Engineering Expert Witness Blog

     We’ve been talking for awhile now about filtration, from fabric filters to cyclones, and how they are most effective when integrated into a local exhaust ventilation system.  These filtration devices are great for the removal of airborne particles like dust, but they don’t do a good job removing chemical vapors like paint fumes, much in the same way as a dust mask wouldn’t have made my spray painting job any less smelly.  This week we’ll focus on filtration capable of addressing the special challenges presented by chemical vapors in the air.

     Chemical vapor contaminants can be separated from good air trapped in a local exhaust ventilation system by way of an air cleaner in a process known as absorption.  In this instance, just like with our smelly goldfish tank, the media can consist of activated carbon, a carbon created by intense heating of substances like bituminous coal, wood, or coconut shell.  The heat removes everything except carbon and creates myriads of tiny pores throughout.   These pores give activated carbon tremendous surface area, meaning lots of nooks and crannies for chemical molecules to get lodged in.  And when I say “lots” of nooks and crannies, I mean it.  One pound of granular activated carbon has enough pores to give it a surface area of 125 acres!  As the air-vapor mixture passes over the huge surface area, chemical vapors are absorbed by combining chemically with the carbon.  Jamb packing surface area into a small space, as activated carbon does, creates a media capable of absorbing vast amounts of chemical molecules for a long time.  As effective as this system is, the carbon pores will eventually become saturated with contaminants, and when it does, it is easily addressed.  Simply replace the media with fresh carbon.

     Another means of removing harmful vapors from the air is through the use of an air cleaner employing temperature as its means of filtration.  I’ll bet you’re asking how that works, and here’s an example you can relate to.  It’s a hot, humid day, and the only thing standing between you and total discomfort is a glass of ice water.  As you eagerly lift the glass to your lips, you notice the glass is wet on the outside, so wet that it’s actually dripping.  In the stupor caused by your heat exhaustion you may for a moment think that the glass is actually leaking, but you soon realize that the water has accumulated on the outside of the glass because the hot, humid air that is making you so uncomfortable has also come into contact with the cold surface of the glass.  When the water vapor in the atmosphere hits the cool of the glass filled with ice, it condenses into droplets.  This condensation process stops when the glass temperature equalizes to that of the temperature in the surrounding air.  Air cleaners can make use of the same phenomenon to filter contaminants.  In their case the contaminated air mixture is cooled to the point where the humidity and chemical vapors present condense together to form a liquid, and the liquid is then drained out for proper disposal.

     That’s it for our look at filters and air cleaners.  To sum things up, remember that there are a variety of factors that have to be considered when selecting filters and air cleaning devices.  These include the volume of air flowing through the system, the concentration of contaminants in the air, the chemical and physical properties of the contaminants, the hazards associated with the contaminants, and the emissions standards established by federal, state, and local environmental regulations.

     Next time we’ll explore the workhorse of a local exhaust ventilation system, its fan.

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