Working Principle of a Screw Compressor

A screw compressor works with a male and female rotor that rotate in the opposite direction. This movement draws in air that is compressed as the space between the rotors and their housing decreases. The compressed air is then displaced to the outlet.

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

The speed of the rotors is optimized at a certain level to minimize mechanical losses (due to heat at very high speed) and volumetric losses (air losses due to very low speed).

There are also fixed-speed and variable-speed drive (VSD) variants. The basic difference is:

• 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

Modern, high-speed, oil-free screw compressors have asymmetric screw profiles. This arrangement results in significantly improved energy efficiency from 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.

One variant is 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, helping remove heat while minimizing leakage in the compression chamber.

Oil-lubricated screw compressor elements can be manufactured for high-pressure ratios. One compression stage is usually sufficient for pressures up to 14 bar, and sometimes 17 bar, albeit at the expense of energy efficiency.

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.

Such inefficiency and waste is generally found at facilities operating multiple shifts and experiencing flow demand fluctuation throughout the day.

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 slow production days, breaks in workflow, or throughout second and third skeleton shifts, VSD technology is especially handy. This type of air compressor saves electricity and money, compared to fixed-speed models.

Since compressed air energy usage accounts for over 80% of an air compressor's costs, it's important to choose efficient equipment.

One of the reasons for the great popularity of rotary compressors is their versatility. There really is the right rotary compressor for every application. Atlas Copco offers screw compressors with three types of drives, each with its own benefits.

• Belt Drive: These compressors are very robust as well as easy to install and maintain. In addition, they have the lowest initial investment cost among rotary screw compressors. This combination makes them a great choice for smaller operations.
• Gear Drive: A gear drive makes compressors quieter. The investment is higher than a belt drive but also more energy efficient. Therefore, if you use the compressor a lot, this investment will pay off over time thanks to energy savings.
• Direct Drive: This is the optimum of rotary compressor technology. The innovative direct drive offers unmatched efficiency and performance. Among the three, they create the least noise. Direct drive rotary screw compressors come in different types - from a basic design for the smaller models to those with ultra-efficient variable speed drives.

If you’re running heavy-duty applications that continue over long periods of time, like in a factory, industrial plant or workshop, then you should consider upgrading to a rotary screw air compressor.

Benefits of a screw compressor in industry:

• They are designed to work 24/7
• They can operate at higher flow capacities while dissipating the heat generated
• They are relatively less noisy than piston compressors
• They receive less wear and tear
• They tend to require less space
• They outperform piston compressors in terms of efficiency at about 15 kW (20 hp)
• They discharge less oil and contaminants with the compressed air