Engineering Expert Witness Blog

     Vacuums are wonderful tools, when used correctly.  And when you think about it, the constituent elements of a household canister vacuum cleaner are similar to those of an industrial local exhaust ventilation system.  My home vac is comprised of five main elements, all of which most of you are familiar with:  a nozzle, hose, filter, a fan located inside the canister to provide suction, and an exhaust hole, also located within the canister, which serves to discharge newly filtered air back into the atmosphere.

     Industrial usage local exhaust ventilation systems also typically consist of five constituent elements, namely, a hood, ducts, an air cleaning device, a fan, and an exhaust stack.  Like my home vacuum its main objective is to suck something in, namely, contaminated air.  Let’s take a closer look at each of the parts.

     The hood is located at the beginning of the local exhaust ventilation system, and like your home vac’s nozzle, it’s positioned in close proximity to the area requiring cleaning.  The objective is of course to capture contaminants at the source.  Now placement of the hood within the work area is very important.  Ultimately it must be far enough away from the source of contamination so as not to interfere with the work that’s being done, yet close enough to prevent contaminants from escaping.

     Hoods that almost completely enclose the work area provide the best control of contaminants.  Trouble is, they can interfere with the work process.  That’s where a specific design of hood, known as a “capture exhaust hood” comes in handy.  This type of hood is attached to a flexible duct that resembles a super-sized vacuum cleaner hose.  This arrangement provides greater flexibility than a huge, all-encompassing hood, and it also allows the hood to be easily positioned anywhere within the workplace as necessary.

     Again, placement of a capture exhaust hood is critical to its effective operation.  Say for instance that a hood is initially positioned a mere two inches from a source of fumes, then someone comes along and bumps it.  It ends up being four inches away from the source, and now it will require around three times the amount of air volume through the system to provide the same degree of capture as it did when it was just two inches away.  If the ventilation system isn’t strong enough to draw in this extra volume of air, fumes will escape into the work area, rendering our cleanup efforts ineffective.

Next time we’ll discuss the second main component in a local exhaust ventilation system, its ductwork.

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