| 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. _____________________________________________ |
Posts Tagged ‘airborne contaminants’
Industrial Ventilation – Local Exhaust Ventilation Exhaust Stack Design
Sunday, June 5th, 2011Industrial 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:
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. _____________________________________________
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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. _____________________________________________
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Industrial Ventilation
Sunday, March 27th, 2011| On a hot, sticky day, what price would you pay for a cool breeze? Imagine for a moment that it’s 90 degrees in the shade and humidity is 85%. There are few human beings that wouldn’t consider this uncomfortable weather, although I have some die hard neighbors who rarely close their windows during the summer to engage the air conditioning, a rather recent modern convenience. My mom told me stories of how it was common in the 1940s for entire Chicago neighborhoods to head to Lake Michigan, spread a blanket, and sleep on the beach to keep cool on the hottest nights. As for myself, I remember being really happy when Dad broke down and finally purchased a window air unit. It was as big as a small refrigerator and took two men to lift. It was loud and drew so much power it frequently “blew the fuse.” It was so much nicer when central air conditioning came along a few years later and we could finally retire that old clunker.
Ultimately, it’s ventilation that makes air conditioning work, the principle here being a continuous circulation of air, exchanging hot for cooled. If you’ll remember, hot air gives up its heat to coils containing coolant, and the newly cooled air is released back into the room. In addition to cooling, another major function of ventilation is to remove odors and refresh the air. Everyone likes a fresh smelling home, but even more importantly, proper ventilation reduces the concentration of contaminants in the air, things which tend to make us sick, like mold. That’s why many states’ building codes require whole house air ventilation systems to be installed in new homes. In industrial settings ventilation performs the same functions, but it’s necessary for other reasons as well. Industrial facilities often house processes that create airborne toxins and other contaminants. These byproducts of manufacturing can be dangerous if allowed to collect unchecked within the confines of a building. Air containing certain concentrations of contaminants, such as vapors emitted by paints and solvents, can ignite, resulting in fire or explosion. For safety of both workers and equipment, fresh air must displace air contaminated with fumes and dust. There are three types of ventilation that can be found in industrial facilities. These include indoor air quality ventilation, dilution ventilation, and local exhaust ventilation. Indoor air quality ventilation provides freshly heated or cooled air to buildings as part of the normal heating, ventilating and air conditioning system, much like we have in our own homes. Dilution ventilation gets its name from the fact that it dilutes contaminated air by displacement, the blowing in of clean air and exhausting of dirty. The last type of ventilation, local exhaust ventilation, captures contaminated emissions at or near the source and exhausts them directly outside. Depending on the type of industrial application one, two, or all three of these ventilation types may be employed to keep air quality safe. Next week we’ll discuss dilution ventilation in detail, followed by local exhaust ventilation, and we’ll gain a better understanding of how they are used to protect worker health and safeguard property. _____________________________________________ |
