Ensuring a clean and pure compressed air supply
Published: 06 May, 2015
With so many potential contaminants in the atmosphere that can play havoc with air purity, Mark Whitmore, general manager at BOGE Compressors, explains how a compressed air supply can remain fit for purpose at all times.
There are many tiny substances and particles in our atmosphere that can penetrate into pneumatic systems powered by compressed air. These include dust, pollen, moisture and compressor oil, and can impact on the integrity and functionality of these devices, as well as affecting the compressed air quality.
The biggest threat to any compressed air system is an airborne one. Our atmosphere is rich with suspended particles of dust, grit and pollen, which in itself is an accepted environmental factor. In a factory however, where product and manufacturing by-products and dust are denser and can be in more plentiful supply, there is an average of 140 million particles per cubic metre of air¹.
Although up to half of these may be less than 2mm in size, and dry particulate matter at low concentrations may not cause an impurity issue, this can become a problem when particles merge with moisture compressor oil or other lubricants and congeals. The result is that this coagulated blend can accumulate and adhere to stationary and moving surfaces inside a pneumatic device, causing valves to stick and seals and parts to wear, leading to costly machine failure and downtime.
It’s not just dry or airborne substances that can cause problems. Water vapour, which naturally occurs in air, will condense when the warm air emitted from a compressor comes into contact with the comparatively cold surfaces of downstream equipment. Drains fitted to a compressor can remove condensate, but this will not effectively break down moisture suspended in the air flow. Consequences can be far-reaching. A moisture build up over time breaks down lubricating oil, creating corrosion on exposed metal surfaces – which will break off, increasing friction between moving parts.
The third common contaminant in the air supply is compressor oil. Not naturally associated with airborne contamination, compressor oil becomes a threat to purity when emitted as an oil vapour. This can condense as a film within valves and cylinders, collecting dirt and grit on its journey, damaging the surface of moving parts. Furthermore, oil that travels from an air compressor into a system becomes oxidised and degraded once it has been subjected to compression heat. The substance produced is usually acidic and appears as a varnish-like substance that possesses properties completely opposed to lubrication.
One part that often experiences the greatest impact by compressor oil are valve seals, as it causes the Acrylonitrile butadiene rubber used within the O-ring and bonded spool devices to expand, leading to the valve eventually becoming stuck in one position.
Further difficulties can be caused by chemical substances, which may be sucked in from the air supply into the compressor. Without treatment or sufficient filtration, these will attack rubber, seals and gaskets, leading to the steady decline in operating efficiency.
The fallout from contamination in food processing
Compressed air is used across many processes and stages of food processing. If contaminated air comes into contact with a food product, taste, appearance, colour and shelf life can be affected, as well as hygiene standards. The combination of dirt, particles and micro-organisms being pulled in, alongside the oils and liquids that can seep through worn seals and O-rings, make food processing susceptible to impurities.
As well as the threat of rust breaking off and getting into the food processing cycle, water condensate must be considered as it can create the right conditions for mould, bacteria growth and spores. Exposing customers to any of these could damage the company’s reputation, in addition to the health implications.
In order to avoid this, the majority of food processing firms keep to the rules of Hazard Analysis and Critical Control Points (HAACP)² as well as undertaking risk analysis, but this doesn’t always extend to the inner workings of a compressed air system. Therefore, most companies specify their own compressed air standard, which is most commonly aligned with ISO8573.1-2010, section 6³, which covers and advises on what is: direct contact with food, indirect contact with food, or non-contact, high risk, and finally, non-contact, no risk.
So what needs to be put in place in order to break down or remove all of the contaminants?
The best barrier to contaminants is filtration. While a fitted filter attached to the compressor intake may catch and remove larger elements, a more robust solution is required in order to remove contaminants under 2-5 microns, as well as water vapour and excess compressor oil that has found its way in.
A higher level of defence is the installation of separate preventative and control measures located downstream of the actual compressor, which include filtration, drying, pressure regulation and where necessary, lubrication.
Filtering out the fragments and fluids
The standard mechanism for filtration works by directing air at high speed over a slatted deflector plate. This creates a vortex that spins both solid particles and moisture droplets away from the air stream, while any liquid or solid matter in solution is collected in the filter base and drained away.
When dealing with smaller particles such as oil mist that tend to be below 0.5 microns, a secondary filter is required. The fine stainless steel mesh has multiple layers of filter tissue combined with adsorbent gauze to catch particles. This forms a film on the filter, rather than penetrating the machine.
Filtration is effective for ensuring air supply remains pure, and depending on the material used and number of layers, it can trap and remove particles as small as 0.01 micron, which encompasses 99.99 percent of contaminants.
Drying and adsorbing moisture
Although most air compressors are supplied with an after-cooler, to remove condensation caused by compression and reducing air temperature to within 10-15 degrees C of ambient temperature, an additional adsorption dryer is often required to combat excess water vapour. Circumstances for this are if parts of the pneumatic system are exposed to areas where temperatures fluctuate, such as pipes running along outside walls. The adsorption dryer works by forcing the moist air through a drying agent such as dehydrated chalk or magnesium, lithium or calcium chloride, creating a solution that can be drained away.
Whereas this method requires a drying agent, there is also the regenerative type of adsorption dryer that adsorbs moisture onto silica gel or activated alumina. These materials do not undergo a chemical reaction as the moisture is bound to the drying agent by adhesive force, or ‘unbalanced molecular attraction’. Whatever the method, it needs to be used in tandem with fine micro-filters, which ensure that any small impurities are blocked from entering a pneumatic system in fine mist form.
In addition to checking and replacing filters, operators need to monitor pressure and lubrication levels. If either of these are overlooked, then there is a greater risk of malfunctioning.
When it comes to filters on any pneumatic device, size really does matter. Using the wrong specification means that particles cannot be collected and this increases the chances of mechanical breakdown or contaminants seeping through.
Before opting for or installing any filtration or drying process, the specific application and demands of the overall pneumatic system need to be considered. This, together with whether the contaminants are airborne, moisture or oil-based, will safeguard your system.
References:
¹ The Pneu Book, page 24 (SMC Pneumatics Ltd)
² Food Standards Agency, http://www.food.gov.uk/business-industry/caterers/haccp
³ http://www.iso.org/iso/catalogue_detail.htm?csnumber=46418
For further information please visit: www.boge.co.uk
