The working principle of a rotary screw compressor

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Rotary screw compressors are one of the most used compressors in industrial applications. In this article we’ll elaborate on its working principle. How is the air compressed and how does the compressor's lubrication function?

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Two types of rotary screw compressors

Essentially there are two types of screw compressors: (oil-)lubricated and ‘dry’ compressors. MARK only offers oil-lubricated compressors, but for the sake of education we’ll explain the working principle of dry screw compressors as well. 

Dry screw compressors, don’t have, as the name might suggest, any lubricants between the screws. The compression process might consist of one or two stages. Each is housed separately. Another difference with lubricated screw compressors is that dry compressors require external timing gears to assure that the rotors operate properly. These timing gears are housed separately, but will need constant lubrication to work. 

Lubricated screw compressors - in most cases the lubricant is oil - have a lubricant between the rotating gears. They don’t require external timing gears since oil is injected between the screws. We’ll explain the working principle of an oil-lubricated air compressor in the next paragraph. 

Read more about the importance of oil lubrication in our article ‘Why oil is used in a rotary screw compressor’. 


The working principle of a rotary screw compressor

In oil- lubricated compressors, there are two separate circuits: the air circuit and the oil circuit.

Working principle of a rotary screw compressor: the air circuit

In the air circuit, athmospheric air is sucked through the fliter and the inlet valve. It is drawn into the compressor and compression starts. After this stage a mix of compressed air and oil (see: oil circuit) flows into the oil separator and air receiver through a check valve. Afterwards the outlet valve ejects the air through a minimum pressure valve and an air cooler

The minimum pressure valve ensures that, during operation, the pressure in the separator tank is above a certain level that is needed for lubrication. The check valve makes sure that the compressed air doesn’t escape. When the compressor stops operating, the check valve and inlet valve close up. That way the compressed air is prevented from going into the air filter.

Working principle of a rotary screw compressor: the oil circuit

Oil is injected directly into the valves to ensure lubrication. Most of that oil is removed in the air receiver/oil separator. Through centrifugal action the oil particles are separated from the compressed air. The oil is collected in a part of the oil separator that functions as a receptacle. The oil circuit also has a bypass valve. This valve ensures that the supply to the oil cooler is shut off when the temperature is below a certain temperature. That way the oil isn’t cooled unnecessarily.

After the air receiver/oil separator, the oil is forced - through air pressure - through the oil filter and stop valve. If the oil temperature is above the set point, the bypass valve lets oil into the oil cooler. The stop valve makes sure that the compressor element isn’t flooded with oil when operations halts or stops.

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