Posts Tagged ‘forensic investigation’

Industrial Ventilation – Local Exhaust Ventilation Filters and Air Cleaners III

Sunday, May 15th, 2011
     I was out in the garage today spray painting, a job I would have preferred to have done outdoors, but alas, it was raining.  It wasn’t a big job, and I probably didn’t spend more than about an hour doing it, but by the time I was done I was all too aware of how noxious the chemical fumes were that were put out by my aerosol spray can.  I had thought that with all the garage doors open and a good cross breeze going through I’d be spared the unpleasant smell.  Now imagine this all on a much larger scale, say an industrial setting, where massive spray painters are used all day long.

     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.


Forensic Engineering Focus On Electrical Fires

Sunday, October 4th, 2009

     Property damage and loss of lives, these are often the result of fires.  But did you know that one of the leading causes of fire is electricity?  Residential electrical fires claim the lives of nearly 500 Americans each year and injure another 2300.  Annually, these fires result in over $800 million in property losses.

     Approximately one third of the nearly 70,000 home electrical fires that occur each year are traceable to design and manufacturing defects in electrical products.  The rest are caused by the misuse and poor maintenance of electrical products, overloaded circuits and extension cords, and incorrectly installed wiring.

     The three components that must be present in order for a fire to manifest and sustain itself are well known.  These components make up the “Fire Triangle,” a potentially lethal combination of heat, fuel, and oxygen.  If any one of these three components is missing from the triangle, a fire can’t be started or sustained.  In the case of an electrical fire, it’s electricity that creates the heat component of the Fire Triangle.


The Fire Triangle

     How does electricity contribute to fires?  One example would be an overloaded extension cord.  Homeowners are sometimes unaware that extension cords must be sized appropriately for their ultimate usage.  If not, they can overheat, particularly if they are damaged.  Damage to cords can result from a myriad of factors, from factory production errors to kinking when heavy furniture is carelessly placed on top of them.

     The same principle holds true for electrical products.  If their internal wiring or a component is insufficiently sized or damaged, overheating can result.  If things get hot enough and there is sufficient airflow (oxygen) and combustible material (fuel) in the vicinity, then the fire triangle is complete.  The fire starts internally and can soon spread to other objects in the area.

     Electrical arcing can occur when an energized electrical circuit is broken.  For example, suppose a wire carrying current is suddenly broken in two.  If the voltage is high enough, the electricity will want to continue to flow through the air across the break to form an electrical arc.  If the power flowing through the arc is great enough, heat can once again complete the Fire Triangle, resulting in fire.

     Forensic engineering analysis of evidence collected from a fire scene often yields telltale signs of overheating due to overloaded electrical circuits or damaged wiring in components.  Under close examination by an experienced professional, even the smallest strand of wire can point to the cause of an electrical fire.

     CSI skills aren’t only employed at crime scenes.  Forensic engineers also use similar techniques to get to the true story of cause and effect.