Why is the Evaporator Position Higher than the Compressor? Several Key Points of Oil Return in the Refrigeration System!

2025-05-06

In the design of a refrigeration system, the layout of the four main components is very particular; especially the relative height positions of the compressor, condenser, and evaporator. At the same time, during the installation process of actual air conditioners or refrigeration units, the positions of these three components also require careful consideration.

Let's first look at several situations
1.1 In household wall-mounted and cabinet air conditioners, the position of the indoor unit is higher than that of the outdoor unit, that is, the position of the evaporator is higher than that of the compressor.
1.2 In a VRV system, the installed position of the indoor unit is higher than that of the outdoor unit, that is, the position of the evaporator is higher than that of the compressor.
1.3 In an air-cooled unit, the position of the evaporator is higher than that of the compressor.
The Evaporator is Higher than the Compressor
As can be seen from the above, generally speaking, the position of the evaporator in a refrigeration system should be higher than that of the compressor. This is a principle that must be followed both in unit design and on-site installation.
For the normal operation of a refrigeration system, the following points need to be mainly considered:
2.1 Are the four main components of refrigeration matched?
2.2 Is the opening degree of the refrigerant and the expansion valve correct?
2.3 Does the air volume (for air-cooled systems), water volume or water temperature (for water-cooled systems) meet the requirements?
2.4 Is the circulation of the compressor's refrigeration oil normal?
The position of the evaporator being higher than that of the compressor is considered from the fourth point, and the reasons are as follows:
The position of the evaporator being higher than that of the compressor is conducive to the oil return of the refrigeration system.
Oil Return of Two Types of Pipelines
3.1 Oil Return of the Liquid Pipe
For the liquid pipe of the refrigeration system (the pipe from the condenser to the inlet of the evaporator), oil return is relatively easy to consider because the refrigerant is in a liquid state, and the compressor's refrigeration oil will follow the liquid refrigerant. So, the oil return is relatively normal, and the pipeline layout basically does not need to be considered.
3.2 Oil Return of the Gas Pipe
The oil return of the gas pipe includes two sections: the exhaust pipe and the suction pipe. Taking the suction pipe as an example, since the refrigerant is in a gaseous state and the refrigeration oil is in a liquid state, there will be problems with oil return at this time. If the flow rate of the gaseous refrigerant is relatively low, it will be difficult for the oil to flow with the refrigerant, and the oil will accumulate in the low-lying parts of the pipe. Over time, the oil in the compressor will gradually decrease, causing compressor failures.
So, if the gas pipe is a descending pipeline at this time, when the compressor stops, the refrigeration oil will also flow directly back to the compressor along the pipeline, and there will be no problem with the oil return of the system.
Therefore, generally speaking, the gas pipe is designed as a descending pipeline to facilitate the oil return of the system.
For small refrigeration systems (such as household air conditioners, etc.), the resistance of the suction and exhaust pipelines can also be increased to increase the flow rate of the refrigerant. Because only when the refrigerant flows quickly can it drive the refrigeration oil to flow, which is convenient for the circulation of the oil and prevents the formation of "dead oil".
Practical Applications
4.1 Small Refrigeration Systems
For small systems, there are two methods to improve the situation:
4.1.1 Increase the flow rate of the suction and exhaust refrigerant: The resistance of the suction and exhaust pipelines can be increased to increase the flow rate of the refrigerant and ensure the circulation of the refrigeration oil. The disadvantage is that excessively increasing the suction and exhaust resistance is not beneficial for refrigeration.
4.1.2 Add an oil return operation mode
Add a bypass circuit at the lowest point of the system pipeline, leading to the compressor suction port, and add a solenoid valve and a capillary tube in the middle; regularly open the solenoid valve to let the refrigerant bring the refrigeration oil back to the compressor suction port.
4.2 Medium and Large Systems
During actual installation, sometimes it is difficult to ensure that the position of the evaporator is higher than that of the compressor. In this case, the method of "adding oil return bends" can be used to deal with it.
4.3 Why Set Oil Return Bends
Regarding the setting of oil return bends in a Freon refrigeration system, the main purpose is to ensure the oil return of the compressor. Oil return bends must be set in the following three situations.
4.
3.1 When the main unit is higher than the terminal (evaporator)
There is an ascending vertical pipe between the evaporator and the main suction pipe. Since the refrigeration oil will not evaporate and vaporize in the evaporator, it accumulates at the bottom. When there is a large amount of refrigeration oil accumulated at the bottom of the evaporator, it will block the suction pipe. If an oil return bend is set at the bottom of the evaporator, the amount of oil accumulated in this bend will not be too much. As long as the bend is about to be blocked, the pressure difference at both ends is sufficient to "pump" out the limited amount of refrigeration oil in the bend until it reaches the horizontal suction pipe at the top, and then it is sucked back by the compressor along the slope.
