Posts Tagged ‘dust’

Industrial Ventilation – Local Exhaust Ventilation Filters and Air Cleaners II

Monday, May 9th, 2011

     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.


Industrial Ventilation – Local Exhaust Ventilation Filters and Air Cleaners

Sunday, May 1st, 2011

     My wife is an aquarist, meaning she keeps aquariums.  Three of them.  Each contains a different variety of fish housed within its own unique liquid environment.  One of these is a 35 gallon tank containing three goldfish.  These fish have two unique characteristics that make them especially noteworthy, they are extremely hardy and extremely dirty.  Hardly a week can go by between tank changes before the water quality starts to deteriorate, evidenced by cloudy, stinky water.  It’s the kind of stink that makes a passerby in the area exclaim, “Who used the bathroom and didn’t turn on the exhaust fan!”  Thank goodness for activated carbon.  With its proper placement inside the aquarium’s filtration system a cleaner, fresher environment is delivered, both to fish inside the tank and the humans who watch them from outside.  Put the carbon in the wrong compartment, however, and the water quality plummets back to its original fetid state within a matter of days.

     As is true with the proper care of goldfish, it is often necessary within an industrial environment to remove contaminants before the air that contains them is once again dispersed into the general environment.  This is where filters and air cleaners come in.  They’re generally placed inside the ductwork, somewhere between the hood and fan.  Their job is to ensure a good, clean outcome, usually through an external exhaust of some sort.  Local exhaust ventilation systems begin with a precisely positioned hood at the source of contamination and end with an exhaust stack located outside the building.  Some airborne contaminants being released from the stack are deemed unsafe for the environment, and outdoor air quality standards promulgated by state and federal Environmental Protection Agencies limit their release back into the atmosphere.  For this reason the proper use of filtration and air cleaners is crucial.

     Airborne contaminants are in the form of dusts and vapors.  If the issue to be addressed comes in the form of dust, then filters and mechanical separators are commonly used.  Filters, like the atmospheric conditions they are meant to address, come in many configurations.  They are typically positioned within the local exhaust ventilation system ductwork, as shown in Figure 1 below.

Figure 1 – Local Exhaust Ventilation System With Filter

     The fan draws in air and dust through the strategically positioned hood, located at the source of contamination, then follows a course through ductwork, passing through a filter along the way.  The filter contains media with holes tiny enough to allow for air to pass through, but small enough to stop dust particles.  The cleaned air is then drawn out of the filter by a fan, which finally exhausts it into an externally positioned stack. 

     Next time we’ll continue our discussion on filtration devices by examining a cyclone.  And no, I don’t mean the famous vacuum cleaner, although the methodology is similar.