Why the specification starts at the point of use, not at the compressor
Many enquiries for oil-free screw compressors begin with a request for a kW rating or a cfm figure. That is the last number to fix, not the first. The air that reaches a tablet coater, a filling nozzle or a fermenter has passed through the compressor, aftercooler, receiver, dryer, filters and several hundred metres of pipe. Each of those can add or remove contamination.
A two-stage dry screw element keeps lubricating oil out of the compression chamber, so the compressor does not add oil to the air. It cannot remove what is already in the intake. In an industrial area where the ambient air carries hydrocarbon vapour from traffic, boilers or nearby process vents, that vapour passes through the compressor and appears downstream. Water vapour, dust and microbes behave the same way. So the honest statement is that an oil-free compressor removes the largest single source of oil risk, and the rest of the system still has to be engineered.
For the machine itself, its stages, coatings, drives and package options, see our page on oil-free rotary screw compressor technology. This page deals with the other half of the job: turning a process requirement into a written air specification that a supplier can quote against and a quality team can accept.
What does ISO 8573-1 actually ask you to specify?
ISO 8573-1 describes compressed-air purity as three separate numbers: solid particles, water and total oil (aerosol, liquid and vapour). A specification is written as, for example, Class 1.2.1 or Class 2.4.1. Each figure is chosen independently, and each one needs its own treatment step.
| Contaminant | What it means in practice | Typical treatment that controls it |
|---|---|---|
| Solid particles | Dust drawn in at the intake, pipe scale, dryer desiccant fines, wear debris | Intake filtration, general and fine coalescing filters, dust filter after a desiccant dryer, sterile filter at point of use |
| Water | Liquid water and vapour, expressed as pressure dew point (PDP) | Aftercooler and separator, refrigerated dryer (typically about +3 °C PDP), desiccant dryer (typically −20 °C or −40 °C PDP) |
| Total oil | Aerosol, liquid and vapour, from the compressor and from the intake air | Oil-free compression, then coalescing filtration and, where specified, activated-carbon adsorption |
| Micro-organisms | Not graded by a class number in Part 1; handled by the user’s own process rules | Low dew point to stop growth, sterile filtration at point of use, validated filter housings |
Class 0 is a special case. It is not “zero oil”. It means a limit tighter than Class 1 that the user or equipment supplier defines and states. Manufacturers who describe a compressor as Class 0 do so on the basis of third-party type testing of that model under defined conditions. That is a statement about the machine on a test bed. Your system still needs measuring at the point of use if your quality system requires evidence.
How to write the specification, step by step
- List every point of use. Separate air that touches product, packaging or patients from utility air for cylinders, valves and blow-off. They rarely need the same class.
- Record the required class for each. Take it from your process specification, equipment maker’s manual, regulatory guidance or customer requirement, and note who owns that requirement. Mark any unknown value as unresolved rather than guessing a generic industry class.
- State the pressure at the point of use. Add pressure drop through dryer, filters and pipe to find the discharge pressure. Most oil-free screw duties are 7–10 bar (101.5–145 psig); every extra bar costs roughly 7% more power, so avoid setting a high pressure just because one user needs it.
- Describe the demand pattern. Normal flow, peak flow, night and weekend load, and batch events such as CIP or tablet press start-up. Logged data beats estimates.
- Define criticality. What happens if air stops for an hour? This decides N+1 standby, dryer redundancy and whether rental cover must be pre-arranged.
- Fix the system boundary. Which equipment is existing, which is new, and who is responsible for each interface: compressor room, treatment, distribution, point-of-use filters.
- Agree verification. Where samples are taken, what instruments are used (dew-point sensor, particle counter, oil vapour test), how often, and what records are kept.
Matching the treatment train to the class
Once the class is known, the treatment train follows logically. For an oil-free screw installation the usual building blocks are:
- Aftercooler and moisture separator. Most packages have them built in. In a 45 °C Telangana or Andhra Pradesh summer the aftercooler outlet will be hotter than the brochure figure, so dryer sizing must use real inlet temperature.
- Wet receiver. Buffers demand, lets condensate settle and smooths load on the dryer.
