First, the concept and classification of liquid blow

The phenomenon that the liquid refrigerant and/or lubricating oil damages the suction valve piece when the gas is sucked into the cylinder of the compressor, and is not discharged quickly after the air is introduced into the cylinder, and is instantaneously high-pressure generated when the piston is compressed near the top dead center. The phenomenon is often referred to as a liquid strike. Liquid hammer can cause damage to compression parts (such as valve plates, pistons, connecting rods, crankshafts, piston pins, etc.) in a short time, and is a deadly killer of reciprocating compressors. Reducing or avoiding the entry of liquid into the cylinder prevents liquid slamming, so liquid slamming is completely avoidable.

Usually, 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 at a higher speed with the suction, the suction valve piece may be excessively bent or broken due to the impact and incompressibility of the liquid; When the liquid that has not been vaporized and discharged in the cylinder is compressed by the piston, the pressure is instantaneous and the deformation and damage of the force-receiving member are caused. These force-receiving members include an intake and exhaust valve plate, a valve plate, a valve plate pad, a piston (top), a piston pin, a connecting rod, a crankshaft, a bearing bush, and the like.

Second, the process and phenomenon

1 suction valve piece breaks

The compressor is a machine that compresses gas. Typically, the piston compresses the gas 1450 times per minute (half-sealed compressor) or 2900 times (full-sealed compressor), ie, the time to complete an inhalation or exhaust process is 0.02 seconds or less. The size of the suction and exhaust apertures on the valve plate and the elasticity and strength of the suction and exhaust valve plates are designed according to the gas flow. From the perspective of the force of the valve piece, the impact force generated when the gas flows is relatively uniform.

The density of a liquid is tens or even hundreds of times that of a gas, so that the momentum of the liquid flowing is much larger than that of the gas, and the impact force generated is much greater. The flow in the inhalation when more droplets enter the cylinder belongs to the two-phase flow. The impact of the two-phase flow on the suction valve piece is not only high intensity but also high frequency, just like the typhoon is mixed with pebbles on the glass window, and its destructiveness is self-evident. The rupture of the suction flap is one of the typical features and processes of liquid slamming.

2 link break

The compression stroke takes about 0.02 seconds and the venting process is shorter. The droplets or liquid in the cylinder must be discharged from the vent hole in such a short period of time, and the speed and momentum are large. The condition of the exhaust valve piece is the same as that of the suction valve piece, except that the exhaust valve plate is supported by the limit plate and the spring piece, and is not easily broken. When the impact is severe, the limit plate will also be deformed and tilted.

If the liquid does not evaporate and drain out of the cylinder in time, the piston will compress the liquid when it approaches the top dead center. Due to the short time, the process of compressing the liquid seems to be a collision, and a metal knocking sound is also emitted from the cylinder head. Compressed liquid is another part or process of liquid slamming.

The high pressure generated by the liquid hitting moment has a great degree of destructiveness. When the familiar link is bent or even broken, other compression members (valve plate, valve plate pad, crankshaft, piston, piston pin, etc.) will also be deformed. Or damaged, but often overlooked, or confused with excessive exhaust pressure. When the compressor is overhauled, it is easy to find the bent or broken connecting rod and give it a replacement, and forget to check whether other parts are deformed or damaged, thus laying a foundation for future failures.

The breakage of the connecting rod caused by the liquid hammer 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 the liquid hammer occurs in a short time. The piston and the crankshaft at both ends of the connecting rod move freely, and generally there is no axle or seizure caused by severe wear. Although the valve chip debris occasionally causes serious scratches on the piston and cylinder face after the suction valve piece is broken, the surface scratch and the lubrication failure cause the wear to be very different. Secondly, the breakage of the connecting rod caused by the liquid hammer is caused by the pressure, and the connecting rod and the broken joint have the pressing characteristics. Although the breakage of the connecting rod after the piston bites the cylinder is also possible, the premise is that the piston must be stuck in the cylinder. The connecting rod after the axle is broken is even more different. The big rod of the connecting rod and the crankshaft are seriously worn, and the breaking force is the shearing force, and the breaking is different. Finally, before the axle is held and the cylinder is bitten, the motor will be overloaded, the motor will be hot, and the thermal protector will operate.

