Rotary Screw Air Compressors

Rotary screw air compressors are best for applications that require continuous use or high CFM. Oil-free rotary screw compressors are also a good choice for any application that requires very clean air. Some industries that commonly utilise rotary screw air compressors are:

• Food and beverage industry
• Hospitals
• Manufacturing

A rotary screw air compressor works by forcing air through a pair of rapidly rotating helical screws (or rotors). Powered by the air compressor motor, the two rotors, one female rotor and one male rotor, move in opposite directions. As the male and female rotors turn, they interlock, creating a series of chambers. This causes a vacuum, which pulls more air into the chamber through the air compressor air intake. As air moved through the successive chambers of the paired rotors, it is squeezed into a smaller volume, resulting in compressed air. Finally, the compressed air exits the screw unit, where it can be captured, dried, filtered, and used or stored.

There are two basic types of rotary screw compressors, oil-injected and oil-free.

OIL-INJECTED ROTARY SCREW COMPRESSORS

• Oil-injected screw compressors are the most common type of rotary screw air compressor and work well for most industrial applications.
• An oil-injected screw compressor utilises oil to lubricate the screw unit and ensure a smooth operation.
• The oil is also used to help cool the compressed air and prevent the unit from overheating. After the air has been compressed, the oil is removed using an oil separator.
• A small amount of oil may be left in the compressed air (oil carryover), which can be further reduced using inline filtration.

OIL-FREE ROTARY SCREW COMPRESSORS

• An oil-free rotary screw air compressor uses a two-stage compression process. Air passes through an intercooler between the two compression stages to bring temperatures down and prevent overheating.
• Oil-free rotary screw air compressors eliminate any oil carryover, producing very clean high-quality compressed air.
• However, oilless rotary screw compressors tend to be more expensive, noisier, and more complicated to maintain compared to oil-injected rotary screw compressors.

A rotary screw air compressor is one of the two types of positive displacement gas compressors, with the other being the piston air compressor. It uses a pair of enmeshed helical screws to compress air. As the screws turn, the air is continually compressed.

Rotary screw air compressors are widely used for applications that require high CFM (cubic feet of air per minute) continuous use (100% duty cycle). Due to this, it is the most commonly used air compression technology for heavy industrial applications such as powering pneumatic production lines and conveyor systems. Rotary screw air compressors are one of the easiest types of air compressors to operate and maintain.

Below are the basic components of a rotary screw compressor:

AIR-END (PUMP)

The air-end is where the work of air compression happens inside the rotary screw air compressor.

AIR INLET VALVE

Where atmospheric air comes into the air-end for compression.

ROTORS

A pair of interlocking helical screws that rotate rapidly to compress air.

COMPRESSION CYLINDER

The housing that contains the rotors. A rotary screw air compressor may have a single compression cylinder or multiple cylinders for multi-stage compression. Oil-free compressors will also have an intercooler between each compression cylinder.

DISCHARGE VALVE

Where compressed air exits the air-end. Air may go from the discharge valve to an air receiver tank or directly to air-using processes.

BEARINGS

Bearings on both rotor ends help the rotors to stay in place and turn smoothly and rapidly. Bearings are typically anti-friction and corrosion-resistant.

HIGH CFM

• Compared to piston air compressors, rotary screw air compressors can produce more compressed air (measured in cubic feet per minute, or CFM) per horsepower. That means they use less energy for producing the same volume of compressed air.

CONTINUOUS USE

• Rotary screw compressors are designed to compress continuously and operate best at 100% duty cycle, making them a great choice for 24/7 production operations.

CONSISTENT AIRFLOW

• You can also count on a rotary screw air compressor for highly consistent airflow, so you can size your air compressor very close to your maximum use scenario.

LOWER OVERALL COST OF OWNERSHIP

• A rotary screw air compressor costs more upfront but will save money in the long run.
• In addition to lower energy costs due to their higher CFM output, rotary screw compressors will generally last longer than piston air compressors.

CLEANER AIR

• A rotary compressor will deliver very clean air with low oil carryover.
• An oil-injected rotary screw compressor will have an oil carryover of ~3 ppm. Oil-free compressors, of course, have no oil carryover.

QUIETER OPERATION

• Compared to piston compressors, screw compressors are overall quieter. This is because the action of the rotary screws is quieter than the action of moving pistons. Additionally, screw compressors often have a sound-damping enclosure.
• A typical rotary screw air compressor will operate at 65-75 dBA, or somewhere between the noise level of a car engine and a vacuum cleaner.

COOLER TEMPERATURES

• Lower operating temperatures translate to less work for coolers and air dryers, resulting in further energy cost savings.
• Internal operating temperatures for a rotary screw air compressor range from roughly around 26°C to 37°C.
• For comparison, a piston compressor typically operates at an internal temperature of roughly 65°C to 93°C

SPACE SAVINGS

• A rotary screw air compressor generally takes up less floor space than a piston air compressor at similar CFM output.
• In manufacturing facilities where floor space is at a premium, this can be a huge advantage.