Engineering Expert Witness Blog

     We’ve been talking about mechanical filtration, like the type used by fish tanks.  Now we’ll consider another type, the “cyclone.”  It’s something which most of us have become very familiar with, thanks to a British bloke and his awesome vacuum that “…won’t lose suction!”  His invention makes use of the principles of cyclone technology, and as effective as it is used in vacuums, it’s equally impressive used in local exhaust ventilation system applications.  A cyclone that has been incorporated within this type of system is shown in Figure 1.

Figure 1 – Local Exhaust Ventilation System With Cyclone

     Here’s how it works.  A local exhaust ventilation system draws in corrupted air by means of a strategically placed hood, and its fan pulls the captured air and dust mixture through ductwork and into the cyclone.  The cyclone is shaped like a cone standing upright on its small end.  A cutaway view is shown in Figure 2.

Figure 2 – Cutaway View of a Cyclone

     When a quickly-moving air and dust mixture gets drawn into the cyclone by the fan, the mixture is forced to spiral down into the cone by the shape of the inlet passage.  Because dust particles are heavier than air molecules, they tend to separate due to centrifugal force.  The heavier dust particles are sent crashing into the sloping sides of the cone.  They then slide down to the bottom of the cone, where they will eventually fall through the bottom and into a waiting trash bin.  The lighter air tends to stay in the center of the cyclone and is eventually drawn out by the fan through the outlet passage.

     Unfortunately, cyclones are not 100% efficient when it comes to removing dust from the air.  Their efficiency depends on many factors, including the shape of the cyclone, the speed of the flow going through it, and the weight of the dust particles.  In any case, there’s always going to be some dust that will escape along with the air that’s being exhausted to the building’s exterior through the exhaust stack.  If necessary, this air can be cleaned further before being released into the atmosphere by the use of additional filtration located within the ductwork between the cyclone and the fan.

     That wraps up our discussion on dust removal through mechanical filtration.  Next time we’ll look at systems capable of removing chemical vapors.

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