22 May, 2022

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.

Buyers' Guide Search
Search for UK supplier by name
Browse by Product Group.
APRIL/MAY 2022 Issue To view a digital copy of the APRIL/MAY 2022 issue of Hydraulics & Pneumatics Magazine, click here.

MARCH 2022 IssueTo view a digital copy of the MARCH 2022 edition of Hydraulics & Pneumatics Magazine, click here.

For a FREE subscription please click here

July/August 2021 Annual Buyers' GuideTo view a digital copy of the JULY/AUGUST edition of Hydraulics & Pneumatics Magazine, click here.

For a FREE subscription please click here

BFPA YearbookTo read the latest BFPA Yearbook, click here
Talking Industry events in 2021Talking Industry Events in 2021
BFPA Training AcademyClick the image to go to the BFPA Training Academy website
Compressed Air & Vacuum Technology Guide 2018To read the official BCAS Compressed Air & Vacuum Technology Guide 2018 click here
Offshore Europe JournalOEE
Super Swivels
Impulse Automation Limited was established in 1960 and is based in the United Kingdom. We are an importer and distributor of mechatronic components used in a wide variety of industry sectors.Impulse Automation Limited
Hy-Pro FiltrationWEB LOCATOR