Airflow and how close to the fume
Airflow rises with fan size, so a compact unit on a 200 millimetre duct suits a single welding bay, while 300 and 400 millimetre fans move enough air for a whole workshop. Whatever the size, suction falls away quickly with distance, so as a rule of thumb keep the open end of the duct within one duct width of the fume.
Extracting fume or blowing fresh air
Ducting on the intake reaches the fume at the work and ducting on the outlet carries it outside, or the same fan can blow fresh air down a duct to where someone is working. Each extra metre of hose and each bend costs some airflow, so for a long run choose a unit with a higher pressure rating, quoted in pascals, which keeps more of its airflow through the ducting.
Venting, power and flammable vapour
Run the outlet outdoors and point it away from windows, doors and air intakes, or the fume drifts back inside. These fans move fume rather than filter it, so there is no filter to replace: check the ducting for splits, clear dust and spatter from the guards, and expect them to draw well under a kilowatt. Where solvent vapour or fine dust could build up to an explosive level, the extractor must be rated for explosive atmospheres.
