Liquid refrigerant migration
Refrigerant migration refers to the accumulation of liquid refrigerant in the compressor crankcase when the compressor is shut down. As long as the temperature inside the compressor is lower than the temperature inside the evaporator, the pressure difference between the compressor and the evaporator will drive the refrigerant to a colder place. This phenomenon is most likely to occur during the cold winter months. However, for air conditioning and heat pump devices, when the condensing unit is far from the compressor, even if the temperature is high, the migration phenomenon may occur.
When the system is shut down, if it is not turned on within a few hours, even if there is no pressure difference, the migration phenomenon may occur due to the attraction of the refrigerated oil in the crankcase to the refrigerant.
If excessive liquid refrigerant migrates into the crankcase of the compressor, serious liquid shock will occur when the compressor starts, resulting in various compressor failures, such as valve disc rupture, piston damage, bearing failure and bearing erosion (refrigerant washes the chilled oil away from the bearing).
Liquid refrigerant overflow
When the expansion valve fails to operate, or the evaporator fan fails or is blocked by the air filter, the liquid refrigerant will overflow in the evaporator and enter the compressor as a liquid rather than steam through the suction tube. When the unit is running, the liquid overflow dilutes the refrigerated oil, resulting in the wear of the compressor moving parts, and the oil pressure reduction leads to the action of the oil pressure safety device, thus making the crankcase lose oil. In this case, if the machine is shut down, the refrigerant migration phenomenon will quickly occur, resulting in a liquid shock when it is started again.
Liquid hammer
When the liquid strike occurs, the metal percussion sound emitted from the compressor can be heard, and the compressor may be accompanied by violent vibration. Hydraulic percussion can cause valve rupture, compressor head gasket damage, connection rod fracture, shaft fracture and other types of compressor damage. When the liquid refrigerant migrates into the crankcase, the liquid shock will occur when the crankcase is turned on. In some units, due to the structure of the pipeline or the location of the components, the liquid refrigerant will accumulate in the suction tube or evaporator during the downtime of the unit, and will enter the compressor in the form of pure liquid at a particularly high speed when it is turned on. The speed and inertia of the hydraulic stroke are sufficient to destroy the protection of any built-in compressor anti-hydraulic stroke device.
Oil pressure safety control device action
In a cryogenic unit, after the frost removal period, the overflow of liquid refrigerant often causes the oil pressure safety control device to operate. Many systems are designed to allow refrigerant to condense in the evaporator and suction tube during defrosting, and then flow into the compressor crankcase at startup causing oil pressure to drop, causing the oil pressure safety device to operate.
Occasionally once or twice the oil pressure safety control device action will not have a serious impact on the compressor, but repeated times in the absence of good lubrication conditions will lead to compressor failure. The oil pressure safety control device is often considered by the operator to be a small fault, but it is a warning that the compressor has been running for more than two minutes without lubrication, and remedial measures need to be implemented in a timely manner.
Recommended remedies
The more refrigerant the refrigeration system is charged, the greater the chance of failure. Only when the compressor and other major components of the system are connected together for system testing can the maximum and safe refrigerant charge be determined. Compressor manufacturers are able to determine the maximum amount of liquid refrigerant to be charged without harming the working parts of the compressor, but they are not able to determine how much of the total refrigerant charge in the refrigeration system is actually in the compressor in most extreme cases. The maximum amount of liquid refrigerant that the compressor can withstand depends on its design, content volume and the amount of refrigerant oil charged. When liquid migration, overflow or knock occurs, the necessary remedial action must be taken, the type of remedial action depends on the system design and the type of failure.
Reduce the amount of refrigerant charged
The best way to protect the compressor from failure caused by liquid refrigerants is to limit the refrigerant charge to the compressor’s allowable range. If this is not possible, the amount of filling should be reduced as much as possible. Under the condition of meeting the flow rate, the condenser, evaporator and connecting pipe should be used as small as possible, and the liquid reservoir should be selected as small as possible. The minimization of the amount of filling requires the correct operation to alert the eyeglass to bubbles caused by the small diameter of the liquid tube and the low head pressure, which can lead to serious overfilling.
Evacuation cycle
The most active and reliable method of controlling liquid refrigerant is the evacuation cycle. Especially when the amount of system charge is large, by closing the solenoid valve of the liquid pipe, the refrigerant can be pumped into the condenser and the liquid reservoir, and the compressor runs under the control of the low-pressure safety control device, so the refrigerant is isolated from the compressor when the compressor is not running, avoiding the migration of refrigerant to the compressor crankcase. It is recommended to use a continuous evacuation cycle during the shutdown phase to prevent leakage of the solenoid valve. If it is a single evacuation cycle, or called non-recirculating control mode, there will be too much refrigerant leakage damage to the compressor when it is shut down for a long time. Although the continuous evacuation cycle is the best way to prevent migration, it does not protect the compressor from the adverse effects of refrigerant overflow.
Crankcase heater
In some systems, operating environments, costs, or customer preferences that may make evacuation cycles impossible, crankcase heaters can delay migration.
The function of the crankcase heater is to keep the temperature of the chilled oil in the crankcase above the temperature of the lowest part of the system. However, the heating power of the crankcase heater must be limited in order to prevent overheating and freezing oil carbon. When the ambient temperature is close to -18 ° C, or when the suction tube is exposed, the role of the crankcase heater will be partially offset, and the migration phenomenon may still occur.
Crankcase heaters are generally continuously heated in use, because once the refrigerant enters the crankcase and condenses in the chilled oil, it can take up to several hours to get it back to the suction tube again. When the situation is not particularly serious, the crankcase heater is very effective for preventing migration, but the crankcase heater cannot protect the compressor from the damage caused by the liquid backflow.
Suction tube gas-liquid separator
For systems prone to liquid overflow, a gas-liquid separator should be installed on the suction line to temporarily store the liquid refrigerant that has spilled from the system and return the liquid refrigerant to the compressor at a rate that the compressor can withstand.
Refrigerant overflow is most likely to occur when the heat pump is switched from the cooling condition to the heating condition, and in general, the suction tube gas-liquid separator is a necessary equipment in all heat pumps.
Systems that use hot gas for defrosting are also prone to liquid overflow at the beginning and end of the defroster. Low superheat devices such as liquid freezers and compressors in low temperature display cases can occasionally cause overflow due to improper refrigerant control. For vehicle devices, when experiencing a long shutdown phase, it is also prone to serious overflow when restarting.
In a two-stage compressor, the suction is returned directly to the lower cylinder and does not pass through the motor chamber, and a gas-liquid separator should be used to protect the compressor valve from the damage of the liquid blow.
Because the overall charge requirements of different refrigeration systems are different, and the refrigerant control methods are different, whether a gas-liquid separator is needed and what size of gas-liquid separator is needed depends on the requirements of the specific system to a large extent. If the amount of liquid backflow is not accurately tested, a conservative design approach is to determine the gas-liquid separator capacity at 50% of the total system charge.
Oil separator
The oil separator cannot solve the oil return fault caused by the system design, nor can it solve the liquid refrigerant control fault. However, when the system control failure cannot be resolved by other means, the oil separator helps to reduce the amount of oil circulating in the system, which can help the system through a critical period until the system control is restored to normal. For example, in an ultra-low temperature unit or full liquid evaporator, the return oil may be affected by defrosting, in which case the oil separator can help maintain the amount of chilled oil in the compressor during system defrosting.
Post time: Sep-07-2023
