How does a rotary screw compressor work?

Previously, we discussed the working principles of piston compressors. This blog will look at how screw compressors, also known as screw air compressors, work. Rotary screw compressors are a type of rotary volume compressor.

Estimated time to read: 7 minutes

March 7, 2023

Rotary screw compressors

The way in which an air compressor compresses or pressurises air will have an influence on the air that it releases. A rotary screw compressor is designed to release a steady, continuous and consistent flow of pressurised air, without any of the fluctuations often associated with piston compressors. This has made them particularly popular in food-packaging plants, automated manufacturing systems and for tools like impact wrenches, jackhammers, sandblasters and, of course, paint-spraying equipment.

How do rotary screw compressors work?

Alup rotary screw compressor

A rotary screw compressor compresses air in what we call the rotary screw element. This contains two ‘rotors’ that spin or – you guessed it – rotate. Nonetheless, you’ll often see the rotors referred to as ‘helical screws’ or even more simply, as ‘screws’. This is perfectly understandable too; the rotors have a ‘thread’ on them, just like you would find on a screw. When a rotary screw compressor draws air in, the air is caught in the gaps between the thread. As the rotor turns, the air moves with the thread towards the outlet. What is the name of this process? ‘Positive displacement'. The lines of thread, or ‘teeth’, are further apart near the air inlet. This allows them to draw more air in. But closer to the outlet, the ‘teeth’ are closer together, meaning the ‘grooves’ – the gaps between them – are smaller. The teeth of one rotor press into the grooves on the other rotor. This compresses or pressurises the air. As the grooves are spaced closer and closer together towards the outlet, the air is compressed more and more as it moves closer to the outlet.

The different types of rotary screw compressors

There are basically two main types of rotary screw compressors. An oil-free screw compressor is mostly used for sensitive processes where the compressed air needs to meet a specific quality standard.

However, oil-injected screw compressors are far more common. They use oil to keep the moving parts lubricated, minimising wear and tear, while also helping to keep operating temperatures low and reduce the noise made by the air compressor. They are slightly more complex, with different circuits for the air and the lubricating oil.

The air circuit

A rotary screw compressor draws air in. The air first travels through a filter, then through an open inlet valve into the compressor element. This inlet valve is effectively a one-way valve. When the compressor is turned off, the valve closes, stopping air and oil from heading into the filter.

The air is compressed in the compressor element as described above.

In an oil-free screw compressor, the compressed air is effectively able to be moved from the compressor to the outlet. But, for an oil-injected screw compressor, oil inevitably ends up mixed in with the compressed air. Since it needs to be removed, the compressed air is passed from the compressor element into an air receiver/oil separator via a check valve. This check valve stops the air and oil from travelling backwards when the compressor is turned off.

When the rotary screw compressor is operating, a minimum pressure valve keeps the pressure in the separator tank above a minimum value. This is required for lubrication. Further downstream, another valve stops compressed air from being vented into the atmosphere when the compressor is turned off. 

Oil-injected screw compressors: the oil circuit

In an oil-injected screw compressor, the air is pushed through the air compressor element and it inevitably picks up oil. This needs to be removed. As such, in an oil-injected screw compressor, the compressed air moves from the air compressor element into an air receiver/oil separator. Here it is spun at super-high speeds that separate the heavier oil from the lighter air using centrifugal force.  

The bottom part of the air receiver/oil separator serves as a kind of oil tank. There is a thermostatic bypass valve in the tank. This means the oil will stay in the tank until a certain temperature is reached. When the temperature is reached, the valve opens, and air pressure forces the oil from the air receiver/oil separator through the oil filter and oil stop valve into the compressor element. When the oil is approximately 15˚C above the set point, it also travels through an oil cooler. The oil stop valve prevents the compressor element from being flooded with oil when the compressor stops. 

Would you like to know more about rotary screw compressors? Are you wondering which one would be best for your unique needs? Get in touch. There’s nothing we love more at ALUP than chatting about air compressors.

Scroll Up ↰

See also

FInd more must-read articles on our compressed air blog!