Third, the cause of liquid hammer

1 back liquid

Generally, liquid return refers to the phenomenon or process of returning liquid refrigerant in the evaporator to the compressor through the suction line while the compressor is running.

For refrigeration systems that use expansion valves, liquid return is closely related to the selection and use of expansion valves. If the expansion valve is too large, the superheat setting is too small, the installation method of the temperature sensor package is incorrect, or the insulation package is damaged, and the expansion valve fails, the liquid may be returned. For small refrigeration systems that use capillaries, the amount of liquid added is too large to cause backflow.

Systems that utilize hot air defrosting are prone to liquid return. Whether using a four-way valve for heat pump operation or a hot gas bypass valve for cooling operation, hot gas defrosting creates a large amount of liquid in the evaporator that may return to the compressor at the beginning of the subsequent cooling operation.

In addition, when the evaporator is severely frosted or the fan fails, the heat transfer deteriorates, and the unvaporized liquid causes the liquid to return. Frequent fluctuations in the temperature of the cold storage can also cause the expansion valve to fail to react and cause liquid return.

Most of the liquid impact accidents caused by liquid return occur in air-cooled (referred to as air-cooled or air-cooled) semi-closed compressors and single-stage two-stage compressors, because the cylinders of these compressors are directly connected to the return air pipe. It is easy to cause a liquid impact accident. Even if no liquid impact is caused, returning to the cylinder will dilute or flush the lubricant on the piston and cylinder wall, increasing piston wear.

For return-to-gas (refrigerant steam) cooled semi-hermetic and hermetic compressors, liquid return rarely causes liquid hammer. However, it will dilute the lubricating oil in the crankcase. Lubricating oils containing a large amount of liquid refrigerant have low viscosity and cannot form a sufficient oil film on the friction surface, resulting in rapid wear of the moving parts. In addition, the refrigerant in the lubricating oil will boil when it is heated during the conveying process, which affects the normal transportation of the lubricating oil. The farther away from the oil pump, the more obvious the problem becomes. If the bearing at the motor end is severely worn, the crankshaft may settle to one side, which may easily cause the stator to sweep and the motor to burn.

Obviously, the liquid return will not only cause liquid shock, but also dilute the lubricating oil to cause wear. The load and current of the motor will increase greatly during wear and will cause motor failure over time.

For refrigeration systems that are difficult to avoid with liquid return, installing a gas-liquid separator and using an evacuation shutdown control can effectively prevent or reduce the hazard of liquid return.

2 with liquid start

When the return air-cooled compressor is started, the phenomenon that the lubricating oil in the crankcase violently foams is called starting with liquid. The blistering phenomenon with liquid start can be clearly observed on the oil sight glass. The root cause of liquid start-up is that a large amount of refrigerant dissolved in the lubricating oil and sinking under the lubricating oil suddenly boils when the pressure suddenly drops, and causes foaming of the lubricating oil. This phenomenon is very similar to the cola foaming phenomenon when people suddenly open a Coke bottle in daily life. The duration of foaming is related to the amount of refrigerant, usually a few minutes or ten minutes. A large amount of foam floats on the oil surface and even fills the crankcase. Once the cylinder is drawn through the intake port, the foam is reduced to a liquid (a mixture of lubricating oil and refrigerant) that can easily cause a liquid impact. Obviously, the liquid blow caused by the start of the liquid only occurs during the start-up process.

Unlike the liquid return, the refrigerant that causes the liquid to start is entered into the crankcase in a "refrigerant migration" manner. Refrigerant migration refers to the process or phenomenon in which the refrigerant in the evaporator is in the form of a gas, enters the compressor through the return line and is absorbed by the lubricating oil, or is mixed with the lubricating oil after condensation in the compressor.

