Common Problems and Solutions of Liquid Refrigerant in Refrigeration System

1. Migration of liquid refrigerant

Refrigerant migration refers to the accumulation of liquid refrigerant in the crankcase of the compressor when the compressor stops. As long as the temperature in the compressor is lower than that in the evaporator, the pressure difference between the compressor and the evaporator will drive the refrigerant to migrate to a colder place. In the cold winter, this phenomenon is likely to happen more often. However, for air conditioning and heat pump devices, when the condensing unit is far away from the compressor, even if the temperature is high, the migration phenomenon may occur. Once the system is shut down, if it is not turned on for several hours, even if there is no pressure difference, the migration phenomenon may occur due to the attraction of refrigerant by the refrigerant oil in the crankcase. If excessive liquid refrigerant migrates into the crankcase of the compressor, serious liquid hammer will occur when the compressor is started, which will lead to various faults of the compressor, such as valve rupture, piston damage, bearing failure and bearing erosion (refrigerant washes the refrigerant oil from the bearing).

2. Overflow of liquid refrigerant

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 overflows and dilutes the refrigerant oil, resulting in the abrasion of the moving parts of the compressor, and the decrease of oil pressure leads to the action of the oil pressure safety device, thus causing the crankcase to lose oil. In this case, if the machine is shut down, the phenomenon of refrigerant migration will occur quickly, which will lead to liquid hammer during restart.

3. Liquid hammer

When the liquid hammer occurs, the sound of metal inside the compressor can be heard, and it may be accompanied by severe vibration of the compressor. The hydraulic shock will lead to the rupture of the valve, the damage of the gasket of the compressor head, the fracture of the connecting rod, the fracture of the machine shaft and the damage of other types of compressors. When the liquid refrigerant migrates into the crankcase, liquid hammer will occur when it is restarted. In some units, due to the pipeline structure or the position of components, the liquid refrigerant will accumulate in the suction pipe or evaporator during the shutdown of the unit, and will enter the compressor in the form of pure liquid at a particularly high speed when starting up. The speed and inertia of liquid hammer are enough to destroy any protection measures of built-in compressor anti-liquid hammer device.

4, oil pressure safety control device action

In a low-temperature unit, after the defrosting period, the overflow of liquid refrigerant often leads to the action of the oil pressure safety control device. Many system designs allow the refrigerant to condense in the evaporator and suction pipe during defrosting, and then the refrigerant flows into the crankcase of the compressor at startup, causing the oil pressure to decrease, resulting in the action of the oil pressure safety device. Occasionally, one or two actions of the oil pressure safety control device will not seriously affect the compressor, but repeated actions without good lubrication will lead to compressor failure. The oil pressure safety control device is often considered as a minor fault by operators, 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 time.

5. Recommended remedial measures

The more refrigerant charge in the refrigeration system, the greater the probability of its 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 manufacturer of the compressor can determine the maximum charge of the liquid refrigerant that will not harm the working parts of the compressor, but it is impossible to determine how much of the total charge of the refrigeration system is actually in the compressor in most extreme cases. The maximum amount of liquid refrigerant that the compressor can bear depends on its design, internal volume and the amount of refrigerant oil. When liquid migration, overflow or liquid hammer occurs, necessary remedial measures must be taken. The type of remedial measures depends on the system design and the type of failure.

1) reduce refrigerant charge.

The best way to protect the compressor from the failure caused by liquid refrigerant is to limit the refrigerant charge within the allowable range of the compressor. If this is not possible, the filling amount should be reduced as much as possible. Under the condition of meeting the flow rate, the condenser, evaporator and connecting pipes should use pipes with small diameters as much as possible, and the reservoir should be as small as possible. After the filling amount is minimized, correct operation is required, and it is necessary to be alert to the bubbles in the sight glass caused by too thin liquid diameter and too low head pressure, which will lead to serious overfilling.

2) Evacuation cycle

The most active and reliable way to control liquid refrigerant is to evacuate circulation. Especially when the charging amount of the system is large, the refrigerant can be pumped into the condenser and the accumulator by closing the solenoid valve of the liquid pipe, and the compressor runs under the control of the low-pressure safety control device, so that the refrigerant is isolated from the compressor when the compressor is not running, thus avoiding the migration of the refrigerant into the crankcase of the compressor. It is recommended to adopt continuous evacuation cycle in shutdown stage to prevent leakage of solenoid valve. If it is an evacuation cycle, or a non-recirculation control mode, excessive refrigerant leakage will damage the compressor when it is shut down for a long time. Although continuous evacuation cycle is the best way to prevent migration, it can't protect the compressor from the adverse effects caused by refrigerant overflow.

3) Crankcase heater

In some systems, operating environment, cost or customer's preference, it is possible that the evacuation cycle cannot be realized. At this time, the crankcase heater can delay the migration. The function of the crankcase heater is to keep the temperature of refrigerant oil in the crankcase higher than that of the lowest temperature part of the system. However, the heating power of crankcase heater must be limited in order to prevent overheating and freezing oil from carbon deposition. When the ambient temperature is close to -18℃, or when the suction pipe is exposed, the effect of the crankcase heater will be partially offset, and the migration phenomenon may still occur. In use, the crankcase heater usually heats continuously, because once the refrigerant enters the crankcase and condenses in the refrigerant oil, it takes several hours for it to return to the suction pipe again. When the situation is not particularly serious, the crankcase heater is very effective in preventing migration, but the crankcase heater cannot protect the compressor from the damage caused by liquid backflow.

4) Gas-liquid separator of suction pipe

For systems prone to liquid overflow, a gas-liquid separator should be installed on the suction pipe to temporarily store the overflow liquid refrigerant in the system and return the liquid refrigerant to the compressor at a rate that the compressor can bear. Refrigerant overflow is most likely to occur when the heat pump is switched from cooling mode to heating mode. Generally, the gas-liquid separator of suction pipe is a necessary equipment in all heat pumps. The system using hot air to defrost is also prone to liquid overflow at the beginning and end of the defroster. Low-temperature devices, such as liquid refrigerators and compressors of low-temperature display cases, occasionally overflow due to improper refrigerant control. For vehicle devices, after a long period of downtime, it is easy to cause serious overflow when they are restarted. In the two-stage compressor, the suction air directly returns to the low-stage cylinder, and does not pass through the electric machine room. A gas-liquid separator should be used to protect the compressor valve from hydraulic impact. Due to the different requirements of the overall charge volume of different refrigeration systems and the different refrigerant control methods, whether or not a gas-liquid separator is needed and the size of the gas-liquid separator depends largely on the requirements of specific systems. If the quantity 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 filling volume of the system.

5) Oil separator

The oil separator can't 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 can't be solved by other methods, the oil separator helps to reduce the amount of oil circulating in the system, and can help the system survive a dangerous period until the system control device returns to normal. For example, in an ultra-low temperature device or a flooded evaporator, the oil return may be affected by defrosting, in which case the oil separator can help maintain the amount of refrigerant oil in the compressor during the system defrosting.