How an oil-free screw compressor works
A screw airend has a male and a female rotor meshing inside a casing. As they turn, air is trapped in the pockets between the lobes and pushed along the rotors into a shrinking space, so it is compressed before it leaves at the discharge port. In an oil-injected screw, oil fills the gaps, carries away heat and lets the male rotor drive the female rotor directly.
Representative images of machine types we sell, service and rent. Models and capacities are confirmed against your duty.
In an oil-free dry screw, the rotors do not touch. Precision timing gears drive both rotors and hold a small clearance between them and the casing. The rotors are commonly coated to resist corrosion and to keep the effective clearance tight. Because there is no oil film to seal those clearances, dry screws run at high rotor speeds, driven through a step-up gearbox, so that internal leakage stays small compared with the flow.
The gearbox and bearings are oil-lubricated. On each rotor shaft, an oil seal and an air seal, with a vented chamber between them, prevent oil travelling along the shaft into the compression space. This is why seal condition is central to a dry-screw overhaul.
Why two stages, and what the intercooler does
Without oil to absorb heat, single-stage compression to normal plant pressure would give a discharge temperature too high for the rotors, coatings and bearings. Dry screws therefore split the work into two stages:
- The low-pressure stage compresses atmospheric air to an intermediate pressure, typically around 2–3 bar(g), a pressure ratio of roughly 3.
- The intercooler cools the air close to cooling-medium temperature and removes condensed water through a separator and drain.
- The high-pressure stage compresses the cooled air to final pressure, typically 7–10 bar(g) (100–145 psig).
- The aftercooler cools the discharge before it leaves the package for the dryer.
Intercooling reduces the work of the second stage and keeps both stage temperatures within limits. A fouled intercooler raises second-stage inlet temperature, which raises power, discharge temperature and coating wear. Stage temperatures and the intermediate pressure are therefore the most useful health indicators on these machines. See two-stage oil-free air compressors for more on staging.
Dry screw or water-injected screw?
A water-injected screw takes a different route: clean water is injected into a single-stage airend to seal and cool. Water’s high heat capacity keeps compression close to isothermal, so one stage reaches plant pressure with a low discharge temperature.
| Point | Two-stage dry screw | Water-injected screw |
|---|---|---|
| Stages | Two, with intercooler | One |
| Rotor sealing | Clearance and coatings | Water film |
| Discharge temperature | Higher; suits heat-of-compression dryers and heat recovery | Low |
| Materials | Coated steel rotors | Stainless steel, polymer or other corrosion-resistant parts |
| Critical utility | Cooling air or cooling water for coolers | Water quality and water treatment |
| Typical size range | Medium to large | Small to medium |
Both are oil-free in the compression chamber. The right choice depends on size, the site’s water quality, whether you want heat recovery or a heat-of-compression dryer, and the service support for that model. See water-injected air compressors.
Comparing oil-free screw offers at your duty
Two packages of the same nameplate size can perform very differently at your pressure and ambient conditions. Compare them on the same basis:
- Free air delivery (FAD) at your working pressure, measured to ISO 1217, in cfm or m³/min, not motor kW or “HP class”.
- Specific power (kW per m³/min or per 100 cfm) at that pressure, for the whole package including fans and dryer if integrated.
- Control type: fixed-speed load/unload or variable speed (VSD). VSD suits a varying demand; a fixed-speed machine suits a steady base load.
- Turndown range on VSD machines; dry screws have a minimum speed below which they unload.
- Cooling: air-cooled or water-cooled, and the design ambient. Ask how the package performs at 45 °C.
- Pressure: each extra bar costs about 7% more power, so set the pressure the process needs, plus realistic distribution losses, and no more.
- Integrated dryer and its PDP, or space and connections for a separate dryer.
- Heat recovery potential for process water or boiler feed.
- Service requirements: consumables, gearbox oil, and the manufacturer’s element overhaul interval.
Applications in Indian plants
| Industry | Typical use of oil-free screw air | Usual treatment |
|---|---|---|
| Pharma and API | Main plant air for coating, granulation, packing, instrument air | Desiccant or heat-of-compression dryer, particulate and sterile filters |
| Food, dairy, beverages | Conveying, packaging, product contact, CIP valves | Refrigerated or desiccant dryer, sterile filters at contact points |
| Textiles | Air-jet looms, spinning | Refrigerated dryer, particulate filters |
| Electronics | Assembly, cleanroom tools | Desiccant dryer, fine filtration |
| Power and process plants | Instrument and service air | Desiccant dryer for instrument air |
Our process-air page, specifying process air from oil-free screw systems, covers air-quality classes and point-of-use acceptance in more detail.
Installation: the details that decide reliability
- Ventilation: an air-cooled package rejects nearly all its input power as heat. Duct hot air out of the room and bring in cool air low down; size for the Indian summer, when ambient can exceed 45 °C.
- Intake: place intakes away from generator exhausts, boiler stacks and dusty roads. Intake vapour ends up in your air.
