How does a Rotary Screw Compressor work?

How does a Rotary Screw Compressor work?

Ultramax Systems on 15th Oct 2026

Short answer: A rotary screw compressor traps air between two meshing helical rotors and their housing. As the rotors turn, each trapped pocket moves along the rotors and gets smaller, so the air pressure rises until the pocket opens to the discharge port. In oil-injected machines, oil cools, seals and lubricates the process, and a separator removes most of it afterwards.

How a rotary screw compressor works

The heart of the machine is the airend. The US Department of Energy (2016) describes it as two intermeshing rotors turning inside a machined housing. One is the male rotor and the other the female rotor, and they rotate in opposite directions (Atlas Copco n.d.).

Air moves through the airend in small pockets, and each goes through the same four stages. First, the lobes unmesh near the inlet port and air flows in to fill the space between them. Further rotation then closes that pocket off from the inlet, trapping the air. As the meshing line moves along the rotors, the pocket shrinks and its pressure rises. Finally, the pocket reaches the discharge port and the compressed air leaves the airend.

Schematic plan view of an oil-injected screw airend. Air enters the inlet port, oil is injected, and pockets between the male and female rotors get smaller and darker in colour as pressure rises toward the discharge port.
Figure 1. Inside the airend: trapped pockets shrink as they travel from the inlet port to the discharge port.

Because the discharge port sits at a fixed position, every airend has a built-in pressure ratio. As Atlas Copco (n.d.) explains, rotor length and pitch set that ratio, and the design of the discharge port also plays a part. Efficiency is typically highest when the built-in ratio suits the working pressure, so a machine run well away from its design pressure does extra work for the same air.

Key components of an oil-injected package

Around the airend sits an air path and a separate oil circuit. Figure 2 shows how air and oil travel through a typical package, and the table summarises each part.

Flow diagram of an oil-injected screw compressor. Air passes the inlet filter, inlet valve, airend, air-oil separator tank, minimum pressure valve and aftercooler. Oil returns from the sump through a thermostatic valve, oil cooler and oil filter back to the airend.
Figure 2. Air path (blue) and oil circuit (gold) in a typical oil-injected rotary screw compressor.
ComponentWhat it does
Inlet filterStops dust in the intake air from reaching the rotors and the oil.
Inlet valveControls how much air the airend draws in, so the machine can load, unload or modulate.
AirendCompresses the air. Oil is injected into the compression space.
Air-oil separator tank and elementThe tank drops out most of the oil. The separator element then removes most of what remains.
Minimum pressure valveHolds pressure in the separator tank at start-up and is often combined with a check valve.
AftercoolerCools the compressed air, so much of its water vapour condenses before the air leaves the package.
Thermostatic valve and oil coolerKeep oil within its design temperature range by varying how much oil passes through the cooler.
Oil filterRemoves particles from the oil before it returns to the airend.

Oil is injected for three reasons. It lubricates the rotors and bearings, it carries away the heat of compression, and it seals the small internal clearances so less air slips back (US Department of Energy 2016). During start-up, a minimum pressure device stops air rushing through the separator too fast and carrying oil with it (US Department of Energy 2016). Oil temperature needs control too. Hoerbiger (n.d.) describes a thermostatic valve that holds it in the design range by varying how much oil passes through the cooler, and some versions combine this valve with a spin-on oil filter.

Oil-injected and oil-free screw compressors

Oil-injected units are the common choice for general industry. Single-stage lubricant-injected packages range from about 3 to 900 hp, with discharge pressures from 50 to 250 psig, which is roughly 3.4 to 17 bar(g) (US Department of Energy 2016). In practice, oil-injected screw elements typically reach up to 13 bar in a single stage (Atlas Copco n.d.).

Oil-free screw compressors have no oil in the compression chamber, so external timing gears keep the rotors in step instead, and the rotors do not touch each other or the housing. Atlas Copco (n.d.) notes that, without oil to absorb heat, these machines typically split the job into two or more stages with cooling between them to reach higher pressures.

Where rotary screw compressors are used

The lubricant-injected screw has become a dominant type of industrial compressor (US Department of Energy 2016). One reason is duty cycle: rotary screw units are designed for continuous operation (Atlas Copco n.d.), which suits sites that need air across long shifts. Compressed air systems can use up to 30% of a site's electricity (Australian Government n.d.), so control and maintenance show up on the power bill.

Pressure vessel rules in Australia

The separator tank holds air and oil at discharge pressure, so it is a pressure vessel, as is the air receiver downstream. Australian regulators rank pressure vessels from hazard level A, the most hazardous, to E, the least, using AS 4343 (SafeWork SA 2021). WorkSafe WA (2022) lists pressure vessels at hazard level A, B or C as plant requiring registration, while gas cylinders and serially produced vessels are among the exclusions. Separately, SafeWork SA (2021) requires regular inspection of pressure equipment by a competent person in line with AS/NZS 3788. Confirm the status of your own vessels with your state regulator or a competent person.

Common mistakes

Many screw compressor problems start with service parts left too long. A loaded or damaged separator element can raise pressure drop and let more oil pass downstream. A blocked inlet filter restricts intake, so the machine delivers less air than it should, and inlet air should not be unnecessarily warm, humid or dirty either (Australian Government n.d.). Follow the manufacturer's oil specification and change interval too.

Another common mistake is assuming the air leaving the package is clean. Typical lubricant carryover from an oil-injected machine is 2 to 5 ppm (US Department of Energy 2016), and the air still carries water and particles. That is why most sites need dryers and filters chosen for the purity class they require (ISO 8573-1:2010). A coalescing filter is the usual next step for oil aerosols.

The NSW Office of Environment and Heritage (2017) notes that the shorter a compressor's load and unload cycles are, the higher its average unloaded power. Finally, avoid comparing machines on motor size alone. Compare free air delivery (FAD) and specific power at the same conditions instead.

Key takeaways

  • Two meshing rotors trap air in pockets that shrink as they move toward the discharge port.
  • In oil-injected units, oil cools, seals and lubricates, then the separator recovers most of it.
  • Air leaving the package still needs drying and filtering to suit your required air quality.
  • The separator tank and air receiver are pressure vessels, so check registration and inspection duties with your state regulator.

Frequently asked questions

Do oil-free screw compressors still need filters?

In most systems, yes. They still deliver water vapour and particles from the intake air. Select dryers and filters for the purity class your application requires (ISO 8573-1:2010), and confirm the choice with a qualified person.

How often should screw compressor filters be changed?

Intervals depend on the model, running hours and site conditions. Follow the manufacturer's service schedule and check differential pressure indicators where fitted.

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References

This article is general information only. Check manufacturer documentation and applicable Australian Standards for your installation.