is that the piston must be stuck in the cylinder. The breakage of the connecting rod after bearing seizure is even more different. The connecting rod big end and crankshaft have serious wear and tear, and the force causing the breakage is shear force, and the broken end is also different. Finally, before bearing seizure and piston seizure, the motor will run overloaded, and the motor will generate serious heat, and the thermal protector will act.