When the compressor is shut down, the temperature will decrease and the pressure will increase. Since the partial pressure of the refrigerant vapor in the lubricating oil is low, the refrigerant vapor on the oil surface is absorbed, causing the crankcase air pressure to be lower than the evaporator air pressure. The lower the oil temperature, the lower the vapor pressure and the greater the absorption of refrigerant vapor. The vapor in the evaporator slowly "migrates" into the crankcase. In addition, if the compressor is outdoors, the weather is cold or at night, the temperature is often lower than the evaporator in the room, and the pressure in the crankcase is low. After the refrigerant migrates to the compressor, it is easily condensed into the lubricating oil.

Refrigerant migration is a very slow process. The longer the compressor downtime, the more refrigerant will migrate into the lubricant. This process proceeds as long as liquid refrigerant is present in the evaporator. Since the lubricating oil that dissolves the refrigerant is heavy, it sinks to the bottom of the crankcase, and the lubricating oil that floats on it can absorb more refrigerant.

In addition to causing liquid slamming, refrigerant migration also dilutes the lubricant. Very thin lubricating oil is pumped by oil to each friction surface, which may wash away the original oil film and cause serious wear (this phenomenon is often called refrigerant flushing). The transitional wear will make the matching gap become larger, causing oil leakage, which will affect the lubrication of the far-reaching part, and will cause the oil pressure protector to operate in severe cases.

Due to structural reasons, the crankcase pressure is much slower when the air-cooled compressor starts, the foaming phenomenon is not very severe, and the foam is difficult to enter the cylinder. Therefore, the air-cooled compressor does not have the problem of liquid-starting liquid strike.

In theory, the compressor is equipped with a crankcase heater (electric heater) to prevent refrigerant migration. After a short downtime (such as at night), maintaining the crankcase heater energized can cause the lubricant temperature to be slightly above the rest of the system and refrigerant migration will not occur. After long-term shutdown (such as a winter), heating the lubricant for several or ten hours before starting the machine can evaporate most of the refrigerant in the lubricating oil, which can greatly reduce the possibility of liquid attack when starting with liquid. Sex can also reduce the damage caused by refrigerant flushing. However, in practical applications, it is difficult to maintain the heater power supply after the shutdown or power the heater for more than ten hours before starting the power. Therefore, the actual effect of the crankcase heater will be greatly reduced.

For larger systems, letting the compressor drain the liquid refrigerant in the evaporator before stopping (called pumping down) can fundamentally avoid refrigerant migration. The installation of a gas-liquid separator on the return line can increase the resistance of refrigerant migration and reduce the amount of migration.

Of course, by improving the compressor structure, it is possible to prevent refrigerant migration and slow down the foaming of the lubricating oil. By improving the oil return path in the return air-cooled compressor, a checkpoint (return pump, etc.) is added to the passage where the motor chamber and the crankcase migrate, and the passage can be cut off after the stop, the refrigerant cannot enter the crank chamber; the intake air is reduced. The passage section of the track and crankcase can slow down the crankcase pressure drop rate at start-up, thereby controlling the degree of foaming and the amount of foam entering the cylinder.

3 too much lubricant

Semi-hermetic compressors usually have oil sight glasses to observe the oil level. The oil level is higher than the range of the oil sight glass, indicating that the oil is too much. The oil level is too high, and the crankshaft and the connecting rod with high speed rotation may frequently hit the oil surface, causing a large amount of splashing of lubricating oil. Once the splashed lubricant enters the intake port and is brought into the cylinder, it may cause a liquid impact.

When installing and debugging large refrigeration systems, it is often necessary to replenish the lubricant properly. However, for systems that do not return oil well, it is dangerous to carefully look for the root cause of oil return. Even if the temporary oil level is not high, pay attention to the dangers that may occur when the lubricant suddenly returns in large quantities (such as after defrosting). Liquid blows caused by lubricating oil are not uncommon.

Fourth, summary

Liquid hammer is a common fault in compressors. A liquid strike occurs, indicating that there must be a problem in the system or maintenance that needs to be corrected. Careful observation of the design, construction and maintenance of the analysis system is not difficult to find the source of the liquid attack. Instead of preventing liquid blows from the root cause, simply repairing or replacing a faulty compressor with a new compressor will only cause the liquid hammer to reoccur.