Common problems and solutions of liquid refrigerants in refrigeration systems

Common problems and solutions of liquid refrigerants in refrigeration systems

1. Migration of liquid refrigerant

　　 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 may occur due to the refrigerant oil in the crankcase attracting the refrigerant.

If an excessive amount of liquid refrigerant migrates into the crankcase of the compressor, severe liquid hammer will occur when the compressor is started, causing various compressor failures, such as valve rupture, piston damage, bearing failure and bearing erosion (The refrigerant flushes 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 and enter 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 oil pressure drop causes the oil pressure safety device to operate, 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 strike

　　 When liquid hammer occurs, a metal impact sound can be heard inside the compressor, and it 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. The hydraulic safety control device operates

　　In a low-temperature unit, after the defrosting period, the liquid refrigerant overflow often causes 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 remedies

　　 The more refrigerant charge in the refrigeration system, the greater the probability of failure. Only when the compressor and other main components of the system are connected together for system testing, can the maximum and safe refrigerant charge be determined. The compressor manufacturer can determine the maximum charge of liquid refrigerant 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 that the compressor can withstand depends on its design, internal volume, and charge of refrigerant 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

　　The best way to protect the compressor from malfunctions caused by liquid refrigerant is to limit the refrigerant charge to the compressor's allowable range. If this is not possible, the charge should be reduced as much as possible. Under the condition that the flow rate is satisfied, the condenser, the evaporator and the connecting pipe should be as small as possible, and the accumulator should be as small as possible. After the filling amount is minimized, correct operation is required. Be aware of the bubbles in the sight glass caused by the liquid pipe diameter being too thin and the head pressure too low, which will cause serious overfilling.

　　7, pump down cycle

　　 The most active and reliable way to control liquid refrigerant is to pump down the cycle. Especially when the system charge is large, the refrigerant can be pumped into the condenser and accumulator by closing the solenoid valve of the liquid pipe. The compressor runs under the control of the low-pressure safety control device, so the refrigerant is in the compressor It is isolated from the compressor when it is not running to avoid the migration of refrigerant to the compressor crankcase. It is recommended to use a continuous pump down cycle during the shutdown phase to prevent leakage of the solenoid valve. If it is a pump down cycle, or it is called a non-recirculation control method, excessive refrigerant leakage will damage the compressor when it is shut down for a long time. Although the continuous pump down cycle is the best way to prevent migration, it cannot protect the compressor from the adverse effects of refrigerant overflow.

　　 8. Crankcase heater

　　In certain systems, operating environments, costs, or customer preferences, the pump-down cycle may not be realized. At this time, the crankcase heater can delay the migration. 　　 The function of the crankcase heater is to keep the temperature of the refrigerated oil in the crankcase higher than the temperature of the lowest part of the system. However, in order to prevent overheating and carbonization of the refrigerating oil, the heating power of the crankcase heater must be limited. When the ambient temperature is close to -18°C, or when the suction pipe is exposed, the effect of the crankcase heater will be partially offset, and migration may still occur.

　　 The crankcase heater is generally continuously heated in use, because once the refrigerant enters the crankcase and condenses in the frozen oil, it takes up to several hours to get it back to the suction pipe again. When the situation is not particularly serious, the crankcase heater is very effective to prevent migration, but the crankcase heater cannot protect the compressor from damage caused by the return of liquid.

　　9, suction pipe gas-liquid separator

　　 For systems prone to liquid overflow, a gas-liquid separator should be installed on the suction pipe to temporarily store the overflowing liquid refrigerant in the system and return the liquid refrigerant to the compressor at a rate that the compressor can withstand.

　　 When the heat pump is switched from cooling mode to heating mode, refrigerant overflow is most likely to occur. Under normal circumstances, the suction pipe gas-liquid separator is a necessary equipment in all heat pumps.

　　The system that uses hot gas to defrost is also prone to liquid overflow at the beginning and end of the defroster. Low superheat devices, such as compressors in liquid freezers and low-temperature showcases, occasionally overflow due to improper refrigerant control. For vehicle installations, after a long period of shutdown, serious overflow is also prone to occur when restarting.

　　 In the two-stage compressor, the air is sucked directly back to the lower cylinder without passing through the motor chamber. A gas-liquid separator should be used to protect the compressor valve from liquid shock.

　　Due to the different overall charging requirements of different refrigeration systems and different refrigerant control methods, whether a gas-liquid separator is needed and what size gas-liquid separator is required to a large extent depends on the requirements of the specific system. If the amount of liquid reflux is not accurately tested, a conservative design method is to determine the capacity of the gas-liquid separator according to 50% of the total system charge.

　　10, oil separator

　　The oil separator cannot solve the oil return failure caused by the system design, nor can it solve the liquid refrigerant control failure. However, when the system control failure cannot be solved by other methods, the oil separator can help reduce the amount of oil circulating in the system, and can help the system survive a period of danger until the system control device returns to normal. For example: in ultra-low temperature devices or flooded evaporators, oil return may be affected by defrost. In this case, the oil separator can help maintain the amount of refrigerated oil in the compressor during system defrosting.