Oil Free Air Compressors What Are They And What Are They For?

At GenAir as part of our hire fleet we have an oil free hybrid drive compressor but what is an oil free air compressor and why might you use one?

Oil free and oil injected air compressors both work in the same way, using rotary screws to compress the air. This video from MAN explains the process with an animated cut away of a screw compressor.

In an oil injected air compressor the compression chamber is injected with oil to lubricate the screws, provide a seal and to help with cooling. The oil and compressed air mixture outputted from the chamber is passed through an oil separator with the oil being recycled through a reservoir and back into the chamber. This process of separating the oil from the air is not 100% efficient and, assuming entirely clean ambient air into the compressor, the air coming out will contain oil in the region of 3 parts per million.

An oil free air compressor does not inject oil into the chamber and relies on much tighter manufacturing tolerances for sealing and to prevent heat build up. As a result, assuming the ambient air going into the compressor contains no oil then the compressed air coming out won’t contain any oil either. Sometimes referred to as class zero air.

The disadvantage of an oil free air compressor is, because of the much tighter tolerances in manufacturing, they cost more than an oil injected machine.

Why use an oil free compressor?

When we talk about oil free air it’s really short hand for contaminant free air and there’s a set of standards for it. ISO 8573-1

There are a variety of applications that require contaminant free air, normally where the air itself is being used in the process rather than the air running equipment, such as food and beverage or pharmaceutical production. These applications will require air that meets standards laid out in ISO 8573-1 such as Class 1 or Class 2 with specified maximum levels of contamination permitted for each class of air. This includes levels of oil, particles and water.

It is possible to meet these ISO standards, without using an oil free compressor, by filtering the output of an oil injected compressor. This is often referred to as “technical oil free” and is maligned by some suppliers as an alternative to using an oil free compressor -sometimes when the only tool someone has is a hammer, everything looks like a nail. Technical oil free most certainly has its place and in certain circumstances using an oil injected machine with filtering can be the better option.

When deciding on the best way to deliver the required quality of air to a process the system needs to be considered as a whole.

At a pharmaceutical production facility, for a example, the starting point in the design of the system is the purity of the air required. Working back from that is the quality of the ambient air that the system will be fed with. In a typical British city the air will contain about 0.5ppm oil plus other contaminants – dust, pollen etc – that will need to be removed and a appropriate filtering and monitoring added to the system. In this example an oil free air compressor will almost certainly be specified. In the event that the production compressor is taken off line for maintenance and a rental compressor is supplied to maintain site production an oil free compressor will be the best, indeed only, fit for this particular system.

An application where an oil free machine might not be the best fit is at a petrochemical site such as an oil refinery where a periodic maintenance procedure is being carried out that requires air of a specified purity. Depending on site conditions the ambient air may already contain more than the 3ppm of oil that an oil injected compressors will output. In this example much more filtering of the air is required and the use of an oil free compressor achieves nothing but increased costs.

Between these two extremes lie applications along a spectrum and whether the best fit is an oil injected or oil free machine will depend on the amount of air needed, the duration of the project and a variety of other factors. In every case it will be possible to produce the quality of air required using either an oil injected or oil free compressor. Choosing the most cost effective option is far from a one size fits all exercise.

A GenAir we have both oil injected and oil free air compressors in a fleet that we have built around the needs of our clients. We have the expertise to advise you on the compressors you need to meet your requirements and we can rent you the air compressor you need, not the one we happen to have.