- Cooling water: on water-cooled machines, treat the water and monitor hardness; scale in intercoolers is a common cause of high stage temperature.
- Separate header: never connect an oil-injected standby or rental machine to the oil-free header.
- Power supply: check voltage, cable sizing and starter type, especially for VSD units.
Common faults and what they usually point to
| Symptom | Likely causes | First checks |
|---|---|---|
| High second-stage discharge temperature | Fouled intercooler, high ambient, low cooling-water flow, worn element | Intercooler approach temperature, room ventilation, water flow |
| Intermediate pressure drifting | Wear in one stage, intercooler leak, faulty sensor | Compare with commissioning values |
| Reduced FAD | Blocked intake filter, worn coatings, leaking check valve | Intake pressure drop, check valve, flow test |
| Gearbox oil loss or oil smell in air | Seal wear, blocked vent, overfilled gearbox | Vents, oil level, seal condition |
| Rising vibration | Bearing wear, gear wear, coupling issue | Vibration trend, oil analysis |
| Water carry-over downstream | Failed intercooler or aftercooler drain, dryer fault | Drains, dryer PDP |
For a structured fault approach see breakdown diagnostics and fault triage.
Maintenance and element overhaul
Routine service covers intake filters, gearbox oil and filter, cooler cleaning, drain checks, sensor and control checks, and trend logging of stage temperatures and pressures. The element itself is overhauled or exchanged at the interval the manufacturer sets for that model, based on running hours and condition. An overhaul typically includes bearings, seals, timing-gear inspection and assessment of rotor coatings. Running the element past its interval, or with a fouled intercooler, increases the risk of rotor contact, which is a much more expensive failure.
When the main machine is out for overhaul, plan oil-free standby cover in advance. See temporary oil-free air supply.
How Reddy Compressors helps
We sell new oil-free screw compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands, and checked used/refurbished units, as an independent multi-brand specialist and not an authorised dealer. We have tie-ups with all major and reputed brands. We rent oil-free units for overhaul periods and shutdowns, service them under AMC, and supply spare parts identified from the machine’s make, model, serial number and nameplate, with fit confirmed before we quote.
Our team of over 20 service engineers and technicians works from our office at Srinilayam, 12-13-200, First Floor, Street No. 1, Tarnaka, Secunderabad 500017, and covers all of Telangana and Andhra Pradesh, including pharma and food clusters around Hyderabad, Visakhapatnam and Sri City. We supply machines and spares across India. Call +91 99590 60718 or +91 92468 88812.
What to send us
- Required flow and pressure at the point of use, and how demand varies over the day.
- Industry and required ISO 8573-1 class.
- Make, model, serial number, running hours and nameplate photo of the existing machine.
- Recent stage temperatures and pressures, if logged.
- Cooling type and room layout photo.
- Whether you need purchase, used, rental or service.
Frequently asked questions
How does an oil-free screw compressor work without oil?
Its two rotors never touch. Timing gears drive both and keep a small clearance, the rotors are usually coated, and they run fast so internal leakage stays low. Heat is handled by compressing in two stages with an intercooler between them, or by injecting clean water into a single stage. Oil lubricates only the gearbox and bearings, sealed off from the air.
Why do oil-free screw compressors have two stages?
Without oil to absorb heat, one stage compressing to 7–8 bar would run far too hot for the rotors and coatings. Splitting compression into two stages, each with a pressure ratio of roughly three, and cooling the air between them, keeps temperatures within limits and reduces the power the second stage needs. Water-injected screws avoid this by cooling with water.
Is an oil-free screw compressor less efficient than an oil-injected one?
Not necessarily at system level. A dry screw may have a slightly higher package specific power than a comparable oil-injected machine, but an oil-injected system needs coalescing and carbon filters whose pressure drop costs energy, at roughly 7% more power per extra bar. Compare specific power at your actual pressure, including treatment losses, rather than relying on brochure figures.
How often does an oil-free screw element need overhaul?
The interval is set by the manufacturer for each model and depends on running hours, operating temperature, intake cleanliness and cooler condition. We do not quote a single figure because it varies widely. Keeping intercoolers clean, intake filters changed and stage temperatures within limits protects the element and helps it reach the manufacturer’s intended interval.
What causes high temperature in an oil-free screw compressor?
The commonest causes are a fouled intercooler or aftercooler, poor compressor-room ventilation in hot weather, low cooling-water flow or scaled coolers, and a blocked intake filter. Worn element coatings or bearings can also raise temperature. Compare stage temperatures and the intermediate pressure with commissioning values to locate the problem before it causes a trip or damage.
Can you supply a used oil-free screw compressor?
Yes, where a checked used or refurbished machine suits your budget and duty. We assess the element condition, running hours, gearbox, coolers and controls, and tell you plainly what has been done and what may be due. For critical pharma or food duty we will discuss whether a used machine is suitable for your validation requirements.
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.