Oil-Free vs Oil-Injected Screw Compressors: How to Choose

"Do we need an oil-free compressor?" is one of the first questions in any compressed air project, and it is often answered by habit instead of by requirement. Oil-free and oil-injected screw compressors are both established technologies. They differ in how the air is compressed, in what they add to the air stream, and in the equipment needed around them. This guide explains the differences and ends with a short checklist for deciding.

What the terms mean

Oil-injected compressors, also called lubricant-injected or oil-flooded, inject lubricant into the compression chamber. The U.S. Department of Energy (DOE) sourcebook lists three functions: the lubricant lubricates the intermeshing rotors and bearings, takes away most of the heat of compression, and seals the clearances between the rotors and the housing.

Oil-free compressors, also called lubricant-free, operate without oil in the compression chamber. The Compressed Air & Gas Institute (CAGI) describes them as machines in which lubricant is needed only for the bearings and timing gears, which are separated from the compression chamber.

How each type works

Oil-injected screw compressors

The mixture of air and lubricant leaves the air end and passes to a sump and separator. Changes in direction and velocity remove most of the liquid, and a coalescing filter removes the remaining aerosols.

The DOE sourcebook says this leaves only a few parts per million of lubricant carry-over, usually in the range of 2 to 5 ppm. CAGI's purity guide gives a wider range of 2.0 to 10.0 ppm by weight for lubricated rotary screw compressors, depending on the age of the machine and the quality of maintenance.

Oil-free screw compressors

The DOE sourcebook describes two kinds:

• Dry-type. The rotors do not touch, and timing gears outside the compression chamber hold their relative positions. Because no fluid removes the heat of compression, most designs use two stages with an intercooler between them, and they run at higher rotor speeds than the lubricant-injected type.
• Water-injected. Water is injected into the compression chamber to seal clearances and remove heat. This allows pressures in the 100 to 150 psig range with a single stage.

Air quality: what "oil-free" does and does not guarantee

An oil-free compressor does not add oil to the air during compression. That removes one source of contamination, but it is not the same as guaranteeing a given air quality.

CAGI states that an oil-free compressor delivers air with an oil content no greater than what it takes in at its intake. Oil-free air also still carries oil from the ambient air and particles from the compressor, so it needs filtration.

The reverse is also true: an oil-injected compressor does not rule out clean air. The DOE sourcebook notes that lubricant-free air can be produced either with lubricant-free compressors or with lubricant-injected compressors that have additional separation and filtration equipment. The trade-off is that the extra equipment has to be maintained. CAGI points out that filters plug up over time, which raises pressure drop and energy use, so they need regular service.

To compare the two on equal terms, state the air quality you need in the language of ISO 8573-1, which defines purity classes for particles, water and oil. Its oil classes are expressed as total oil (liquid, aerosol and vapour) in mg/m³: Class 1 is ≤ 0.01, Class 2 is ≤ 0.1, Class 3 is ≤ 1 and Class 4 is ≤ 5, at reference conditions. Class 0 is a special case. It is set by the user or supplier and must be more stringent than Class 1, so the limit has to be stated; the label alone does not tell you the oil content. Our companion article, ISO 8573-1 Explained: Compressed Air Purity Classes, covers the details.

Cost and energy

The DOE sourcebook makes a general comparison. Lubricant-free rotary screw and reciprocating compressors usually have higher first costs, lower efficiency and higher maintenance costs than lubricant-injected compressors. It adds that the separation and filtration needed with a lubricant-injected machine reduces efficiency somewhat, especially if the system is poorly maintained.

Individual products vary. Compare complete systems on total cost of ownership: purchase price, energy, filter elements, service intervals and downtime risk, not the compressor alone.

Which application needs which type

The DOE sourcebook identifies whether lubricant-free air is required as one of the main factors in determining air quality. Some practical reference points:

A short selection checklist

1. Write down the required purity class for particles, water and oil at each critical point of use.
2. Check the intake environment. Oil vapour or dust in the ambient air will reach any compressor.
3. Estimate running hours and load variation, since energy cost accumulates over time.
4. Compare total cost, including filters, dryers, maintenance and monitoring.
5. Plan verification. Ask which test method and standard the supplier's air-quality statement is based on. For oil, ISO 8573-2 covers liquid and aerosol measurement and ISO 8573-5 covers vapour.

Common misunderstandings

• "Oil-free means no oil in the air." Ambient oil and system contamination can still be present, and a purity class has to be measured, not assumed.
• "Oil-injected cannot supply clean air." It can, with suitable treatment, provided the treatment is maintained.
• "Class 0 is the same as zero oil." It is a limit agreed between user and supplier that must be stricter than Class 1; always ask for the stated figure.

Talk to HUADA

HUADA is the flagship industrial air power brand of Fujian Huade Group, founded in Quanzhou, Fujian, China in 1985 and exporting to 20+ countries. If you are unsure which technology fits your process, send us your required air quality, running hours and site conditions. To start, request a quote through the inquiry form on our Contact page.