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Rotary Screw Compressors: Your Guide

Displacement Compressors Compressor Types Compressors Compressed Air Wiki

When looking for the perfect compressor, it’s important to understand how the compressor works and what types there are. In this article, you’ll find out all about rotary screw air compressors, how they work and why you’d need one.

What is a rotary screw air compressor?

The inner workings of an oil-lubricated screw compressor

A rotary screw air compressor is one of the two types of positive displacement gas compressor. It uses two rotors to create the pressure needed to compress air. They are one of the easiest types of air compressor to use and maintain.

The other type of positive displacement compressor is the reciprocating or piston compressor.

Equivalent to positive displacement compressors, are dynamic compressors, like centrifugal compressors. The former compresses air by forcing gas into a chamber under high pressure, the latter by accelerating the air inside the mechanism.

How does a rotary screw compressor work?

The main screw compressor parts are the male and female rotors, which rotate in opposite directions. This draws in air that is compressed as the space between the rotors and their housing decreases.

Each screw element has a fixed, built-in pressure ratio that is dependent on its length, the pitch of the screw and the form of the discharge port. To attain maximum efficiency, the built-in pressure ratio must be adapted to the required working pressure.

Oil-lubricated vs. oil-free screw compressors

The inner workings of an oil-free screw compressor

There are two types of rotary screw air compressors: 

In oil-lubricated screw compressor technology, the male rotor drives the female rotor

In oil-free compressor technology, a timing gear drives both rotors for harmonic running, with minimum calculated clearance between both elements.

Oil-free compressor

Modern, high-speed, oil-free screw compressors have asymmetric screw profiles, resulting in significantly improved energy efficiency, due to reduced internal leakage.

External gears are most often used to synchronize the position of the counter-rotating rotors. As the rotors don’t come into contact with each other, nor with the compressor housing, no lubrication is required inside the compression chamber. Consequently, the compressed air is completely oil-free.

The built-in pressure ratio is limited by the limiting temperature difference between the inlet and the discharge. This is why oil-free screw compressors frequently work in several stages and interstage cooling to reach higher pressures.

 

Oil-lubricated compressor

This variant is also known as an oil-flooded compressor or an oil-injected compressor. The technical name, however, is oil-lubricated.

Oil-lubricated rotary screw compressors, inject oil into the compression chamber. This oil cools and lubricates the compressor element.

It also helps to remove the heat from the compression process, and aids minimising leakage in the compression chamber.

Oil-lubricated screw compressor elements can be manufactured for high pressure ratios, with one compression stage usually being sufficient for pressure up to 14 and even 17 bar, albeit at the expense of reduced energy efficiency.

Fixed speed compressors vs. variable speed drive compressors (VSD)

The basic difference between the two technologies can be found right in the name:

  • Fixed speed compressors run at a continuous fixed speed
  • Variable speed drive (VSD) technology turns the motor at the appropriate speed in relation to the amount of air that is required

Fixed speed compressors

Fixed speed compressors are very efficient when continuously operating at 100% capacity, but most facilities don’t require that amount of productivity. Before the motor comes to a complete stop, it will continue to run while no air is being produced, consequently wasting energy and money. 

Inefficiency and waste can especially be visible at facilities that operate multiple shifts and or have great flow demand fluctuation throughout the day.

 

Variable speed drive compressors (VSD)

VSD compressors increase the speed of the motor as the need for air increases, thus supplying more flow/CFM. If the demand decreases, the motor will automatically slow down and only use the required energy to provide appropriate flow. 

On slower production days, break throughout the day, or during second and third skeleton shifts, the VSD technology comes especially handy, as it saves electricity and money.

Considering that compressed air usage is costly, as it accounts for over 70% of Total Cost of Ownership in electricity costs, utilizing VSD technology can help you achieve savings of 35-60%

Advantages and usage of rotary screw compressors

In short, the most important benefits of rotary screw air compressors are:

 

  • Energy efficiency

  • Low noise output

  • Small footprint 

  • Continuous operation at temperatures of up to 46 degrees C

  • Low oil carry-over (as little as 3 ppm) in oil-lubricated machines

  • Zero loss of capacity over time

  • Life expectancy between 60,000 – 80,000 hours.

  • Reliability

One of the main benefits for users is the absence of a duty cycle. An oil-injected rotary screw compressor could run for the entire length of a working day and suffer no ill effects. As a matter of fact, it would be beneficial.

 

This is particularly important for manufacturing, where a stopped compressor will impact production, and sectors where air demand is difficult to predict or control.

 

Unlike piston compressors, rotary screw compressors have no valves or other mechanical forces that can cause unbalance. This allows a screw compressor to:

 

  • Operate at high speeds

  • Combine a large flow rate with small exterior dimensions

 

Another significant difference from piston technology, is that oil-injected compressors have as many varied uses as there are industries. This type of compressor is usually picked by users needing large volumes of medium pressure air.


Not sure what type of rotary screw air compressor you need? Get in touch with Atlas Copco’s experts for personal advice and a tailored quote.

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