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The Three E’s in Dust Collection: Exposure, Efficiency, and Emissions

February 20, 2018
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The components of an industrial air filtration system. Staying within thresholds for exposure and emissions are the performance factors that really matter and filter efficiency is just one component in getting there.
The components of an industrial air filtration system. Staying within thresholds for exposure and emissions are the performance factors that really matter and filter efficiency is just one component in getting there.
Beyond a filter rating, determine whether your dust collector can remove the contaminants you're sending to it, and can do so in a steady-state condition.
Beyond a filter rating, determine whether your dust collector can remove the contaminants you're sending to it, and can do so in a steady-state condition.

When it comes to controlling industrial dust, the equipment market has long focused on the efficiency rating of filters. But Chrissy Klocker, an applications engineering manager with Donaldson Co., says two other “E’s” — exposure and emissions — are more important measures of effective dust collection. We discuss with Klocker what she calls the “three E’s” of industrial ventilation.

Question: Why does filter efficiency alone fall short in evaluating dust collection?

Klocker: Car buyers look at the listed mpg. In dust collection, it’s the MERV rating of the collector’s filter. That’s the easy button. However, what are you potentially exposing your employees to and does it present any hazards? Ultimately, you need to be conscious of what’s in your facility and how you’re physically capturing it with your hoods. Then, what is the quality of the filtered air you’re either emitting back into the building or exhausting outside? Staying within thresholds for exposure and emissions are performance factors that really matter—and filter efficiency is just one component in getting there.

Question: How do I evaluate exposure and emissions?  

Klocker: To understand exposure, you need to know the properties and concerns of the materials you either use or produce in your facility. Then you need to understand where and how your employees can potentially become exposed to those materials. The best way to understand these issues is to engage a certified industrial hygienist to review the properties and concerns of the materials you use or produce in your facility, look at various job groups, and then take samples of the air your employees breathe. This will allow you to detect average or peak concentrations of contaminants they’re exposed to while performing certain tasks.

For emissions, a firm that monitors air quality can perform stack testing to measure the material being discharged in your filtered air at the outlet of a collector. Some facilities are mandated to provide continuous emission monitoring by the EPA. OSHA and other standards may apply to the quality of filtered air you intend to return or discharge from your facility, so you will potentially have a variety of specific test methods and emission limits.

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Question: What components of my industrial ventilation system should I review?

Klocker: If you’re concerned about exposure and emissions, you need to look at more than the performance of the collector. You have to look at all the components of your system and work your way back. Ask these questions along the way:

       Hoods: Am I controlling all nuisance dust generation points—and are those hoods designed to be effective and efficient?

       Ducts: What is a logical network of ducts to effectively convey the dust or fumes from each hood into a collector and then to a discharge point? Are the ducts sized to ensure design air volumes are drawn from each hood, and are all ducts handling enough air to avoid any material settling in ducts?

       Fan: Am I pulling the right design air volume through the ducts to capture the dust at the hoods and then keep it moving to the collector?

       Collector: Can the collector remove the contaminants from the air volume I’m sending to it, and can it do so in a steady-state condition?

       Exhaust: What besides filtered air am I exhausting either back into my building or outdoors?  Are there remaining particulates, vapors, or gases? What other contaminants (besides the particulate) is in the filtered air? Do any of those pose concerns?

Question: How important are hoods and ducts in total system performance?

Klocker: Placement and design of hoods is the first and probably most critical step. Everything that happens downstream is dependent on how well the first component performs. If your hoods aren’t designed or operated properly, they will limit the opportunity to achieve dust control. The dust collector can only filter the air stream that is brought to it. So if the hood captures 20 percent of the dust, the system will perform at a maximum of 20 percent. It doesn’t matter how much more you invest in the other components.

Question: What are the key considerations in hood-and-duct design?

Klocker: First, audit your facility to figure out where all fugitive dust emissions are occurring. What are the sources? Is it a powder transfer point, the sacks you’re emptying, or the door to the mixer that opens every five minutes? You might discover new fugitive dust generation points where you need to add controls. Second, think beyond hood size. Engaging the right experts with experience in industrial ventilation is key. Determine whether you’re drawing sufficient control volume at each exhaust ventilation hood to contain and control fugitive dust at each source. Determine how far you must move the air to reduce energy consumption and consider how you can minimize changes in direction. A typical 8-in. duct requires more than 3 in. of static energy per 100 ft of straight length. In addition, changes in direction consume static energy as well. If you need a hood a long way from a collector, that increases the fan motor size you need.

