90% of compressor failures are caused by these three reasons

Common compressor failures include: motor burnout, liquid hammer, and lack of oil caused by refrigeration oil

Today, the editor will analyze the causes of these three major failures in detail, hoping to provide some help to you during maintenance.

Liquid hammer

The phenomenon that liquid refrigerant and/or lubricating oil damages the suction valve plate when it is sucked into the compressor cylinder with gas, and the phenomenon of instantaneous high hydraulic pressure generated by being compressed when the piston approaches the top dead center after entering the cylinder without being quickly discharged during the exhaust process is usually called liquid hammer.

Liquid hammer can cause damage to the compression force-bearing parts (such as valve plates, pistons, connecting rods, crankshafts, piston pins, etc.) in a very short time, and is a fatal killer of reciprocating compressors. Reducing or avoiding the entry of liquid into the cylinder can prevent the occurrence of liquid hammer, so liquid hammer can be completely avoided.

Process and phenomenon

(1) Suction valve plate fracture

The compressor is a machine that compresses gas. Normally, the piston compresses the gas 1450 times per minute (semi-hermetic compressor) or 2900 times per minute (fully hermetic compressor), that is, the time to complete an intake or exhaust process is 0.02 seconds or even shorter. The size of the intake and exhaust apertures on the valve plate and the elasticity and strength of the intake and exhaust valve plates are designed according to the gas flow. From the perspective of the force on the valve plate, the impact force generated when the gas flows is relatively uniform.

(2) Connecting rod fracture

The compression stroke takes about 0.02 seconds, while the exhaust process is even shorter. The droplets or liquid in the cylinder must be discharged from the exhaust hole in such a short time, and the speed and momentum are very large. The exhaust valve plate is the same as the intake valve plate, except that the exhaust valve plate is supported by a limit plate and a spring plate and 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 piston will compress the liquid when it approaches the top dead center. Since the time is very short, this process of compressing the liquid seems to be a collision, and a metallic knocking sound will also be heard in the cylinder head. Compressed liquid is another part or process of liquid hammer phenomenon.

Cause Analysis

Obviously, the liquid that can cause compressor liquid hammer is nothing more than the following sources:

1) Liquid return, that is, liquid refrigerant or lubricant flowing back from the evaporator to the compressor;

2) Foam during liquid startup;

3) Too much lubricant in the compressor.

1) Liquid return

Generally, liquid return refers to the phenomenon or process that 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 improper selection and use of expansion valves. Liquid return may be caused by over-selection of expansion valves, too low superheat setting, incorrect installation method of temperature sensor or damaged insulation wrapping, and failure of expansion valves. For small refrigeration systems using capillary tubes, excessive liquid addition will cause liquid return.

Liquid return is prone to occur in systems that use hot air to defrost. Whether the heat pump is operated with a four-way valve or the refrigeration is operated with a hot gas bypass valve, a large amount of liquid will be formed in the evaporator after the hot gas defrost, and this liquid may return to the compressor at the beginning of the subsequent refrigeration operation.

Liquid return will not only cause liquid hammer, but also dilute the lubricating oil and cause wear. The load and current of the motor will increase greatly during wear, which will cause motor failure over time. For refrigeration systems where liquid return is difficult to avoid, installing a gas-liquid separator and using pump-down control can effectively prevent or reduce the harm of liquid return.

2) Liquid start

Unlike liquid return, the refrigerant that causes liquid start enters the crankcase in the form of "refrigerant migration". Refrigerant migration refers to the process or phenomenon that when the compressor stops running, the refrigerant in the evaporator enters the compressor in the form of gas through the return gas pipeline and is absorbed by the lubricating oil, or condenses in the compressor and mixes with the lubricating oil.

After the compressor stops, the temperature will drop and the pressure will increase. Since the refrigerant vapor partial pressure in the lubricating oil is low, it will absorb the refrigerant vapor on the oil surface, causing the crankcase pressure to be lower than the evaporator pressure. The lower the oil temperature, the lower the vapor pressure, and the greater the absorption of refrigerant vapor. The vapor 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 the indoor evaporator, and the pressure in the crankcase is also low. After the refrigerant migrates to the compressor, it is also easy to be condensed and enter the lubricating oil.

