| 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 ‘exhaust stack’
| 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.
| When something is said to be the “heart of the operation,” one usually imagines that it is integral to whatever is being discussed, and it is probably centrally located. The human heart fits this description well. This amazing organ, centrally located within your chest cavity, moves blood, nutrients, oxygen, and carbon dioxide through your body with amazing efficiency. During a twenty four hour period it can pump as much as 2,000 gallons of blood through 6,000 miles of arteries, veins, and capillaries.
At the heart of a local exhaust ventilation system is its fan. Like the human heart, it is a model of efficiency. It first creates a vacuum in the intake hood, which is strategically located at a pollution source, pulling in contaminated air and leading it through ductwork. Sometimes the fan leads the air to a filter or other air cleaning equipment, but eventually the dirty air is exhausted through a stack leading outdoors.
There are two main types of fan, axial and centrifugal. You’re probably most familiar with the axial type, because they’re the type commonly used in tabletop, box, and oscillating fans in your home. These have blades that look like a propeller on an airplane, and they work by drawing air straight through the fan. As helpful as they are within a personal setting, axial fans are not typically used in local exhaust ventilation systems because the electric motor that drives the blades is in the path of airflow. This setup can create a problem if the air flowing over the motor contains dust and flammable vapor. Dust can cause the motor to get dirty and overheat. Flammable vapor can ignite if the motor wiring fails and creates an electrical arc.
Because of the technical difficulties presented by an axial type fan, centrifugal fans are what are most often used in industrial settings. One such fan is shown in Figure 1.
Figure 1 – Centrifugal Fan
The blades of a centrifugal fan are fully enclosed in air tight housing. This housing keeps any dust or fumes from leaking out into the building. The electric motor that drives the fan can be safely located outside of this housing, where it is dust-free and there are no flammable vapors. If you look inside the housing you will see that the moving part, known as the impeller, resembles a squirrel cage. See Figure 2.
Figure 2 – Centrifugal Fan Impeller
This impeller is made up of many blades, set up within a wheel configuration. When an electric motor causes the wheel to rotate, air is made to move off the blades and out of the impeller due to centrifugal force. This air is sent crashing into the fan housing, shown in Figure 1, which is curved like a spiral to direct the air into an outlet duct which is connected to ductwork that leads to the exhaust stack. As air leaves the impeller, more air rushes into its center from the inlet duct to occupy the empty space that’s been created. Hence, as long as the motor keeps spinning the impeller, air will flow through the fan.
In order for all this to work effectively, the centrifugal fan must be the right size, one that is capable of providing enough suction to capture contaminated air at the hood source, then overcoming the resistance to air flow that is presented by ductwork, filters, and other air cleaning devices. Because air resistance factors such as these impede the fan’s ability to move air through the system, the fan must be of sufficient strength make up for these factors. To size up the right centrifugal fan for the job, engineers must calculate the resistance to airflow that is expected to be encountered, and to do this they use data supplied by manufacturers of component parts, as well as tabulated data that is readily available in engineering handbooks. Just as a lawn mower engine won’t provide sufficient energy to power a car, an undersized fan won’t be able to move air through a system which is beyond its capacity limit.
Next time, we’ll finish our series on local exhaust ventilations systems by looking at the last component in the system: the exhaust stack.
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.
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.
| I’m a husband who occasionally does a little vacuuming, at least of the areas I’m responsible for messing up. It’s not one of my favorite activities, and I particularly hate it when I’m in a hurry and I don’t have enough time to move things out of the way. That’s when an accident is prone to happen, and I end up sucking something besides dirt into the hose. The extra work I’ve just created for myself results in my having to open up the vacuum bag and start sifting through the debris. In the end, I sometimes end up making a bigger mess than the one I started out with.
Vacuums are wonderful tools, when used correctly. And when you think about it, the constituent elements of a household canister vacuum cleaner are similar to those of an industrial local exhaust ventilation system. My home vac is comprised of five main elements, all of which most of you are familiar with: a nozzle, hose, filter, a fan located inside the canister to provide suction, and an exhaust hole, also located within the canister, which serves to discharge newly filtered air back into the atmosphere.
Industrial usage local exhaust ventilation systems also typically consist of five constituent elements, namely, a hood, ducts, an air cleaning device, a fan, and an exhaust stack. Like my home vacuum its main objective is to suck something in, namely, contaminated air. Let’s take a closer look at each of the parts.
The hood is located at the beginning of the local exhaust ventilation system, and like your home vac’s nozzle, it’s positioned in close proximity to the area requiring cleaning. The objective is of course to capture contaminants at the source. Now placement of the hood within the work area is very important. Ultimately it must be far enough away from the source of contamination so as not to interfere with the work that’s being done, yet close enough to prevent contaminants from escaping.
Hoods that almost completely enclose the work area provide the best control of contaminants. Trouble is, they can interfere with the work process. That’s where a specific design of hood, known as a “capture exhaust hood” comes in handy. This type of hood is attached to a flexible duct that resembles a super-sized vacuum cleaner hose. This arrangement provides greater flexibility than a huge, all-encompassing hood, and it also allows the hood to be easily positioned anywhere within the workplace as necessary.
Again, placement of a capture exhaust hood is critical to its effective operation. Say for instance that a hood is initially positioned a mere two inches from a source of fumes, then someone comes along and bumps it. It ends up being four inches away from the source, and now it will require around three times the amount of air volume through the system to provide the same degree of capture as it did when it was just two inches away. If the ventilation system isn’t strong enough to draw in this extra volume of air, fumes will escape into the work area, rendering our cleanup efforts ineffective.
Next time we’ll discuss the second main component in a local exhaust ventilation system, its ductwork.