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Picture of rotary screws in air compressor
Oil-Free Compressors Oil-Injected Air Compressors

Principles of the Rotary Screw Compressor


Machines are oftentimes a marriage of straightforwardness and complexity; this certainly holds true for the rotary screw air compressor. Simple in design, yet precision engineered to deliver with great efficiency, rotary screw air compressors are the workaholics of the industrial world. They deliver a constant supply of energy all day, every day, without interruption. And when properly sized, they are one of the more efficient forms of compressed air delivery around.

How Does a Rotary Screw Air Compressor Work?

The principal behind a twin rotary screw compressor was developed in the 1930’s. The twin elements of the compressors include male and female parts rotating in opposite directions. Air fills the space between the rotors and, as they rotate, the volume between them and the surrounding housing decreases, squeezing or compressing the air into a smaller space; the length, pitch of the screw, and the form of the discharge port collectively determine the pressure ratio. Beyond that, there are no valves, or other mechanical forces that can cause unbalance, which allows a screw compressor to operate at high speeds while combining a large flow rate with small exterior dimensions – it packs a good punch for its size.

Oil-Free Screw Compressors vs. Oil-Injected Screw Compressors

Rotary screw air compressors are available in two primary technologies: oil free and oil injected. Here’s a breakdown:

Oil Free Rotary Screw Compressors

External gears synchronize the position of the counter-rotating screw elements, and, because the rotors do not come in contact and create friction, no lubrication is needed within the compression chamber. As a result, the compressed air is oil-free. Precision engineering within the housing keeps pressure leakage (and drops) from the pressure side to the inlet at a minimum. And because the internal pressure ratio is limited by difference in air temperature between the inlet and discharge ports, oil-free screw compressors are frequently built with several stages and inter-stage cooling to maximize the pressure reach. The gearbox driving the mechanism does contain lubricants; oil-free refers to the compression chamber itself, and the delivered air is free of foreign contaminants beyond those found inherently in the air that passes through the intake.

Oil-Injected Rotary Screw Compressors

In liquid-injected rotary screw air compressors, a liquid is injected into the compression chamber to accomplish the following: to cool and lubricate the compressor elements’ moving parts; to cool the air that is being compressed in the chamber; and to help minimize leaks from returning into the chamber during discharge. Oil is the most commonly-used liquid used due to its lubricating and sealing properties, though water and other polymers are also used. The oil is then separated and passes through a filter and cooler before it cycles back into the process again. The compressed air can still be hot and oftentimes is run through a cooler, depending on end usage.

 Want more information about screw compressors? Visit us at www.atlascopco.com/air-usa or download our new whitepaper, Your Essential Guide to Rotary Screw Compressors!


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