Common problems and solutions of liquid refrigerants in refrigeration systems
【】1. Liquid refrigerant migration
Refrigerant migration refers to the accumulation of liquid refrigerant in the compressor crankcase when the compressor is stopped. As long as the temperature in the compressor is lower than the temperature in the evaporator, the pressure difference between the compressor and the evaporator will drive the refrigerant to migrate to a colder place. In cold winters, this phenomenon z* easily occurs. However, for air conditioners and heat pump devices, when the condensing unit is far from the compressor, migration may occur even if the temperature is high.
Once the system is shut down, if it is not turned on within a few hours, even if there is no pressure difference, migration phenomenon may occur due to the refrigerant oil in the crankcase attracting the refrigerant.
If excessive liquid refrigerant migrates into the crankcase of the compressor, severe liquid hammer will occur when the compressor is started, leading to various malfunctions of the compressor, such as valve rupture and piston damage , Bearing failure and bearing erosion (refrigerant washes out the frozen oil from the bearing).
2. Liquid refrigerant overflow
When the expansion valve fails, or the evaporator fan fails or is blocked by the air filter, the liquid refrigerant will overflow in the evaporator. It enters the compressor through the suction pipe in the form of liquid instead of vapor. When the unit is running, the liquid overflow dilutes the refrigerating oil and causes the wear of the moving parts of the compressor. The drop of oil pressure causes the action of the oil pressure safety device, which causes the crankcase to lose oil. In this case, if the machine is shut down, refrigerant migration will occur quickly, which will cause liquid shock when restarting.
3. Liquid hammer
When liquid hammer occurs, a metal impact sound can be heard inside the compressor, which may be accompanied by severe vibration of the compressor. Liquid hammer can cause valve rupture, compressor head gasket damage, connecting rod fracture, crankshaft fracture, and other types of compressor damage. When the liquid refrigerant migrates into the crankcase, liquid hammer will occur when the machine is turned on again. In some units, due to the piping structure or the position of the components, the liquid refrigerant will accumulate in the suction pipe or the evaporator during the shutdown of the unit, and will enter the compressor in the form of pure liquid and at a particularly high speed when the unit is turned on. . The speed and inertia of the liquid hammer are sufficient to destroy any built-in compressor anti-liquid hammer protection measures.
4. Oil pressure safety control device action
In a low-temperature unit, after the defrosting period, liquid refrigerant overflow will often cause the oil pressure safety control device to act. Many systems are designed to allow refrigerant to condense in the evaporator and suction pipe during defrosting, and then these refrigerants flow into the compressor crankcase when starting up, causing the oil pressure to drop, causing the oil pressure safety device to act.
Occasionally once or twice the action of the oil pressure safety control device will not have a serious impact on the compressor, but repeated times without good lubrication will cause the compressor to malfunction. The oil pressure safety control device is often regarded as a minor fault by the operator. They do not know that this is a warning, indicating that the compressor has been running for more than two minutes without lubrication, and remedial measures need to be implemented in time.
5. Recommended remedial measures
The more refrigerant charge in the refrigeration system, the greater the probability of failure. Only when the compressor and the other main components of the system are connected together for system testing, can it be determined that z* is large and at the same time a safe refrigerant charge. The compressor manufacturer can determine the maximum charge of liquid refrigerant z* that does not cause damage to the working parts of the compressor, but cannot determine how much of the total charge of the refrigeration system is actually in the compressor in most extreme cases. The maximum charge of liquid refrigerant z* that the compressor can withstand depends on its design, internal volume, and charge of refrigerating oil. When liquid migration, overflow or liquid strike occurs, necessary remedial measures must be taken. The type of remedial measures depends on the system design and the type of failure.
6, reduce refrigerant charge
protect the compressor from malfunctions caused by liquid refrigerant z The best way is to limit the refrigerant charge to the compressor allowable Within range. If this is not possible, the filling volume should be reduced as much as possible. Under the condition that the flow rate is satisfied, the condenser, evaporator and connecting pipe should be as small as possible, and the liquid reservoir should be as small as possible. After the filling amount z* is reduced, correct operation is required. It is necessary to be aware of the bubbles in the sight glass caused by the liquid pipe diameter is too small and the head pressure is too low, which will cause serious overfilling.
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