bitzer compressors capacity control
compressor for condensing units

In discussions about compressor maintenance and performance, an aspect that cannot be overlooked is the circulation and return of oil. Compressors require oil to lubricate moving parts, reduce friction, prevent wear, and dissipate the heat generated during operation.

Oil Discharge and Circulation

Ideally, we would hope to retain 100% of the oil within the compressor. However, this is often unrealistic, as, with few exceptions, all oil-lubricated compressors will discharge oil into the airflow.

The oil discharged from the compressor into the gas exists in two main forms: as fine oil droplets (mist) within the airflow, and as liquid oil crawling along the pipe walls under the push of the gas velocity. Unlike refrigerant, oil does not change from liquid to gas state. It adheres to the pipe walls, forming a very thin liquid film that can be carried by the refrigerant along the entire length of the pipeline and enter all components in the circuit.

Factors Influencing Oil Return

To ensure the system’s efficient operation and extend equipment life, once oil leaves the compressor, it must circulate through the system and return to the compressor’s crankcase. This process is influenced by several factors:

  1. Oil/refrigerant miscibility (the degree to which oil mixes and moves with the refrigerant)
  2. Oil viscosity (the thickness of the oil)
  3. The refrigerant velocity throughout the circuit.
Danfoss oil balance system

Methods of Compressor Oil Return

There are two main methods of compressor oil return: one is through an oil separator, and the other is through a return pipe. Oil separators are commonly installed on the compressor’s exhaust pipeline, widely used for their excellent oil return effect and speed.


However, if compressor oil return fails, a large amount of lubricating oil can accumulate in the evaporator piping. When the oil film increases to a certain extent, it directly affects the system’s cooling performance.

On the other hand, if there is a shortage of refrigerant oil, the machine cannot operate, necessitating continuous replenishment of refrigerant oil. This leads to an accumulation of lubricating oil in the system, creating a vicious cycle, increasing operating costs, and reducing operational reliability.


To avoid these issues, regularly monitoring the oil level, checking for leaks, and ensuring the proper oil viscosity are important steps in maintaining efficient compressor oil return. Additionally, selecting the correct type of refrigerant oil for the system and following the manufacturer’s recommendations can enhance oil circulation. By taking appropriate measures and maintenance strategies, oil loss can be minimized, ensuring the long-term reliability and efficiency of the compressor and the entire refrigeration system.


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