35th Anniversary
COMPRESSORS & COMPRESSED AIR FEATURE PREPARE YOUR AIR FOR PURITY
Impurities in the atmosphere mean potential problems for compressed air systems. Ray Lawton, project sales manager at BOGE, explains how common threats can be avoided
A
lthough they may be too small to see, many unwanted substances and particles in our atmosphere can easily be drawn into compressed air systems. These contaminants include dust, pollen, micro- organisms, moisture and compressor oil, which can all have an adverse affect on air quality and the operation of valves, cylinders and ancillary devices. The biggest contamination threat to a
compressed air system is from airborne particles of dust, grit and pollen. In a factory, it is estimated that each cubic metre of untreated ambient air can contain up to 180 million particles of dirt as well as between 50 and 80% suspension of water vapour and oil. During the compression process the concentration of these particles increases; at a pressure of 10 bar, for example, the effective density of contaminants will increase to around two billion particles per cubic metre. Although many of these may be less than 2mm in size, and dry particulate matter at low concentrations may not cause an issue, problems can occur if particles combine with moisture, compressor oil or other lubricants to form an abrasive or corrosive solution. Over time, this can accumulate and adhere to surfaces inside pneumatic devices, causing valves to stick and seals and moving parts to wear, leading to machine failure. Water vapour is also an issue. Naturally occurring in air, it can condense when the warm air emitted from a compressor comes into contact with the relatively cooler surfaces of downstream equipment. Drains fitted to a compressor or elsewhere within a compressed air system can remove condensate, but will not
effectively eliminate vaporised droplets of moisture suspended in the air flow. Moisture will eventually break down lubricating oils, leading to corrosion on exposed metal surfaces, with increased friction and wear between moving parts. Compressor oil, emitted as an oil vapour, can also create a problem as it can condense as a film within valves and cylinders. This film then acts as a trap for particles of dust and other contaminants, causing surface damage to moving parts. Furthermore, compressor oil that has been heated during the compression process will oxidise and can become acidic. If this is transmitted downstream then it can adversely affect the properties of other lubricants and the integrity of rubber seals and gaskets. There are various solutions to the
problems, one of which is a filter attached to the compressor intake, which will normally be adequate to remove larger airborne particles. However, there are limitations on the size of filter, as it is important to prevent restrictions in air flow to the compressor. As a result, downstream air preparation devices will also be required to remove smaller particles, together with oil and moisture. Many filtration devices use a mechanical coalescing system that removes aerosols of water and oil, plus solid particulates at levels that can be as low as 0.01 micron in size. These devices should be installed in pairs and in series, so that the first unit acts as a general-purpose filter to protect the second high-efficiency filter from bulk contamination. This ensures a continuous supply of quality compressed air with low operational costs and minimal maintenance.
Refrigerant dryers cool air to just above freezing point, so that water and oil aerosols condense out
Although the majority of air
compressors are supplied with an after- cooler, which removes most of the condensation caused by compression, an additional dryer is often required to eliminate downstream water vapour. Dryers are classified as refrigeration,
Ray Lawton offers suitable solutions to contamination threats in compressed air systems
Adsorption dryers use a desiccant material to remove water vapour
membrane or adsorption, each of which should be protected by upstream coalescing filters. In a refrigeration system, air is cooled to just above freezing point so that water and oil aerosols condense out, while a membrane dryer separates water vapour from the air stream. By comparison, an adsorption dryer uses a desiccant material such as activated alumina to remove water vapour, with dryers generally having a twin tower arrangement, with one tower in operation while the second is regenerating. Finally, it should be recognised that
different applications require different levels of compressed air treatment. It is therefore important to match the air treatment system to the specific needs of the application, including factors such as volume air flow and air quality.
BOGE Compressors
www.boge.co.uk
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