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, inherent pressure ratio determined by the screw’s length and pitch, along with the design 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).

• Air intake & control – Filters incoming air and regulates loading and unloading based on system pressure.
• Compression screw element – Interlocking screw elements inside the compression chamber generate pressure and airflow.
• Lubrication & separation – Oil filters and separators support cooling, sealing, and clean air delivery in oil-injected units.
• Drive & bearings – The motor and bearings power and stabilize rotation while minimizing friction and wear.
• Cooling & moisture removal – Aftercoolers and water separators reduce heat and remove condensate before air enters the system.

Modern oil-free compressors use high-speed, asymmetrical screw profiles to compress air with reduced internal air leakage compared to conventional screw designs, which improves compression performance. External timing gears keep the rotors perfectly synchronized so they never touch each other or the compressor housing. Because no oil is used in the compression chamber, the system delivers completely oil-free compressed air, protecting products and processes from contamination.

To manage heat and achieve higher pressures safely, oil-free screw compressors typically use multiple compression stages with interstage cooling rather than a single high-pressure stage.

Fixed speed compressors are very efficient when continuously operating at 100% capacity, but most facilities don’t require that amount of productivity. To maintain the correct pressure level, the compressor will alternate between load and unload condition. During unload 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. If the demand decreases, the motor will automatically slow down and only use the required energy to provide appropriate flow.

As the power consumption of the VSD compressor is adjusted to any fluctuation in air demand, this type of compressor can reduce electricity use and associated operating costs compared to fixed speed models operating under similar, variable-demand conditions.

Since in many industrial installations the energy used to produce compressed air accounts for a large share of an air compressor's total lifecycle cost, it is important to choose equipment with high energy efficiency performance.

Dual-speed combine elements of both technologies. They operate at two distinct speeds: a full-speed mode for high air demand and a lower speed mode during unload.

This second speed is what sets them apart. Unlike fixed-speed compressors under similar operating conditions, dual-speed models can start under pressure, reduce blow-off and unload losses, and consume less energy when running unloaded. As a result, in our internal tests under typical industrial load profiles, they have delivered energy savings of up to 20% compared to fixed-speed compressors of similar capacity

• Belt Drive: These compressors are very robust as well as easy to install and maintain. In addition, they typically have a lower initial investment cost than other rotary screw compressor drive types, which can make them a practical choice for smaller operations. As belts are the least efficient drive type, they are typically only found in smaller power sized compressors.
  
• Gear Drive: For high-demand operations, a gear drive can offer long-term benefits. It typically runs quieter than a belt drive, optimizes element speed to improve efficiency, and can reduce energy use over time, which may offset the higher upfront cost depending on operating conditions.
• Direct Drive: Direct drive combined with a variable speed drive represents a high-performing rotary compressor configuration. Its design provides high efficiency and performance, and it is typically the quietest option of the three drive types described here.

Different compressors operate in distinct ways. Some suit occasional or low-demand tasks, while others are built for continuous use in demanding environments, with each option offering a different balance of efficiency, cost, noise level, capacity, and maintenance. What are the main points to be aware of while choosing an air compressor?

Choosing the right compressor depends on three key factors: flow, pressure, and application. Together, they define how much air you need, the force required to perform the task, and how the compressor will be used over time. To size your compressor accurately and avoid costly under- or oversizing, use our air compressor sizing guide.

While rotary screw compressors typically involve a higher upfront investment, the cost of ownership is often largely driven by energy use. In certain industrial applications, energy can account for a substantial share, in some cases up to around 80% of a compressor's estimated lifetime cost, depending on operating hours and electricity prices.

Variable Speed Drive (VSD) screw compressors can reduce this by matching motor speed to actual air demand, and in our internal studies under typical industrial load profiles, they have delivered energy savings of up to 60% compared to fixed-speed models. Depending on local energy prices and operating hours, users may offset the higher purchase price within a year through lower energy bills and associated reductions in indirect CO₂ emissions from electricity consumption.

With oil-free, oil-lubricated, fixed speed and VSD options available, choosing the right technology depends on your application, duty cycle and efficiency goals. Our compressed air specialists can assess your requirements and recommend the optimal solution.