Due to structural reasons, the decrease in crankcase pressure when the air-cooled compressor is started will be much slower, the foaming phenomenon is not very intense, and it is difficult for the foam to enter the cylinder. Therefore, there is no problem of liquid shock when the air-cooled compressor is started.

In theory, installing a crankcase heater (electric heater) on the compressor can effectively prevent refrigerant migration. After a short period of shutdown (such as at night), keeping the crankcase heater powered 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 more than ten hours before starting can evaporate most of the refrigerant in the lubricating oil, which can greatly reduce the possibility of liquid hammer during liquid startup and the harm caused by refrigerant flushing. However, in actual applications, it is difficult to maintain the power supply of the heater after shutdown or to power the heater for more than ten hours before starting. Therefore, the actual effect of the crankcase heater will be greatly reduced.

Of course, by improving the compressor structure, the migration of refrigerant can be prevented and the foaming of lubricating oil can be reduced. By improving the return oil path in the return air cooling compressor, adding checkpoints (return oil pump, etc.) to the channel where the motor cavity and the crankcase migrate, the passage can be cut off after shutdown, and the refrigerant cannot enter the crankshaft cavity; reducing the channel section between the intake duct and the crankcase can slow down the speed of the crankcase pressure drop when starting, thereby controlling the degree of foaming and the amount of foam entering the cylinder.

3) Too much lubricating oil

Semi-hermetic compressors usually have an oil sight glass to observe the oil level. If the oil level is higher than the oil sight glass range, it means that there is too much oil. If the oil level is too high, the high-speed rotating crankshaft and connecting rod big end may frequently hit the oil surface, causing a large amount of lubricating oil to splash. Once the splashing lubricating oil enters the intake duct and is brought into the cylinder, it may cause liquid hammer.

When installing and debugging a large refrigeration system, it is often necessary to add lubricating oil appropriately. However, for systems with poor oil return, it is necessary to carefully find the root cause of the oil return. It is dangerous to add lubricating oil blindly. Even if the oil level is not high temporarily, you should also pay attention to the dangers that may be caused by the sudden return of a large amount of lubricating oil (such as after defrosting). Liquid hammer caused by lubricating oil is not uncommon.

Oil shortage and insufficient lubrication

The compressor is a complex machine that runs at high speed. Ensuring sufficient lubrication of the compressor crankshaft, bearings, connecting rods, pistons and other moving parts is a basic requirement for maintaining the normal operation of the machine. For this reason, compressor manufacturers require the use of specified brands of lubricating oil and require regular inspection of the lubricating oil level and color. However, due to negligence in the design, construction and maintenance of the refrigeration system, it is common for the compressor to lack oil, oil coking and deterioration, return dilution, refrigerant flushing, and the use of inferior lubricants to cause insufficient lubrication of moving parts. Insufficient lubrication can cause wear or scratches on the bearing surface. In severe cases, it can cause shaft seizure, piston jamming in the cylinder, and the resulting bending and breaking of the connecting rod.

Lack of oil

Lack of oil is one of the most easily identifiable compressor failures. When the compressor lacks oil, there is little or no lubricant in the crankcase.

The compressor is a special air pump. When a large amount of refrigerant gas is discharged, it also carries away a small amount of lubricant (called oil running or oil running). Oil running in the compressor is inevitable, but the speed of oil running is different. There is about 2-3% lubricant oil in the exhaust of semi-sealed piston compressors, while it is 0.5-1% for scroll compressors. For a 6-cylinder compressor with a displacement of 100m3/hr and a crankcase oil storage capacity of 6 liters, 3% oil loss means about 0.3-0.8 liters/minute of oil loss, or the compressor will run for more than ten minutes without oil return.

If the lubricating oil discharged from the compressor does not return, the compressor will be short of oil.

