The quest for oil-free air

The term 'oil-free compressor' refers only to the compression chamber, not the compressor system as a whole or the resulting compressed air quality. Also bear in mind that 'oil-free' is a vague statement used within the compressed air industry and is dependent upon what is drawn into the compressor air intake. Purification equipment will be required in both the compressor room and at point of use to ensure 'oil free air'.

To achieve oil-free compressed air we need comply with the quality classes specified in standard ISO 8571-1. These provide the precise definitions of oil free air. Compressor manufacturers have different ranges of 'oil-free’ or oil-cooled systems but these invariably need primary or secondary downstream purification systems.

Air compressors draw in huge amounts of atmospheric air, which fills a compressed air system with contaminants such as water vapour, micro-organisms, atmospheric dirt and oil vapour.

When the air has passed through the compression stage, the after-cooler will condense water vapour into the compressed air in both a liquid and aerosol form. This will then pass into the system piping that distributes the compressed air and into an air receiver or some form of storage device; pipework and receiver also contribute to large amounts of contamination. They also act as a means of cooling the warm, saturated compressed air which in turn causes more condensation, adding liquid water into the system. 

This saturated air and liquid water can lead to corrosion, pipescale and, in some cases, microbiological growth.

These are: 

•  Liquid water  

•  Water vapour 

•  Water aerosols

•  Oil

•  Oil vapour 

•  Oil aerosols

•  Atmospheric dirt 

•  Microorganisms

•  Rust 

•  Pipescale

Liquid oil and oil aerosols are introduced by an oil- lubricated compressor. The filtration equipment needed to remove the remaining contaminants also removes the liquid oil and oil aerosols.

Filtration

When installing an ‘oil free’ compressor, it is quite common to omit one level of downstream filtration.

Filters in a compressed air system will typically be a coarse and a fine filter set-up. It is often believed that one of these filters is a particulate filter and that the other is an oil removal filter. In reality, the two filters used are coalescing filters; they combine to remove varying levels of contaminants (water aerosols, oil aerosols, atmospheric dirt, microorganisms, rust & pipescale). The first filter is a ‘general purpose filter’ which protects the ‘high efficiency filter’ from bulk contamination. 

All compressed air systems need filtration and to achieve this we have a number of filters which we can install to remove most of the contaminants. However to remove oil from a system we need to install some form of catalytic convertor, for example the Bekokat.

This works by converting gases/ hydrocarbons by the process of oxidation, producing less harmful gases typically carbon dioxide and water vapour. This operation is not dependent on the temperature and humidity of the compressed air to be treated, but will continuously supply air with a residual oil content of less than 0.003mg/m3

The basic formula for this conversion is   CnH2n+2 +O2  = H2O + CO2.

The Bekokat is also suitable for the retrofitting of compressor stations. A compressor station with existing oil-lubricated compressors can easily be transformed into an oil-free compressed-air provision system without the need for investment in new oil-free compressors. Bekokat systems are both effective and environmentally friendly.