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Technology, Oil-Free Air Compressors

Principles of the Rotary Screw Compressor


There’s a famous quote about sausage and not wanting to see how it’s made, the point being that sometimes it better to appreciate a product for what it is because, if you were to see how it got from points A to B, it may lose some of it’s appeal (the original quote, it’s attribution widely disputed, actually compared the ways both sausages and laws are made).

Encased meat and legal decrees aside, there can be great satisfaction in understanding how things are made, to take a peek and see what goes on behind the curtain. Machines are engineering marvels, often times a marriage of straightforwardness and the complex. That holds true with the rotary screw compressor. Simple in design, yet precision engineered to deliver with great efficiency. Rotary screw air compressors are the workaholics of the industrial world, delivering 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.

The principal behind a twin rotary screw compressor was developed in the 1930s. 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.

Here are some of the primary advantages of rotary screw compressor power delivery:

  1. Shockless, non-wearing compression technology
  2. Hundreds of fewer wearing parts to maintain or replace
  3. Simplified maintenance procedures
  4. Extremely low oil-carryover (as little as 3 ppm)
  5. Less overall oil consumption
  6. Proven reliability in harsh environments
  7. Smaller physical footprint, typically by 50 percent
  8. No “unbalanced” forces, less vibration transfer
  9. Lower noise outputs to meet OSHA requirements
  10. Less heat generation
  11. Significant reduction in weight, typically by 50 percent
  12. Zero reduction in capacity over time

Rotary screw air compressors are also available in two primary applications: 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 cool and lubricate the compressor elements moving parts, to cool the air being compressed in the chamber, and to help minimize leaks from returns into the chamber during discharge. While oil is the most common liquid used today because of its lubricating and sealing properties, water and other polymers are sometimes 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 often times is run through a cooler, depending on end usage. There’s a pretty good animation of the rotary screw animation process here and here. Check out Atlas Copco's oil-injected rotary screw compressors here.

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