What is a rotary screw air compressor?

April, 20 2022 IAS Admin

Rotary screw air compressors are a newer, more advanced type of air compressor. As the name would suggest, a rotary screw compressor uses rotary movements to compress the air – this type of air compressor continues to grow in popularity and is now one of the most widely used compressed air technologies. Renowned for their durability and reliability, rotary screw air compressors are often found in demanding industrial environments that require compressed air 24/7.

While the upfront cost of a rotary screw compressor is generally higher than a traditional reciprocating (piston) compressor, they have several benefits that make them far more efficient to run and maintain, making them a more cost-effective option for long-term compressed air use.

How does a rotary screw air compressor work?

When thinking of an air compressor, most people would likely picture the traditional reciprocating air compressor which uses a reciprocating piston inside a cylinder to compress air. It does this by reducing the volume inside the cylinder with a reciprocating motion. This compressed air is then transferred to an air-receiver tank ready for use.

Reciprocating air compressors have been around since the industrial revolution and have a relatively low up-front cost which has made them a popular choice for the likes of workshop environments.

How a reciprocating compressor functions
Diagram of how a reciprocating air compressor works. Source

In comparison, a rotary screw air compressor operates in a completely different way. This type of compressor works by using two spiral rotors (rotary screws) that turn in opposing directions inside a metal housing. The air enters in through the top of the housing at one end via an inlet valve, where it is mixed with oil and travels down through the rotating rotary screws.

Because the two rotary screws rotate in unison and are in full contact with each other, oil is necessary to keep the rotors lubricated and spinning freely. This oil is later separated from the compressed air before it exits the system for use. As the air moves down through the rotors, there is a significant reduction in volume which results in the air being compressed before it exits through the other end of the housing. This compressed air is then transferred to an air-receiver tank ready for use. This housing unit with the spiral rotors inside is commonly known as the ‘air-end’ on a rotary screw compressor.

How a rotary screw compressor functions
Diagram of how a rotary screw air compressor works. Source

Flow process of a rotary screw compressor

There are several steps involved in the flow process of a rotary screw compressor, as outlined below.

  • Step 1: Screws turn rapidly in opposing directions.
  • Step 2: Screw revolutions create a vacuum, pulling atmospheric air into the inlet valve through the air filter.
  • Step 3: Air enters the air-end where it is trapped, mixed with oil and compressed as it moves down the screw chamber.
  • Step 4: Compressed air mixed with oil exits the air-end via the discharge port.
  • Step 5: Compressed air mixed with oil enters the oil separator tank where the oil is separated from the compressed air.
  • Step 6: Oil collected in the oil separator tank is moved to an oil cooler where it is cooled down.
  • Step 7: Oil-free compressed air exits the filtration system and enters the air-receiver tank where it is stored for use.
  • Step 8: The cooled oil is sent to an oil filter where any remaining debris and contaminants are removed.
  • Step 9: The filtered oil is recycled back into the air-end for lubrication via the scavenge return line.

For a visual understanding of how a rotary screw compressor works, please check out this excellent animated video that MAN Diesel & Turbo have created which explains the basic principles of the rotary screw.

Size and capacity

The capacities of a rotary screw compressor are typically rated by either horsepower (HP), kilowatt (KW), or standard cubic feet per minute (CFM). Another unit of measurement that is important is pounds per square inch (PSI) which can vary depending on the application.

Most common rotary screw compressors are manufactured to produce a continuous supply of 116 PSI (8 Bar) or 145 PSI (10 Bar) pressure however special rotary screw compressors are also manufactured for applications that require higher pressure than this.

Rotary screw compressors are commonly available in anything from 5 HP up to 500 HP and can produce airflows of 18 cfm to 2500 cfm respectively. Generally, the most pressure that can be obtained using single-stage screw compression (one set of rotors) is around 250 PSI, however by using a 2-stage screw compressor (two sets of rotors), pressures of up to 600 PSI can be obtained.

It is important to note that a screw compressor is designed and manufactured to operate at a certain pressure level and generally won’t be able to cover a large range of output pressures. For example, a 116 PSI (8 bar) screw compressor won’t be sufficient to produce 145 PSI (10 bar) pressure output as its ‘air-end’ and systems are different, and therefore won’t be able to produce higher pressure without running into problems. Having said this, there are now more advanced rotary screws available that can efficiently produce a range of pressures from 8 to 14 Bar, however as the pressure is increased, the CFM output will decrease.

Applications and industries suited for screw compressors

Rotary screw compressors are built for continuous or near-continuous compressed air supply up to 24 hours a day, 7 days a week and as such, they are used in larger industrial applications. The introduction of small variable-speed drive (VSD) rotary screw compressors, also provides smaller industrial applications with a more efficient compressed air solution too.

Made to only supply the exact amount of compressed air required and with the ability to automatically switch off and on when required, smaller rotary screw compressors are now a fantastic and efficient alternative for environments that may have relied on a traditional reciprocating air compressor. As such, rotary screw compressors have now become the air compressor of choice for many different applications.

Rotary screw compressors are required in industries such as food and beverage packaging plants and automated manufacturing facilities where there is a continuous demand for compressed air. Manufacturing facilities that have frequent intermittent demand for compressed air such as smaller timber joineries also use rotary screw compressors and sometimes when the demand is less frequent, a variable-speed rotary screw compressor will be the preferred option to help save on energy costs.

Portable rotary screws compressors are also commonly used in the construction industry to provide compressed air to pneumatic operated oil pumps, jackhammers, sandblasters, industrial paint systems and many other construction tools. These compressors are mounted on tow-behind trailers and powered by diesel engines. It is a common and necessary machine for most construction and civil work sites around the world.

PneuTech variable-speed drive rotary screw compressors

PneuTech variable-speed rotary screw compressor

As industrial compressed air suppliers, we know just how valuable an efficient rotary screw air compressor is, which is why we stand by the range from PneuTech.

PneuTech’s RSCRV and RSCRPM series of variable-speed rotary screw air compressors have achieved well-deserved renown in the marketplace for their unparalleled performance, energy-saving efficiencies and long-term reliability. With cutting edge technology and proven engineering processes, the permanent magnet range will bring further efficiencies to your factory, by more accurately matching air supply to the demand, with as little as 120 seconds of turn-down time. Significant power savings can also be achieved – as high as 60 percent in some cases!

Want to know more about this range of energy-efficient air compressors? Learn more in the free brochure below.

Learn about our entire range of permanent magnet air compressors. Download brochure

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