How an oil-injected screw compressor works
Two meshing helical rotors, a male and a female, trap air at the inlet end and carry it along the airend as the space between the lobes shrinks. Oil is injected into the compression chamber in large quantities. The air and oil leave the airend together and pass through the oil system, which separates the oil and returns it to the airend.
Representative images of machine types we sell, service and rent. Models and capacities are confirmed against your duty.
- Air enters through the intake filter and the inlet (suction) valve, which controls loading and unloading.
- The airend compresses the air while injected oil absorbs heat, seals clearances and lubricates bearings.
- The air-oil mixture enters the separator vessel, where most oil drops out by direction change and gravity.
- The separator element removes fine oil mist; a scavenge line returns collected oil to the airend.
- The minimum pressure valve holds internal pressure for oil circulation and stops backflow.
- The aftercooler lowers air temperature, condensing much of the water vapour.
- Oil passes through the oil cooler, thermostatic valve and oil filter before being injected again.
The package then delivers warm, saturated air with a small residual oil content, which downstream treatment must handle.
The three jobs of the oil
| Function | What it does | What goes wrong if it fails |
|---|---|---|
| Cooling | Absorbs compression heat so discharge stays at about 75–95 °C | High discharge temperature trips, oil breakdown, varnish |
| Sealing | Fills rotor and housing clearances, reducing internal leakage | Lower FAD and poorer specific power |
| Lubrication | Protects bearings and, in some designs, the rotor contact | Bearing wear, noise, vibration, airend failure |
Because the oil does so much, it is a component of the machine, not merely a consumable. Its condition decides airend life.
Lubricant: specification, compatibility and change intervals
Use the oil type the manufacturer specifies or a lubricant whose technical compatibility has been verified. Two oils with the same viscosity grade can have very different base stocks, additive packages and oxidation lives, and mixing incompatible oils can cause foaming, varnish or sludge.
- Follow the manufacturer’s change interval, shortened for high ambient, dust or heavy duty.
- On our rental fleet, hire jobs are serviced every 300 running hours with an oil change, because site duty is harsh.
- Oil analysis can show oxidation, water ingress, wear metals and contamination before damage occurs.
- Always change the oil filter with the oil, and flush the system when changing lubricant type.
- Dark oil, sludge, a burnt smell or rising discharge temperature are signs the oil has degraded.
Separation and oil carry-over
The separator affects both air quality and energy. A well-functioning separator element leaves a small residual oil content in the delivered air, typically a few mg/m³ in a healthy machine, depending on the design. That figure rises when something is wrong.
| Symptom | Possible cause | What we check |
|---|---|---|
| Oil in downstream air or filters | Separator element damaged or saturated, scavenge line blocked | Element condition, scavenge orifice and sight glass |
| High oil consumption | Overfilled sump, foaming oil, low minimum pressure | Oil level, oil type, minimum pressure valve |
| Rising power at same flow | High separator pressure drop | Differential across element versus limit |
| Oil at intake on stopping | Inlet valve not sealing | Inlet valve and blowdown sequence |
Separator pressure drop is an energy issue. As the element loads, the compressor must generate higher internal pressure to deliver the same outlet pressure, costing power on every running hour. Replace elements at the specified interval or differential limit.
Cooling and temperature control
Normal oil-injected screw discharge temperature is about 75–95 °C. A thermostatic valve bypasses the oil cooler when cold so the oil warms up quickly; this keeps the temperature above the point where water would condense in the oil. Too low a temperature leads to water in the oil and emulsification; too high shortens oil life and triggers trips.
- Clean the oil cooler and aftercooler regularly; dust and oil film are common in Indian plants.
- Duct hot cooler exhaust out of the room and provide cool fresh air.
- Check the thermostatic valve if the machine runs too cold or too hot.
- Check fan, motor and belt condition on air-cooled units.
A cooler that is partly blocked is a reliability problem, not a cosmetic one. In April to June, many high-temperature trips in Telangana and Andhra Pradesh trace back to blocked coolers and hot rooms.
Oil-injected does not mean poor air quality
An oil-injected compressor delivers some oil, water and particles. With the right treatment train it can meet the ISO 8573-1 class that most industrial users need.
- Refrigerated dryer, typically giving a pressure dew point of about +3 °C, for general plant air.
- Desiccant dryer, typically −20 °C or −40 °C PDP, for instrument air and outdoor lines.
- General-purpose and high-efficiency coalescing filters to remove oil aerosols.
- Activated-carbon filters where oil vapour and odour must be minimised.
- Oil-water separator for condensate, which from these machines contains oil and must not go straight to drain.
