Posts Tagged ‘fresh air intake’

Industrial Ventilation – Local Exhaust Ventilation Exhaust Stack Design

Sunday, June 5th, 2011
     Imagine how nasty it would be if some of the dirty water draining into your sink was allowed to leak back into the fresh water coming from your faucet.  Yuck!  Now imagine looking up at a factory’s exhaust stack and noticing that it’s located just inches from the intake pipe, the one that’s supposed to suck in fresh air for the workers inside.  Even a novice can see that this is an unhealthy situation.  Some of the airborne contaminants exiting the building are sure to be sucked right back in.

     Last week we discussed the importance of location with regard to intake and output pipes, an integral part of a local exhaust ventilation system.  The placement of these stacks is governed by various industry standards that present guidelines to insure that ventilation systems work properly and protect the health of people in the workplace.  These guidelines have been determined by scientific study to equate to a safe minimal standard, as determined by a body of experts that have come together to form a consensus committee on the subject. 

     Generally speaking, the standards set recommend that exhaust stacks extend upward a minimum of 10 feet above the highest point of the roof.  As for discharge velocity, the rate at which contaminated air blows out of the stack, the standards recommend that operation take place at a minimum of 3,000 feet per second.  Separation distances between exhaust stacks and air intakes vary according to dilution requirements set out in the standards, but basically the separation must be great enough so that airborne contaminants leaving the exhaust stack get safely diluted by outside air so they will pose no hazard should they ever reach the air intake ducts.  This combination of height, velocity, and distance factors allows contaminated air to be dispersed far enough from the building to avoid downdrafts created by wind passing over the roof, thereby preventing undesirable consequences like the smoke that re-entered my house through its fireplace on windy days.

     One device that is sometimes incorporated into the scenario to keep workplace air clean is the inclusion of rain caps on the roof.  These look like conical shaped hats, and they’re supposed to keep rain from falling into the exhaust stack.  It seems like a good idea, but they unfortunately do not work very well.  To begin with, they don’t do a good job of keeping out rain, especially when it’s driven by strong winds.  Another drawback is that they can actually direct contaminants exiting from exhaust stacks back down to the roof and into the building’s fresh air intake ducts.  Yet another drawback of rain caps is that they often result in the local exhaust ventilation system fan working harder than it has to   because the contaminated air slams into the rain cap, thereby slowing its rate of exit and causing it to lose velocity energy.  This means a fan must be selected to work harder to compensate for the resistance.

     Well, that’s it for our series on local exhaust ventilation systems.  Next time we’ll switch gears and discuss how those outlet covers in your home with the cute little red and black buttons work to protect you against death by electrocution.  They’re usually positioned near water sources and are known as “Ground Fault Circuit Interrupters,” or GFCI.  I’ll be discussing topics like this on an upcoming show to be featured on The Discovery Channel, where I’ll be acting as a subject matter expert.  The series, titled “Curious and Unusual Deaths,will cover a wide range of potential threats that are present in our everyday environments.


Industrial Ventilation – Local Exhaust Ventilation Exhaust Stacks

Sunday, May 29th, 2011
     I like to bring the outdoors inside by the inclusion of natural elements, lots of wood, stone, and gurgling water.  I once lived in a house with a very impressive looking natural stone fireplace.  On calm days it was a pleasure to throw on a few logs and start a nice crackling fire.  But shortly after moving in I discovered that under certain conditions the smoke would back up in the chimney and actually flow back down into the house, creating a smelly, sooty mess.  This usually resulted in me having to open all the doors and windows to vent the place out.  The first time it happened I thoroughly investigated.  Was anything blocking the chimney?  If not, what was the problem?  A little outdoor surveying brought the issue to light.  The fireplace chimney was not built high enough above the roofline, so that when the wind blew, it created downdrafts along the roof that worked against the smoke, forcing it back down into the chimney.

     The phenomenon at play with my stone fireplace is similar to one sometimes facing industrial ventilation applications.  A fireplace chimney functions very much like an exhaust stack on a local exhaust ventilation system, its function being to efficiently discharge contaminants from the building, most typically in a vertical fashion.  At a minimum, exhaust stacks must be designed to provide sufficient dilution of airborne contaminants when they are released into the atmosphere, while adhering to applicable environmental standards.  Dispersion into the atmosphere scatters contaminating molecules into a huge playing field, the sky, thereby reducing concentrations to safe levels.  Just as the vast ocean is capable of absorbing enormous amounts of pollutants from oil spills and the like, the atmosphere at large is equally capable.

     To keep contaminated air moving out of the exhaust stack while achieving the highest amount of atmospheric dispersion, the following factors must be taken into consideration during the ventilation system design process:

  1. The exhaust stack height.
  2. The distance between the exhaust stack and air intakes on the roof, and sensitive locations where windows and doors are located, keeping an eye on their separation distance from the exhaust stack.
  3. Exit velocity and vertical momentum of the contaminated air flowing out of the stack. 

     These factors are addressed for various types of airborne contaminants through standards published by the National Fire Protection Association (NFPA), the American National Standards Institute (ANSI) in conjunction with the American Industrial Hygiene Association (AIHA), and the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE).

     Next time, we’ll take a closer look at their recommendations and the standards they’ve set up to prevent undesirable incidents such as the one I encountered with my natural stone fireplace.