- Dryer. A refrigerated dryer gives a typical PDP of about +3 °C, suitable for many utility and packaging duties. A heatless or heat-of-compression desiccant dryer gives −20 °C or −40 °C, used where pipes run through cold rooms, where moisture would affect powders, or where microbial growth must be suppressed.
- Filtration. General and high-efficiency coalescing filters still matter, because pipe scale, desiccant dust and intake hydrocarbons are not stopped by the compressor. A dust filter after a desiccant dryer is essential.
- Activated carbon. Added only where the oil-vapour class or an odour/taste requirement calls for it, usually in food and beverage.
- Point-of-use sterile filters. For product-contact air in pharma, dairy and breweries, with housings and element integrity testing that match your validation approach.
A heat-of-compression dryer pairs naturally with an oil-free screw, because the hot discharge air (often well above 150 °C before the aftercooler on a dry screw) can regenerate the desiccant. It saves purge air but needs a stable load profile; discuss it at the design stage rather than as a retrofit.
Utility air or process air: one network or two?
Large plants often run one oil-free compressor house and treat all the air to the strictest class. That is simple to manage but expensive, because every litre of tool air is dried and filtered to product-contact standard. The alternative is a split system: a common oil-free supply dried to the utility requirement, with extra drying and sterile filtration only on the product-contact branches.
| Approach | Advantages | Watch-outs |
|---|---|---|
| Single network, highest class everywhere | One standard, simple validation, no risk of cross-connection | Higher dryer and filter running cost; full-flow desiccant purge losses |
| Common supply, point-of-use polishing | Treatment cost concentrated where it is needed | More filter housings to maintain and record; clear labelling needed |
| Separate oil-free and oil-injected systems | Lowest capital cost for large utility loads | Strict segregation and no interconnection; oil-free air must never be backed up from the oil-injected side without a documented risk review |
Sizing the oil-free screw package
With the specification written, the compressor selection becomes a sizing exercise rather than a guess:
- Free air delivery (FAD). Base it on measured demand plus leakage allowance, not on the sum of nameplate consumptions.
- Control. A variable-speed dry screw suits a fluctuating load; a fixed-speed load/unload unit suits a steady base load. Many plants combine one of each.
- Cooling. Air-cooled packages need clean, cool room ventilation; water-cooled packages need cooling-water quality control, because intercooler and aftercooler fouling raises stage temperatures.
- Purge and losses. Add desiccant dryer purge and drain losses to the compressor FAD; otherwise the plant runs short at peak.
- Standby. For critical duties, N+1 compressors and dryers, with interconnecting valves that allow one train to be serviced while the other runs.
Oil-free screw packages cost more to buy than oil-injected units of the same flow. The justification is lower risk and lower downstream treatment: no oil-removal filters loaded with oil, no oily condensate to treat, and no product recall caused by an oil event. That comparison is site-specific, so we work through it with your actual data rather than a standard percentage.
Acceptance and ongoing verification
A specification is only useful if it is checked. At commissioning, record discharge pressure, PDP at the dryer outlet and at the furthest point of use, differential pressure across each filter, and any particle or oil test your quality system needs. Keep the instrument calibration records with the results.
After handover, trend the same values. Rising dryer outlet dew point, rising filter differential pressure and rising stage discharge temperatures are early signs of trouble. On a dry screw, rising second-stage temperature can point to intercooler fouling or element wear; on the treatment side, rising PDP often means a failed drain, overloaded dryer or saturated desiccant.
Condensate from an oil-free compressor contains no lubricating oil from the machine, but it can still carry hydrocarbons drawn in from the atmosphere and is often acidic. Handle it according to your site’s effluent rules rather than draining it straight to ground.
Indian site conditions that change the specification
- High ambient temperature. Summer intake air at 40–45 °C, and above 45 °C on open sites, reduces mass flow and loads aftercoolers and dryers. Size dryers on correction factors for real inlet and ambient temperature.
- Dust. Industrial estates around Hyderabad, Vizag and Sri City can be dusty, especially during construction. Intake filters need a sensible location and change interval.
- Humidity. Coastal plants in Vizag, Kakinada and Nellore see high monsoon humidity, which raises the water load on dryers.