Where air contacts food, beverages or pharmaceutical products, or where any oil contamination would be unacceptable, oil-free compression is often preferred. The choice should be made from the required air-quality class and the consequence of a treatment failure, not from habit.
| Point | Oil-injected screw with treatment | Oil-free screw with treatment |
|---|---|---|
| Purchase cost | Lower | Higher |
| Typical uses | General plant air, tools, fabrication, many process duties | Food, pharma, electronics, sensitive processes |
| Oil risk | Depends on filters and maintenance | No oil introduced in compression |
| Condensate | Contains oil, needs separator | Oil-free but still needs proper handling |
Choosing an oil-injected screw compressor
Selection follows the same rules as any compressor, with a few points specific to oil-injected machines:
- Establish demand from logged data as FAD at your working pressure, with minimum, average and peak values.
- Set the required pressure from the point of use plus genuine treatment and pipe losses; each extra bar costs about 7% more power.
- Choose fixed speed for steady loads and variable speed for varying loads, or combine them.
- Specify the treatment train for the ISO 8573-1 class each user needs.
- Check the package for high-ambient capability, cooler access, and the availability of oil, filters and separators in India.
- Compare specific power in kW per m³/min at your pressure, measured to a recognised standard such as ISO 1217.
Drive arrangement also matters. Belt-drive packages allow speed and pressure variants from one airend but need belt tension checks; direct and gear-driven units remove belt losses and maintenance. Air-cooled machines suit most rooms; water-cooled units suit very hot or dusty rooms where cooling water is reliable. A well-maintained used or refurbished oil-injected machine can also be good value for standby duty, provided airend condition, bearing noise and running hours are checked.
Common faults and their usual causes
- High discharge temperature: blocked cooler, low oil, wrong oil, failed thermostatic valve, hot room.
- Oil carry-over: separator failure, overfilled sump, blocked scavenge line, foaming.
- Low FAD: worn inlet valve, blocked intake filter, belt slip, airend wear.
- Frequent load/unload: undersized receiver, narrow pressure band, leaks.
- Noise or vibration: bearing wear, coupling or belt problems, motor faults.
Detailed troubleshooting is covered on our breakdown diagnostics page.
How Reddy Compressors helps
Reddy Compressors has served industry from Tarnaka, Secunderabad since 2002. As an independent multi-brand specialist, not an authorised dealer, we sell new and used/refurbished oil-injected screw compressors and rent and service machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan and others. We have tie-ups with all major and reputed brands. More than 20 service engineers and technicians cover Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India.
- Service, AMC and overhaul, including oil, filter and separator changes and cooler cleaning.
- Diagnosis of overheating, oil carry-over and low output.
- Spare parts identified from make, model, serial number, nameplate photo or the old part, with fit confirmed before quoting.
- Selection of dryers and filters to reach the ISO 8573-1 class your users need.
- Rental compressors from our own fleet, 50–2,000 cfm and 2–30 bar, during repairs.
What to send us
- Compressor make, model, serial number and a nameplate photo.
- Running hours and date of last service.
- Oil type in use and when it was last changed.
- The fault or concern, with any alarm codes and discharge temperature readings.
- Air quality requirement for your users.
Frequently asked questions
What is an oil-injected rotary screw compressor?
It is a screw compressor that injects oil into the compression chamber to cool the air, seal the clearances between the rotors and lubricate the bearings. The oil is then separated from the air and recirculated. It is the most common type of industrial compressor for plant air at about 7–13 bar.
Why is there oil in my compressed air?
Oil in the air usually means the separator element is damaged or saturated, the scavenge line is blocked, the oil level is too high or the oil is foaming. Downstream coalescing filters may also be overdue. Check the separator differential, scavenge sight glass and oil level, then the filters, before assuming a major fault.
How often should I change the oil in a screw compressor?
Follow the manufacturer’s interval for the oil type in use, and shorten it in hot, dusty or heavy-duty conditions. On our rental fleet, hire jobs are serviced every 300 running hours with an oil change because site duty is severe. Oil analysis can help set a safe interval for your plant.
Can I mix different compressor oils?
It is best not to. Oils with the same viscosity grade can use different base stocks and additives, and mixing them can cause foaming, varnish or sludge. If you change oil type, drain the system fully, flush it as the oil supplier advises and replace the oil filter and, where appropriate, the separator element.
Is an oil-injected compressor suitable for food or pharma?
It can be in some applications with a suitable treatment train, but many food and pharmaceutical users specify oil-free compression to remove the risk of oil contamination if filtration fails. The decision should come from the ISO 8573-1 class required, the consequence of contamination and your quality system, not from purchase price alone.
What temperature should an oil-injected screw compressor run at?
Normal discharge temperature is about 75–95 °C. Much lower than this can cause water to condense in the oil; much higher shortens oil life and approaches the high-temperature shutdown. Persistent high readings usually point to blocked coolers, low oil, a faulty thermostatic valve or poor room ventilation.
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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.