When compared to dust extraction, fume extraction in an industrial environment presents a different set of considerations for the engineer.
There are in effect two types of ‘fume’.
Weld fume and similar contaminants contain particulates, and in many cases are more particulate than gas. Weld fume is a smoke, and smoke mainly consists of fine particles, with some gases mixed in – however it may well be that we are mainly concerned with the particulate matter when considering filtration. In this case, from a filtration perspective we can almost consider fume and smoke as a dust extraction issue – filtration will use particle filters, potentially bags or cartridges as previously discussed, and HEPA filtration will effectively remove a satisfactory level of this particulate. This is most relevant to the fabrication industry and we address these issues using our FPW type AirBench downdraft benches.
Filtration of Gases
In many cases though, when discussing fume extraction, we are really talking about gases. Filtration of gases is a more complex subject. If we do want to capture gases, rather then directly exhaust to atmosphere, we will generally consider carbon filtration. Standard carbon filters work through the process of adsorption – this is notably different to absorption. Absorption is a process through which substances ’soak’ into another material, much like a sponge; whereas adsoption consists of individual particles of a substance adhering to the surface of the capturing material. Carbon is a very effective adsorbing material because it has a huge surface area in relation to the actual size of the material – the surface is highly porous providing a large area for substances to adhere to.
Carbon Filtration and it’s Complexities
However, carbon filtration brings it’s own complexities. While particles can be captured with a fine enough filtering media, knowing which materials will be captured effectively by a carbon filter is largely a process of testing. Standardised lists are available from carbon manufacturers showing various chemicals and how well captured they are by a carbon filter – however these lists will always be limited and in many cases the substance we wish to capture is an unknown (ie, has not been tested on carbon filters). There are also many substances which we know will not be captured effectively; and in some cases there are specialist blends of carbon available designed to capture these chemicals (formaldehyde is a good example of a substance that requires specialist treated carbon blends). In our own dust, mist and fume filtration/extraction product ranges we offer standard activated carbon filtration in light and heavy duty configurations, our ‘E’ and ‘J’ type carbon filters; and also a blended carbon/activated alumina chemisorb media, our ‘R’ type filters. These configurations are available in our AirBench downdraft benches and in many cases also in our VertEx Cross-Draught range.
We must also consider the volume of fume we need to capture. In some cases the volume of carbon required and the additional processing to handle the contaminated carbon filters has a greater environmental (and monetary) impact than simply discharging the fume to atmosphere at high level, for dilution. Care should be taken to comply with discharge requirements; fume should be discharged through correctly installed ductwork at a given height above the building eaves. An additional consideration in this case is to ensure compliance with the solvent emissions directive – applicable when larger volumes of solvent are in use. Exhaust to atmosphere dust extractors are less common, as in most cases dust can be effectively filtered and disposed of.
One final consideration which – while relevant to dust – is more relevant to fume, is explosion risk. While not applicable for welding fume and smoke, for pure gases and solvents we are faced with potential explosion risk even at relatively low volumes. If a potentially explosive atmosphere is present this must be assessed even before installing equipment to handle dust and fume. This is an even more specialist area and most dust and fume extraction companies are not able to advise on explosion risk or zoning under ATEX; so in most cases if you believe there is a potential issue you should approach a specialist consultant. We are able to offer ATEX rated downdraught bench units for certain applications but generally only for dust risk areas – our FX downdraught benches and VX VerTEX cross-draught booths are our product ranges designed to cope with dust in area where this is a risk of explosion.
In conclusion, as with the various options available for filtration of dust, we must consider multiple options and approaches for fume extraction; including various types of filtration or even no filtration at all. A capable engineer with experience of these subjects will be able to advise on the correct filtration solution for your process. Contact us for an on-site assessment and demonstration of our products, or simply to discuss your processes at an early stage for another opinion.