What Are the Common Problems of Oil Return in Compressors?

2025-04-30

Mechanism Analysis of Compressor Oil Shortage
During the operation of the system, the lubricating oil is discharged from the compressor together with the refrigerant and then returns to the compressor after circulation. Therefore, where there is the inflow and outflow of the refrigerant, there is also the inflow and outflow of the lubricating oil.
There are essential differences between the properties of the refrigerant and those of the lubricating oil. The refrigerant exists in two phases during the system circulation process, namely the liquid refrigerant and the gaseous refrigerant, while the lubricating oil is basically in a liquid state. When the refrigerant changes from a liquid state to a gaseous state, the lubricating oil will separate out from the refrigerant. Under the influence of many factors, they are likely to be stored in a certain component or structural point, resulting in the lubricating oil being unable to flow back to the compressor smoothly, causing the scroll compressor to be short of oil. If the oil shortage is not solved for a long time, it will lead to insufficient lubrication of the internal moving parts of the compressor, resulting in failures such as dry burning, which will greatly accelerate the damage of the scroll compressor.
Ensuring Oil Return - In Terms of Installation
When the compressor discharges the refrigerant, it will also discharge a small amount of refrigeration oil. Even with an oil carryover rate of only 0.5%, if the oil cannot return to the compressor through the system circulation, taking a 5HP compressor as an example, the circulation volume is approximately 330kg/h under ARI conditions, and all the oil in the compressor can be carried out within 50 minutes, and the compressor will be burned out in about 2 to 5 hours.
Therefore, in order to ensure that the compressor operates without oil shortage, the following two aspects should be focused on:
Ensure that the refrigeration oil discharged from the compressor returns to the compressor;
Reduce the oil carryover rate of the compressor.
I. Ensure that the Refrigeration Oil Discharged from the Compressor Returns to the Compressor
The flow rate of the refrigerant in the suction pipe (about 6m/s) should be ensured so that the oil can return to the compressor. However, the maximum flow rate should be less than 15m/s to reduce the pressure drop and flow noise. For the horizontal pipe, there should be a downward slope along the flow direction of the refrigerant, about 0.
8cm/m.
Prevent the refrigeration oil from remaining in the evaporator.
Ensure that the oil return hole of the gas-liquid separator is appropriate. If it is too large, it will cause wet compression, and if it is too small, there will be insufficient oil return, and the stagnant oil will be in the gas-liquid separator.
There should be no parts in the system that can cause the oil to remain.
Ensure that there is enough refrigeration oil in the compressor in the case of long piping and high drop. Usually, a compressor with an oil level sight glass is used to confirm that frequent starting of the compressor is not conducive to oil return.
II. Reduce the Oil Carryover Rate of the Compressor
When the compressor is shut down, ensure that the refrigerant does not dissolve into the refrigeration oil (use a crankcase heater).
Avoid over-wet operation because excessive oil carryover may be caused by foaming.
Install an oil separator device inside.
The foaming of the oil inside the compressor makes it easy for the oil to be carried out of the compressor.
III. Long Piping and High Drop
When the length of the piping is greater than the allowable value, the pressure loss in the piping will increase, reducing the amount of refrigerant in the evaporator and leading to a decrease in capacity. At the same time, when there is oil remaining in the piping, the compressor will be short of oil, resulting in compressor failures. When the refrigeration oil in the compressor is insufficient, the same brand of refrigeration oil as that when the compressor leaves the factory should be added from the high-pressure side.
IV. Set Necessary Oil Return Bends
When the drop exceeds 10m to 15m, an oil return bend should be set on the gas pipe side.
Necessity: When the compressor is shut down, avoid the refrigeration oil attached in the piping from returning to the compressor, causing the liquid compression phenomenon. On the other hand, to prevent the compressor from being short of oil due to poor oil return in the gas pipe.
Interval for Setting Oil Return Bends: Set an oil return bend for every 10m drop.
V. Ensure Appropriate Viscosity of Refrigeration Oil
The refrigeration oil and the refrigerant are mutually soluble. When the compressor is shut down, almost all of the refrigerant dissolves in the refrigeration oil. Therefore, a crankcase heater needs to be installed to prevent dissolution.
During operation, the refrigerant containing liquid should not be allowed to return to the compressor, that is, ensure that there is superheat in the compressor suction.
During startup and defrosting, the liquid return phenomenon should not occur.
Avoid operation under an excessive superheat state to prevent oil deterioration.
The size of the oil return hole of the gas-liquid separator should be appropriate: If the hole diameter is too large, the liquid refrigerant will be sucked in, causing over-wet operation; if the hole diameter is too small, the oil return will not be smooth, and the oil will remain in the gas-liquid separator.
Ensuring Oil Return - In Terms of Equipment
I. Selection of Lubricating Oil
The lubricating oil mainly plays the roles of lubrication, sealing, cleaning, heat dissipation, and rust prevention in the scroll compressor. Selecting good lubricating oil is not only beneficial to improving the reliability of the scroll compressor but also greatly improves the performance of the air conditioning system.
II. Selection of Components in the System
Oil Separator:
The oil separator is generally installed on the exhaust pipe. The separation of the oil and gas is achieved through a rapid pressure drop, and then the oil returns to the oil storage tank of the compressor through the oil return capillary tube. Currently, there are three widely used types of oil separators:

The oil separator with a float ball. When there is oil accumulated in the oil separator, the float valve set inside will open, allowing the oil to return to the compressor;
The oil separator that manually returns the oil to the compressor. When the oil accumulates in the oil separator, the oil return valve needs to be manually opened to make the oil return to the compressor;
The oil separator without a float valve inside. Although this type of oil separator has a simple structure, the requirements for the size of the oil return piping are very strict.
The gas-liquid separator is one of the most critical components affecting oil return. It is generally installed between the suction port and the compressor.
The gas-liquid separator has two key indicators, namely the oil return hole and the balance hole.
When designing and selecting, an appropriate gas-liquid separator must be selected according to the needs of one's own system. In the gas-liquid separator of the oil-deficient system, there is basically oil storage. Currently, there are many manufacturers producing gas-liquid separators. General air conditioning manufacturers only simply select them without designing suitable gas-liquid separators according to the needs of their own systems, which is likely to cause oil accumulation in the gas-liquid separator. However, some companies with research and development capabilities will develop gas-liquid separators suitable for the system according to their own needs when developing characteristic products.
Another key component is the connecting pipe between the indoor and outdoor units. Currently, many manufacturers have developed multi-connected units. However, as the length of the oil return pipe increases, the difficulty of oil return gradually increases. How to achieve good oil return when a long connecting pipe is configured is a problem worthy of consideration.
III. System Control
System control mainly involves oil return control and oil equalization control.
Oil Return Control:
In a multi-connected unit system, under the condition of partial load operation, oil accumulation will occur in the un-operated load. The more un-operated loads there are and the longer the operation time is, the more oil will accumulate outside the compressor, and the less lubricating oil will flow back into the compressor.
When the system operates to a certain controlled index (this index can be the oil level, operation time, temperature, etc.), the oil return system will work.
By adjusting controllable factors such as the overall unit load, refrigerant flow rate, working frequency, motor, and system air volume, the flow rate and pressure of the refrigerant in the system are adjusted, increasing the flow rate of the refrigerant in the compressor and driving the lubricating oil to flow back.
When the monitoring system detects that the oil quantity meets the operation requirements of the compressor, it will enter normal load operation, and so on in a cycle.
Oil Equalization Control:
Electric Control of the Oil Equalization System:
Oil equalization occurs in a multi-connected unit system. Similarly, detection points such as the oil level can be designed in the system. When the system detects that a certain compressor is short of oil, the oil equalization system can balance part of the lubricating oil from the oil-rich compressor system to the oil-deficient compressor system. If the second compressor also becomes short of oil, an oil equalization process will be restarted through detection, and so on until the oil quantity in all compressor systems is balanced.
Optimized Structural Design of the Oil Equalization System:
Optimized structural design is also beneficial to oil return. Currently, the commonly used technology is oil and gas balance technology.
Theoretically, the oil pressure and air pressure in the crankcase of each parallel compressor can be guaranteed, but in practice, it is not very ideal. Due to the influence of factors such as the design and processing of the balance pipe, the installation of the unit, and the oil pumping volume of each compressor, the oil pressure and air pressure in the crankcase of each compressor will vary. Therefore, when using this oil return method, it is necessary to control the above aspects well, and do not use more than three compressors.
Another oil return structure adopts a non-equilibrium technology, which is a patented structure of Danfoss. The pressure in the system flow path decreases sequentially, so a pressure gradient is established in the compressor. The lubricating oil first flows into the upstream compressor. When the oil level is higher than the bottom of the connecting pipe, it will overflow under the action of the air flow and pressure difference and enter the next compressor. If the oil quantity is normal, each compressor can obtain sufficient lubricating oil.
IV. Influence of System Speed and Pressure on Oil Return
The change of system operating conditions has a great influence on the flow rate, pressure, and phase state of the refrigerant inside the scroll compressor system. During the operation of the system, the refrigerant and the lubricating oil are almost mutually soluble.
The greater the flow rate and pressure of the refrigerant in the pipeline, the more favorable it is for the return of the lubricating oil.
The oil return control generally changes the refrigerant flow rate of the unit by controlling the unit frequency. When the unit frequency increases, the flow rate of the refrigerant passing through the compressor per unit time is greater, and the flow rate and density of the refrigerant during its flow in the pipeline are increased. Then the return speed of the lubricating oil will naturally be accelerated.
During the installation process of the multi-connected unit, based on structural requirements, the connecting pipe between the indoor and outdoor units may exceed the size recommended by the manufacturer. As the length of the connecting pipe increases, the pressure loss of the system will be greater, and the flow rate of the refrigerant in the system will also slow down, which is extremely unfavorable for the oil return of the system. The lubricating oil will separate out from the refrigerant with slow flow and attach to the inner wall of the pipeline, causing the lubricating oil to accumulate in some parts where oil is likely to be stored, so that the lubricating oil cannot fully flow back into the compressor. The following operations are recommended:
Try to select lubricating oil with a lower pour point, which is beneficial to the flow of the lubricating oil in the pipeline;
Select an oil separator and a gas-liquid separator suitable for the system. The influence of the length of the connecting pipe on oil return cannot be ignored. When the connecting pipe is too long, corresponding treatments should be made, such as adding lubrication, etc.;
Consider the oil return factor as much as possible in the initial design stage and optimize the system oil return through structural design;
The frequency plays a crucial role in the flow rate and velocity of the refrigerant. As the frequency increases, the flow rate and velocity will also increase, and the oil return quantity will also increase.