Question: Can a hood-and-duct network be expanded once it’s in?

Klocker: Yes, but it is more than just dropping in another duct. It’s easy to say my collector can handle an additional 1000 cfm so I can pick up another source. But what you may not realize is when you tap the duct for that source, you’re creating a path of lower resistance and you may end up diverting air needed at the original sources. There’s a delicate balance to manage from a design standpoint, and if you try to tap into what’s already there, you can throw that balance off. It’s a good idea to have an industrial designer advise on any remodeling.

Question: When should the efficiency of my dust collector be on my radar?

Klocker: There certainly is an important role for the collector. In order to provide consistent and predictable performance, collectors must be selected and sized for the duty they need to perform. This includes being able to deliver design air volume at a predictable energy cost. The filtration efficiency of collectors influences the amount of cleaning energy necessary for stable operation, and how much fan energy is necessary for continued operation. If your system struggles to maintain design flow, or cleans excessively, it may mean you want to look at new technology because you don’t have the basic flow capacity or the efficiencies to deliver the performance you need.

Question: What does the filter rating say about dust collector performance?

Klocker: A rated filter efficiency reports how a filter performed against a specific test dust under a specific set of conditions--much like the vehicle mpg rating we mentioned earlier. And just like the mpg sticker, you may not experience the same performance on another dust under other conditions. For many filters, the reported efficiency is evaluated under initial conditions with no dust on the filter. However, a filter in a collector is often pulse-cleaned under heavy loads. In this environment, it has to handle not only the dust entering the collector, but all the dust that has accumulated on the filter (dust cake). You can’t simply assume, “I’m capturing all but 1% of the inlet dust load.” With regenerative cleaning systems, the collector reaches a stable operation point where the concentration of dust that the filters are working with is thousands of times greater than the inlet loading. That’s why you need to monitor exposure and emissions.

Question: What should I focus on when I’m selecting or upgrading my dust collector?

Klocker: An equipment supplier is doing you a disservice if they let you get wrapped up in MERV ratings. Evaluate a collector in terms of what it achieves at its stable set point. The collector needs to deliver consistent, predictable performance. That means it has to remove the contaminants you send into it and do that effectively. The measures of that performance goal include the level of emissions. Your supplier should help you understand what the dust-load demands may produce in terms of emissions, and how to achieve emissions goals in both a cost- and energy-effective way.

Question: What costs come into play during the evaluation process?

Klocker: You’ll want to look at energy consumption, maintenance, and filter life. Consider the air volume you need and how to get the most efficient system to deliver it. That will inevitably present choices. One set of equipment may deliver a reasonable filter life, but a small increase in collector size may reduce filter change frequency, reduce fan energy costs because the collector runs at a lower pressure drop, and may reduce cleaning energy costs because filters clean less frequently and more effectively. Layout will also come into play; that is, should you centralize with ducts to one central collector, or use multiple point-of-use collectors to draw adequate air? The latter option might be less expensive to operate if the duct path is long or complicated, or if sections of the facility do not operate at the same time as other areas. A good design will take all those things into account.

Question: What expertise should I seek out in my system planning?

Klocker:  Engaging an industrial hygienist up front in the process will help you evaluate the materials and any potential employee exposures in your facility and how best to address them. Best practices are contained in the Industrial Ventilation Design Manual published by the American Conference of Governmental Industrial Hygienists. To implement these standards in your own unique facility, it’s advisable to work with a qualified industrial ventilation designer.

Chrissy Klocker is applications engineering manager with the Industrial Air Filtration of Donaldson Company, Inc. For the past five years, she has instructed at the Industrial Ventilation Conference in Lansing, MI, where she also serves on the conference planning committee. Klocker holds a B.S. in Civil Engineering from North Dakota State University and a Master’s of Business Communication from the University of St. Thomas in St. Paul, MN. For more information, visit www.donaldson.com.

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