The airend: two rotors doing the work
The heart of the compressor is the airend. It holds two rotors in a close-fitting housing:
- The male rotor has convex lobes and is usually driven by the motor.
- The female rotor has concave flutes and meshes with the male rotor.
The rotors never quite touch each other or the housing; small clearances are held by precision bearings. As they turn, the space between the lobes, flutes and housing forms a series of pockets.
- Suction. At the inlet end, the lobes unmesh and open a pocket, which fills with air drawn through the intake filter.
- Trapping. As the rotors turn, the pocket closes off from the inlet port. The air is now sealed in.
- Compression. The pocket moves along the rotors towards the discharge end and gets smaller as the lobes mesh more deeply. Pressure and temperature rise.
- Discharge. The pocket reaches the discharge port, which is shaped so the air leaves at about the designed pressure.
Because many pockets are moving at once, delivery is smooth and continuous, with none of the pulsation of a piston compressor.
Built-in volume ratio: why port shape matters
A screw compressor has a fixed built-in volume ratio, set by the shape and position of the discharge port. This gives a built-in pressure ratio. When the system pressure matches it, the air leaves smoothly. When the system pressure is lower, the air is over-compressed and then expands at the port, which wastes energy. When it is higher, air flows back into the pocket at discharge, which also wastes energy.
That is why manufacturers offer different airend designs or port arrangements for 7.5, 10 and 13 bar duties, and why running a compressor far from its design pressure can reduce efficiency.
Oil-injected and oil-free screws
| Feature | Oil-injected screw | Oil-free (dry) screw |
|---|---|---|
| Role of oil | Injected into the airend to seal clearances, cool and lubricate | No oil in the compression chamber |
| Rotor drive | Male rotor drives female rotor through the oil film | Timing gears keep rotors apart |
| Stages | Usually one stage for plant air | Usually two stages with an intercooler |
| Discharge temperature | Typically about 75–95 °C | Much higher inside each stage, cooled by intercooler and aftercooler |
| Air treatment | Separator plus downstream filtration and dryer | Dryer and particle filtration; no oil removal needed for compression oil |
| Typical use | General industry, construction, drilling | Pharma, food and beverage, electronics, textiles |
The rest of this page follows an oil-injected screw, the most common type in Indian industry. Oil-free designs are covered on their own pages.
The complete air path through the package
- Intake filter. Removes dust before air reaches the rotors. In dusty Indian sites, a clean filter is the first protection for the airend and oil.
- Inlet valve. Opens to load the compressor and closes to unload it. On modulating machines it throttles part-way.
- Airend. Air and injected oil are compressed together.
- Separator tank. The air-oil mixture enters a vessel where most oil drops out by change of direction and speed. A separator element removes the fine oil mist that remains.
- Minimum pressure valve. Holds pressure in the separator so the oil keeps circulating and the separator works properly, then opens to the system.
- Aftercooler. Cools the compressed air, which condenses much of its water vapour.
- Moisture separator and drain. Removes the condensed water.
- Downstream treatment. Receiver, dryer and filters bring the air to the quality the process needs.
The oil circuit
Oil is pushed around the circuit by the pressure in the separator tank, with no separate oil pump on most designs.
- Hot oil leaves the separator tank and goes to a thermostatic valve.
- When the oil is cold, the valve sends it straight back to the airend so the machine warms up quickly.
- When the oil is hot, it goes through the oil cooler first.
- The oil passes through the oil filter and is injected into the airend and bearings.
- A scavenge line returns the oil collected in the separator element to the airend.
The thermostatic valve keeps discharge temperature in the normal range. If oil runs too cool, water condenses in it; if it runs too hot, it breaks down quickly. Both damage the bearings.
Drive, cooling and controls
Drive. The motor turns the male rotor through a coupling (direct drive), belts and pulleys (belt drive) or gears (gear drive). A variable speed drive (VSD) changes motor speed to match air demand.
Cooling. Air-cooled packages use a fan to blow air over the oil cooler and aftercooler. Water-cooled packages use plant cooling water. Almost all of the electrical energy used ends up as heat, so the room needs good ventilation, especially in summer.
Control modes.
- Load/unload: the inlet valve opens fully to load and closes to unload. The motor keeps running unloaded for a set time, then stops if demand stays low.
- Modulation: the inlet valve throttles to reduce output. Simple but uses relatively more power at part load.
- Variable speed: motor speed follows demand, which suits plants whose air use varies a lot.
- Sequencing: a master controller shares load between several compressors.
Protection. The controller shuts the machine down on high discharge temperature, motor overload, phase failure and, on some models, high separator pressure difference. Portable diesel screws also stop on low engine oil pressure and high coolant temperature.
