Cool, clean and dry

Published:  06 November, 2015

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. Peter Fearon of IMI Precision Engineering explains how to plan an efficient compressed air filtering system.

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.

Solid contaminants

Solid particles range from heavy dirt particles of 1000 (1mm) down to dust particles imperceptible to the human eye by way of comparison, a human hair is 75 across and some systems can be clogged by silt particles as small as 1-10. Atmospheric air in industrial environments typically contains some 140-150 million dirt particles per cubic metre, with 85% of these measuring less than 2 in size. Intake filters on a compressor are designed to stop solids entering and potentially damaging the apparatus but do not remove the smallest particles those too small to be captured enter the air stream and travel unrestricted into the distribution system.

Contamination can also arise within the distribution and pipework system itself. The products of corrosion rust and metal particles can be created by the presence of water and gases such as carbon dioxide inhaled by the compressor. These interact to form weak acids which can corrode metal pipework and pneumatic components. Carbon contamination can also occur through the action of the heat of compression on the lubricating oil, or by the normal wear patterns of carbon piston rings used in some types of oil-free compressors. Particles of solid debris can also originate from the mechanical fixing of the metal pipework and components within the system itself.

Water and vapour contamination

Heat is generated as a result of compressing atmospheric air, which means the air can hold water vapour as it passes through the compression stage. The ability of air to hold vapour is also dependent upon its pressure the higher the pressure, the greater the amount of water vapour which will condense into liquid water. Due to the effects of heat and pressure, compressed air contains more water vapour per volume than ambient air, as it cools water condensate is produced but is fully saturated with water vapour at 100% relative humidity.

Atmospheric air at a temperature of 30C and 90% relative humidity contains 27.3g of water vapour per m. When compressed to 7 barg (8 bara), the air will only be able to hold 3.8g of water vapour at 30C, meaning that the remaining water vapour condenses, producing 23.5g/m of condensate. An 18kW compressor operating 4,000 hours a year at will therefore produce up to 16,000 litres of condensate which is 43 litres or the equivalent of about a keg of beer per day.

As the compressed air travels into the air storage receiver and the distribution pipework, it passes through a heat exchanger to cool this allows water vapour to condense into liquid water which can be removed by an effective condensate management drainage system. However, once all liquid water has been removed, the remaining air is still saturated with water vapour. This is then carried downstream into the distribution system where water vapour can condense at potentially any point and cause problems including corrosion of metals in the system or lubricant wash-out from pneumatic components. If the temperature of the system drops below 0C, condensate can also freeze which can damage pipes and components through the expansion of water solidifying and even cut off the air supply.

Oil contamination

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 Nm/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.

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.

One size doesnt fit all

Due the varied properties of contaminants solid particles, liquid water, water vapour and oil vapour 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.

These filters can however be adapted to remove solid particle contamination, utilising a replacement filter element with a sufficient micron rating for the type and size of particles most likely to be present. This method removes both particles from the original atmospheric air as well as those which arise as a result of compressor or pipework wear. As an additional benefit, general purpose filters fitted with replacement filter elements also act as a pre-filter to the high efficiency coalescing filters downstream.

Coalescing filters themselves are suited to remove small and submicron particles, water mists and oil aerosols. Utilising the three main mechanisms of filtration diffusion, inertial impaction and direct interception they are typically installed as a combination of pre-filter and after-filter units, each with its own automatic drain to expel liquid emulsions into a condensate management system. For systems with oil-lubricated compressors or oil in the inlet air, pre and coalescing filters are needed.

Of all the contaminants to be dealt with, water condensate is the most critical due to the sheer volumes that must be removed, and the fact that as the compressed air travels through the distribution system, further condensate will occur. For high purity compressed air the pressure dew point (PDP) of the air must be reduced by drying the compressed air to levels suitable for its intended use. The PDP is the temperature at which further condensate will occur so for critical applications, the lower the PDP, the better for example, at a PDP of below -30C, corrosion and bacterial growth are eliminated.

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