There are two ways to return oil to the compressor, one is oil separator oil return, and the other is oil return pipe oil return.

The oil separator is installed on the compressor exhaust pipeline, which can generally separate 50-95% of the waste oil, with good oil return effect and fast speed, greatly reducing the amount of oil entering the system pipeline, thereby effectively extending the oil return time. For cold storage refrigeration systems with particularly long pipelines, full liquid ice making systems, and freeze-drying equipment with very low temperatures, it is not uncommon for more than ten minutes or even dozens of minutes after startup to not return oil or the amount of oil returned is very small. Poorly designed systems will have the problem of compressor oil pressure being too low and shutting down.

When the internal oil return problem occurs, it can be observed that the oil level of the compressor continues to drop until the oil pressure safety device is activated. After the compressor stops, the oil level of the crankcase recovers quickly. The root cause of the internal oil return problem is cylinder leakage, and the worn piston assembly should be replaced in time.

The wear caused by lack of oil in the compressor is generally uniform. If there is little or no lubricating oil, there will be severe friction on the bearing surface and the temperature will rise rapidly within a few seconds. If the power of the motor is large enough, the crankshaft will continue to rotate, and the crankshaft and bearing surface will be worn or scratched, otherwise the crankshaft will be locked by the bearing and stop rotating. The reciprocating motion of the piston in the cylinder is the same. Lack of oil will cause wear or scratches. In severe cases, the piston will be stuck in the cylinder and cannot move.

Insufficient lubrication

The direct cause of wear is insufficient lubrication. Lack of oil will definitely cause insufficient lubrication, but insufficient oil lubrication is not necessarily caused by lack of oil. The following three reasons can also cause insufficient lubrication: the lubricating oil cannot reach the bearing surface; the lubricating oil has reached the bearing surface, but the viscosity is too low to form an oil film of sufficient thickness; the lubricating oil has reached the bearing surface, but it has decomposed due to overheating and cannot play a lubricating role.

Liquid return is a common system problem. One of the major hazards of liquid return is diluting the lubricating oil. After the diluted lubricating oil reaches the friction surface, the viscosity is low and it cannot form a protective oil film of sufficient thickness, which will cause wear over time. When the amount of liquid return is relatively large, the lubricating oil will be very thin, not only can it not play a lubricating role, but it will also dissolve and flush the original oil film, causing refrigerant flushing.

Due to various reasons (including the compressor startup stage), the temperature of the friction surface that does not receive lubricating oil will rise rapidly, and the lubricating oil will begin to decompose after exceeding 175°.

"Insufficient lubrication-friction-high surface temperature-oil decomposition" is a typical vicious cycle. Many vicious accidents including connecting rod seizure and piston cylinder seizure are related to this vicious cycle.

Insufficient lubrication and oil shortage can be seen in the disassembled compressor. Oil shortage generally manifests as large-scale, relatively uniform surface damage and high temperature, while insufficient lubrication is more likely to be caused by wear, scratches and high temperature in some specific parts, such as bearing surfaces far away from the oil pump.

Conclusion and Suggestions

Oil shortage can cause serious lubrication shortage. The fundamental reason for oil shortage is not the amount and speed of oil running in the compressor, but the poor oil return of the system. Installing an oil separator can quickly return oil and extend the compressor's oil return-free operation time.

The design of the evaporator and return air pipeline must take oil return into consideration. Maintenance measures such as avoiding frequent starts, timed defrosting, timely replenishment of refrigerant, and timely replacement of worn piston components can also help return oil.

Liquid return and refrigerant migration will dilute the lubricating oil, which is not conducive to the formation of oil film; oil pump failure and oil line blockage will affect the oil supply and oil pressure, resulting in oil shortage on the friction surface; high temperature on the friction surface will cause the lubricating oil to decompose and lose its lubricating ability.

"The root cause of oil shortage lies in the system. Therefore, replacing only the compressor or certain accessories cannot fundamentally solve the oil shortage problem.