22 July, 2024

Cool, clean and dry – a guide to compressed air purification

01 December, 2015

Peter Fearon of IMI Precision Engineering explains how to plan an efficient compressed air filtering system, with a focus on oil contamination.

Compressed air produced by a compressor is hot, wet and dirty; however compressed air applications require cool, clean and dry air for efficient and reliable performance. Contaminants can be present in compressed air from the outset due to the nature of atmospheric air, but can also enter the system in the form of compressor wear and from the system itself. Effective treatment of the compressed air is therefore vital and only by understanding the various types of contamination which must be reduced or eliminated, can the correct treatment be specified.

As compressed air begins life as atmospheric air – which contains a mixture of water vapour, hydrocarbon vapour and sub-micron solid particulates – part of the post-compression process must include the removal of these substances.

As most compressors are oil lubricated, the possibility of oil contamination exists. The majority of lubricant is removed by an oil separator before being recirculated within the compressor system. However, oil aerosols will enter the compressed air system as a fine mist or vapour. A lubricated compressor of 160 Nm³/h (100scfm) capacity, while a relatively small machine, may introduce as much as 8 litres of oil per year into the system. Having been subjected to high temperatures during the compression process, it becomes oxidised and acidic and therefore very aggressive as a contaminant.

Risk of illness

Even within oil-free compressors, contamination is a possibility if atmospheric hydrocarbon oil vapours enter the system. If this occurs the vapour will cool and condense into liquid oil, which can cause blockages and taint manufactured products. Oil vapours, if released back into atmospheric air, can also cause illness among operators.

Due the varied properties of contaminants, one single solution will not meet all needs. Effective compressed air treatment can only be achieved through using the correct grades and sizes of particulate and coalescing filters, dryers and condensate management, each installed at the correct point in the distribution system. The volume of air flow at each stage much be factored in to compressed air treatment specification, as undersized or inappropriate compressed air treatment equipment that cause high pressure drops are a prime cause of high energy and maintenance costs as well as unnecessary downtime.


General purpose filters (also known as water separators) are able to remove bulk water or liquid oil condensate from compressed air systems. However, as effective as they are, they are not able to remove water or oil aerosols and vapours because they are of simple mechanical design. While this is a highly efficient and cost effective method of water separation, protecting the downstream particulate and coalescing filters, it is not a suitable method of oil aerosol and vapour removal.

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