If there is a concern that the ascending vertical pipe is too long to pump the oil to the top, it should be considered to set an oil return bend at a certain height difference interval (such as 8 meters) in the ascending section of the suction pipe, so that the refrigeration oil can return to the main unit step by step in sections.
4.3.2 When the main unit is lower than the terminal (evaporator) and the height difference is large
Although the refrigeration oil can automatically flow back to the main unit along the slope without an oil return bend, at this time, there is a concern that too much oil return may cause "liquid hammer" in the main unit. Therefore, an oil return bend is set at a certain height difference interval (such as 8 meters) in the main suction pipe, so that the refrigeration oil can return to the main unit step by step in sections.
4.3.3 During low-load operation
The refrigeration oil accumulates in the oil return bend. Due to the limitation of the flow rate, relying only on "the pressure difference at both ends when the bend is about to be blocked" to drive the oil return has a very small effect. The key point is to increase the output capacity of the compressor and the suction speed within a certain period.
If the suction speed can be increased to a relatively large value, there is no need to add an oil return bend. However, in reality, considering the impact of the internal heat exchange effect during low-load operation, an excessive increase in the compressor's output is likely to cause some problems in system control, such as too low low pressure and no superheat in the suction. This means that the increase in the suction speed is limited. When there is a large height difference, it is necessary to use oil return bends to return the oil step by step!
(Note) For a small household one-to-one system, the distance for setting the oil return bend should be set according to the capacity of the compressor itself, and it cannot be simply borrowed!
4.4 Design Principles of Oil Return Bends
The design principles of the oil return bends of an air conditioner are:
4.4.1 Ensure that the pressure drop of the pipeline of the air conditioner's oil return bend does not exceed the allowable value.
4.4.2 Ensure that the lubricating oil can return to the compressor along with the Freon vapor.
4.4.3 Prevent liquid Freon and lubricating oil from flowing into the compressor when the evaporator load decreases or the compressor stops, and prevent liquid hammer when the compressor restarts.
The design principle of the exhaust pipe: It is the same as that of the return air pipe, that is, when designing, it is necessary to consider controlling the pressure drop, ensuring oil carrying, preventing liquid hammer, and preventing noise and vibration.
4.5 Installation Technical Requirements of Oil Return Bends
In a fluorine system, when the height difference between the indoor and outdoor units exceeds a certain height (such as 6 meters or 8 meters), in order to ensure the smooth oil return of the compressor, an oil return bend should be installed every 6-8m from bottom to top in the vertical pipe sections of the high-pressure gas pipe and the low-pressure gas pipe. The oil return bend is made by using two "U" - shaped bends or one "return" - shaped bend; the height is 3-5 times the diameter of the pipe.
The specific settings and requirements for the making of oil return bends are as follows:
4.5.1 When the outdoor unit is below the indoor unit, there is no need to add oil return bends at the lowest and highest points of the vertical pipe.
4.5.2 When the outdoor unit is above the indoor unit, it is necessary to add an oil return bend and a check bend respectively at the lowest and highest points of the vertical pipe.
4.5.3 The dimensions of the oil return bend should be made according to the relevant requirements.
Two Special Situations
5.1 When the position of the condenser is higher than that of the compressor, and the ambient temperature of the condenser is higher than that of the compressor
The U-shaped bend formed by the exhaust pipe first bending downward and then upward is set to prevent the gaseous working medium from condensing into a liquid or the lubricating oil entrained in the exhaust from flowing back to the compressor and causing a liquid hammer accident after the compressor stops; at this time, the oil and the liquid working medium will be carried into the oil separator or the condenser by the exhaust gas and brought back to the compressor through the oil return or the working medium circulation.
When the position of the condenser is lower than that of the compressor, the setting of the U-shaped bend cannot prevent the oil from entering the condenser, and an oil separator should be considered to be installed at this time.
5.2 When the position of the compressor is lower than that of the evaporator
Setting an oil return bend and an ascending vertical pipe at the outlet of the evaporator to make its height exceed the top of the evaporator, and matching it with a horizontal return air pipe with a slope towards the suction port of the compressor can indeed prevent liquid hammer when the compressor stops. But it can also ensure oil return during operation.
The key to ensuring oil return is to maintain a certain flow rate in the ascending vertical pipe. Although the oil bend increases the resistance of the return air, the diameter of the ascending vertical pipe can be reduced and the diameters of other pipe sections can be appropriately enlarged. In this way, both the oil return is ensured and the pressure drop of the entire return air pipeline can be controlled not to exceed the allowable value.
This is for low-temperature working conditions. For air conditioning working conditions, due to the relatively high return air pressure and large return air flow rate, the requirement for carrying oil can generally be met, and there is no need to consider setting an oil return bend, except in special cases (such as a long pipeline with large resistance, the evaporator is below the compressor and the height difference is large, etc.).