Rotary screw type air compressors are the most popular kind of air compressor in the realm of mass production. Whenever you pass by a factory, pressing plant or large building in which manufacturing takes place, chances are that the whole process is being driven with these compressors.

Read more below from rotary air compressor expert Kaishan.

Rotary Compressor Working Principal

Rotary compressors contain two helical rotors within the housing that interlock. Ambient air comes into the compressor through the inlet valve. Air is then trapped between the two rotors. There, the screws turn, and this increases the pressure of the air by reducing its volume.

Some rotary screw air compressors do consist of only one screw, but these are not as widely used in the industrial realm, where the full power of two screws is essential for large-scale productions. Single-screw rotary models are more often used in refrigeration.

In the design of these compressors, the assembly that consists of the housing and rotors is known as the air end. In all types of rotary compressors, the air end is where incoming ambient air is compressed.

Oil vs. Non-Oil Rotary Compressors: What’s the Difference?

Some rotary screw compressors use oil while others do not, but all compressors need to filter out the oil present in the ambient air. In compressors that use oil, the motor drives the male rotor, which in turn drives the female rotor. The oil forms a film between the two rotors and also serves as a sealant and coolant for the compression chamber.

In an oil-free compressor, no oil is used to drive the compression process. The two rotaries in an oil-free model are controlled by gears. Without oil to serve as a chamber sealant, compressors of this type are not as capable of reaching high levels of pressure. These oil-free compressors are less efficient as they are also liable to run hotter due to the lack of cooling oil.

Due to these limitations, oil-free rotary screw compressors are mostly confined to special types of use. Though rare, there are certain oil-free models that employ water instead of oil as a coolant.

The air end serves another function besides the compression of air, as this is where oil is compressed within the air. After the air end stage is complete, the newly compressed air passes into the sump — also known as the separator tank — where oil is extracted from the air. The spinning motion effectively shakes the oil particulates from the compressed air so that the latter can be pure once it reaches its end point.

The process of oil separation is assisted with baffles. Once the air has passed through the separator tank, rarely more than three-parts per million (3 ppm) of the oil remains. Afterward, the air passes through a cooler and onward to the endpoint, whether that happens to be a pneumatic tool or an air-powered machine.

Depending on the temperature of the now-separated oil, a thermostatic valve treats the oil appropriately. The purpose here is to prevent the oil from becoming either hot or cold. If the oil gets hot, it will fry and wear down the internal machinery. If the oil is cold, there will not be sufficient temperature to separate it from all the water extracted from the air during the compression stage.

Air is not allowed into the system until it has enough pressure to be self-lubricated. If the oil contains too much water, the air end will not function properly.

In a rotary screw air compressor with a stationary blade, the driving shaft has an eccentrically mounted roller inside the pump chamber. Within this chamber, the blade divides the inlet and outlet valves. The blade itself is sandwiched by the roller surface and interior body of the air compressor.

As the roller moves, the blade goes up and down to the rotary motion. As such, the compressor consists of three parts that move — the blade, the roller and the shaft. Each of these moving parts is lubricated. In the cylinder, vapors of low temperature and pressure are compressed to high temperature and pressure. This is all made possible by the motion of the roller.


We have been providing world-class equipment to manufacturers all over the world for the last 60 years. To learn more about our compressors or to place an order, contact Kaishan Compressor today.

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