Why screw compressors suit continuous duty
A screw airend has no valves, piston rings or reciprocating parts in the compression chamber. The rotors turn steadily, so there is little vibration and the machine can run loaded for long hours. That is why screw compressors dominate factory plant air and portable site work. The trade-off is that a screw is designed around close clearances and clean oil: dust, wrong oil, overheating and long periods of running cold all wear the bearings and rotors. Good intake filtration, correct oil, clean coolers and a ventilated room are what give a screw compressor its long life.
Key performance terms
| Term | Meaning | Why it matters |
|---|---|---|
| FAD (free air delivery) | Air actually delivered, referred to inlet conditions, measured to ISO 1217 | The true capacity figure; compare machines on FAD, not motor kW |
| Working pressure | The pressure at which the FAD is delivered | FAD falls as pressure rises; about 7% more power per extra bar |
| Specific power | Input power per unit of FAD, in kW per m³/min | The best single efficiency comparison at a given pressure |
| Discharge temperature | Air-oil temperature leaving the airend | Typically about 75–95 °C; a rising trend shows cooling or oil problems |
| Pressure dew point (PDP) | Temperature at which water condenses at line pressure | Set by the dryer, about +3 °C for a refrigerated dryer |
What wears, and what we check
Understanding how the machine works explains what needs service:
- Air filter, oil filter, separator element and oil, which carry the dust, oil mist and heat.
- Inlet valve, minimum pressure valve and thermostatic valve, which control loading, oil flow and temperature.
- Coolers, which foul with dust and oil and cause high temperatures.
- Airend bearings and shaft seal, which wear with running hours, poor oil and dust.
- Drive belts, coupling or gears, and motor bearings.
- Drains, which must clear condensate reliably.
On our own hire compressors, we change oil and service every 300 running hours, because site work is dusty and continuous. Stationary plant machines follow the manufacturer’s intervals, adjusted for site conditions.
How Reddy Compressors helps
Reddy Compressors, based at Tarnaka, Secunderabad, and serving industry since 2002, is an independent multi-brand specialist, not an authorised dealer of any brand. We have tie-ups with all major and reputed brands. We sell new and used/refurbished rotary screw compressors, rent them from our own fleet, and service and repair them for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands.
- Help choosing the right type, pressure and control for your plant.
- Service, AMC, breakdown repair and overhaul through more than 20 service engineers and technicians covering Telangana and Andhra Pradesh.
- Spare parts identified from the machine’s nameplate and supplied across India.
- Air audits to measure demand, pressure and specific power.
What to send us
- Compressor make, model, serial number and nameplate photo.
- Working pressure and typical running hours.
- What the air is used for and any air-quality requirement.
- Any fault codes, alarms or symptoms.
Frequently asked questions
How does a rotary screw compressor work in simple terms?
Two meshing screw-shaped rotors turn inside a close-fitting housing. Air is drawn into the space between the lobes at one end, trapped, and carried along the rotors into a space that keeps getting smaller, so its pressure rises. It leaves at the other end. In oil-injected machines, oil seals and cools the rotors and is then separated from the air before it leaves the package.
Why does a screw compressor need oil?
In an oil-injected screw, oil seals the small clearances between the rotors and housing, removes most of the heat of compression and lubricates the bearings. Without it, the airend would leak back more air and overheat. The oil is separated from the air in the separator tank and element. Oil-free screws use timing gears and two stages instead, with no oil in the compression chamber.
What is the normal operating temperature of a screw compressor?
For an oil-injected screw compressor, discharge temperature is typically about 75–95 °C. The thermostatic valve keeps the oil warm enough to drive off moisture and cool enough to protect it from breaking down. A temperature that rises over time usually means blocked coolers, low or wrong oil, a faulty thermostatic valve or poor room ventilation.
What is the difference between loaded and unloaded running?
When loaded, the inlet valve is open and the compressor delivers air. When unloaded, the inlet valve closes and the machine runs without delivering air, using much less power, but still not zero. If demand stays low, the motor stops after a set time. Long unloaded periods waste energy, which is why VSD or sequencing control can help.
Is a screw compressor better than a piston compressor?
For continuous duty and larger flows, a screw compressor usually gives smoother delivery, lower maintenance per running hour and quieter operation. Piston compressors suit intermittent use, small flows and very high pressures, and cost less to buy. The right choice depends on duty cycle, pressure, flow and the hours the machine will run each year.
How often should a screw compressor be serviced?
Follow the manufacturer’s service manual, based on running hours and adjusted for dust, heat and duty. In dusty, hot conditions, filters and oil need changing sooner. On our own hire compressors on continuous site work, we change oil and service every 300 running hours. An AMC keeps service on schedule and records the machine’s condition.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.