- Power quality. Voltage dips and frequent trips are hard on VSD drives and controllers; specify suitable protection.
- Pharma and food clusters. Genome Valley, the Hyderabad pharma belt and the food parks in Andhra Pradesh are audit-driven environments. The specification and records need to survive an inspection, not just a commissioning day.
How Reddy Compressors helps
We help you write the air specification before anyone quotes a machine. Our engineers visit the plant, list the points of use, log pressure and demand where needed, and check the existing compressor house, dryers, filters and drains. We then propose an oil-free screw package from the brand that best suits the duty, new or used/refurbished, with the dryer and filtration that the specification actually needs.
For shutdowns, validation runs or compressor breakdowns we can arrange temporary oil-free air through our rental engineering service, with treatment matched to the class. Once the system is running we offer service and AMC on oil-free screws, dryers and filters, and supply spares identified from the machine’s make, model, serial number and nameplate, confirming fit before we quote.
Reddy Compressors is an independent multi-brand specialist based at Srinilayam, 12-13-200, First Floor, Street No. 1, Tarnaka, Secunderabad 500017, serving industry since 2002. We sell new and used/refurbished oil-free compressors, rent them, service them under AMC or on call, and supply spares for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes. We are not an authorised dealer of any brand; brand names and trademarks belong to their owners. We have tie-ups with all major and reputed brands. Our team of more than 20 service engineers and technicians covers the whole of Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India. Call +91 99590 60718 or +91 92468 88812, WhatsApp +91 99590 60718, or email reddycompressors@gmail.com.
What to send us
- List of points of use with the required air class and pressure, or the documents that define them
- Existing compressor, dryer and filter make, model and nameplate photos
- Demand data or a description of shifts, peaks and batch events
- Compressor room layout, ambient conditions and cooling method
- Criticality: what stops if air stops, and any standby requirement
- Location of the plant and whether you need purchase, rental or both
Frequently asked questions
Does an oil-free screw compressor give me Class 0 air at the point of use?
Not automatically. A Class 0 rating describes how a compressor model performed in third-party type testing. At your point of use, the result also depends on intake air quality, the dryer, filters, pipework and maintenance. If your process needs Class 0 or Class 1 oil, specify it at the point of use and agree how it will be measured during commissioning and afterwards.
Do I still need a dryer with an oil-free compressor?
Yes. Oil-free compression has nothing to do with water. Air leaving any compressor is saturated with moisture after the aftercooler. A refrigerated dryer typically gives about +3 °C pressure dew point, and a desiccant dryer typically gives −20 °C or −40 °C. The water class you specify under ISO 8573-1 decides which type you need and where it sits in the system.
Why do I need filters if the compressor is oil-free?
Because the compressor only stops oil being added inside the machine. Dust, pipe scale, desiccant fines and hydrocarbon vapour drawn in from the surrounding air still reach the network. Coalescing and particle filters remove them, a dust filter is needed after a desiccant dryer, and sterile filters at the point of use protect product-contact air in pharma, dairy and beverage plants.
What pressure should I specify for an oil-free screw compressor?
Start with the highest pressure any critical point of use genuinely needs, then add pressure drop through dryers, filters and pipes. Most plants land between 7 and 10 bar (about 100–145 psig). Each extra bar costs roughly 7% more power, so if only one machine needs high pressure, a local booster may be cheaper than running the whole plant higher.
Is oil-free screw worth the extra cost over oil-injected with filters?
It depends on your risk and your running costs. Oil-injected compressors with good filtration can achieve low oil classes, but they rely on filters being changed on time and on separators never failing. Oil-free removes that dependency, reduces oily condensate and filter loading, and lowers contamination risk. For product-contact air the risk usually decides; for utility air the running cost comparison does.
Can I rent an oil-free compressor while my plant compressor is overhauled?
Yes. Temporary oil-free air is common during element overhauls, shutdowns and validation work. The rental package should match your pressure, flow and air class, and include the dryer and filtration your specification requires. Tell us the dates, the duty and the connection point, and we will propose a suitable electric or diesel oil-free set with treatment.
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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.