Choosing the correct type of dust extractor – Filtration options
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Dust extractor systems are available in a huge range of types, specifications, sizes, and capacities. Where should you start when considering the correct type of dust extraction system?
We have previously considered capture zones, and the range of approaches available, varying from very high velocity, high pressure on-tool type systems, through medium pressure arms and similar products; to high volume, low velocity systems such as downdraught benches and dust control booths. Capture zones and hoods are however only part of the solution.
We also need to consider how we will capture and dispose of dust – the filtration system (or in some cases, the lack of filtration system).
The simplest dust control system will consist of a capture hood, an air mover (a fan, usually), and an exhaust. In some applications, it can be acceptable to simply duct contaminants to atmosphere with no filtration – however, this is usually most applicable to solvent fume applications. Dust filtration options, however, are many and varied.
A typical large dust extractor will include filters and a cleaning mechanism; these may consist of bag filters which can be shaken; bag filters with a compressed air cleaning system; or cartridge filters (cylinders) which are typically cleaned using compressed air. Bag filters are generally the oldest example of filtration systems for dust extractors; but are still in use in many simple dust extraction systems. Bags require support on frames; the cleaning mechanism usually consists of a motor with eccentric cam to shake the filter. Compressed air cleaning for bag filter systems is a more modern development but has its own complications; not least of which is the need for a compressed air supply. Cartridge filters are a more modern solution allowing for large filter areas in a relatively small space; cleaning through high velocity jets of compressed air introduces a shock wave in the filter which dislodges the dust cake from the filter surface. Either of these filter types is typically applied to large installations – either servicing multiple operator stations where dust is generated, or more typically handling process dust, for example in woodworking factories or food production plants.
Both these filter types bring their own complications; not least of which is the generation of a dust cloud in operation meaning the unit must be assessed for explosion risk and appropriate protection measures introduced – usually a explosion relief panel requiring external siting of the unit – and so a requirement for weather protection and ducting is introduced.
Smaller dust extraction systems are able to use panel type filters without cleaning mechanisms – a common solution in both downdraught benches and dust control booths. A lack of cleaning mechanism generally removes the clouds of dust from the interior of the dust extractor, and so also removes much of the explosion risk – meaning systems with panel-type filtration can often be located internally. In the right process this can be a huge advantage; while cleaning is more manual, there are large potential savings from installing internally, lack of weather proofing, lack of explosion proofing, no ducting, and recirculation of heated or cooled air within the factory. In the current climate of energy efficiency, reducing our heating costs by recirculating air can be a major consideration. Panel type systems can be more readily applied to manual processes where dust volumes are generally lower – from GRP and other composite work, through to manual mixing and hand sanding type processes.
Another dust extractor option to consider for more flammable or explosive dusts is wet filtration. While there are many varieties of wet filter, the general approach required is to violently mix contaminated air with water. This mixing can be achieved through self-induced spray (where the air movement driven by the fan forces air/water mixing); or by a separate spray system (where air is moved through curtains of spray generated by pumps). Wet filtration almost entirely removes the need for alternative explosion protection through wetting dusts and containing the dust within water, but as with all system types there are negatives – including corrosion, increased humidity, and a need to consider legionella risks from standing water. Wet filtration is used mainly for white metal work, and the most common application is with titanium dusts – often in the aerospace industry.
As with any major plant investment, the correct choice of dust extractor is best accomplished in consultation with an expert in the field. HSE provide some helpful documentation that you can use to help choose a supplier – for example INDG408 (https://www.hse.gov.uk/pubns/indg408.pdf) gives a good introduction. A good supplier will be able to provide advice and articulate clear advantages of their dust extraction system over alternatives; and will be open and honest when confronted with a problem for which their own equipment is not suited.