In today’s episode of the Dust Safety Science podcast, Niels Pedersen, Senior Application Engineer at Nederman in North Carolina, answers more questions about enclosureless dust collectors. This episode, which is Part 2 of a two-part series, is based on an ‘Ask Me Anything’ session held in the Dust Safety Academy on October 8, 2020. Today, he answers the following questions:
- How do you determine CFM requirements at your facility?
- Should companies inspect their ducting?
- How do you monitor pressure drop for enclosureless dust collectors?
- Is there a collection limit that enclosureless dust collectors can’t handle?
- Can you incorporate load sensors into the bag so that you know when they’re full?
- Do you need an abort gate between the cyclone and the enclosureless dust collector?
How Do You Determine CFM Requirements at Your Facility?
Niels replied that the equipment supplier should generally provide this information.
“Often, the equipment supplier has already put pick-up on the equipment, and it may be four inches, three inches, or whatever. So we will generally calculate the area of that opening and generate 4500 feet per minute velocity through that opening. So you are simply determining you have the duct size and as you assign the ductwork, you start adding areas together to maintain the 4500-foot design velocity in your system.”
He explained that this rule is basically an industry standard, as velocities should not go much below 4000 feet per minute.
Should Companies Inspect Their Ducting?
“The requirement from NFPA is that ducting should be inspected annually,” Niels replied. “You should have a flow measurement taken in your duct to see that you actually have that [required] pipe velocity. This goes somewhat beyond enclosureless dust collectors because it’s an industry standard for dust collection, and you should be doing that.”
He pointed out that equipment is often changed, and dust collectors are commonly regarded as machines that don’t produce income.
“[Managers] say, “Oh, we have some suction here, but let us go online and buy some ductwork and tie it into the line.” Then suddenly your system is out of balance, and … your velocity drops below 4000 feet per minute. That is a nest for any glowing embers to settle in and build. With that constant amount of oxygen getting in, it’s just like starting a campfire. That’s why you have to keep up the velocity and make sure dust doesn’t settle in the ductwork.”
How Do You Monitor Pressure Drop for Enclosureless Dust Collectors?
Niels explained that enclosureless dust collectors don’t typically monitor pressure drop. The only pressure is the positive pressure from the fan. As a result, there can be a relative differential pressure between outside and what the fan generates, but that doesn’t reveal anything about the dust collector’s operation. Dust collectors that tend to clog up and build become inefficient, so it is important to choose dust collectors that are somewhat self-cleaning and do not have a huge chance for significant dust buildup in the filter media section.
Is There a Collection Limit That Enclosureless Dust Collectors Can’t Handle?
Niels replied that NFPA 654 has a 10-kilo limit, so the bags may have to be emptied several times, while NFPA 664 requires daily emptying.
In addition, you cannot use any mechanical or automatic means of removing the dust. It must be a fixed system. You won’t put it on a motor or some other machine that produces high amounts of shavings because you have to clean it constantly. Then it isn’t functional or practical.”
Can You Incorporate Load Sensors Into The Bag So That You Know When They’re Full?
Anything is possible,” Niels said. “It is possible to place cells underneath, but if you have six bags for the 5000 CFM, it means you have to put six load cells underneath, which are probably going to cost more than the dust collector.”
He recommended that the container should have a window or some way of viewing the interior, but conceded that most operators will know when emptying needs to be done.
“Operators will know that they have to do it once a day now. So we’re going to be very realistic – they may not always do it once a day, but they do have a good feel for when they will need to empty the containers.”
Do You Need an Abort Gate Between the Cyclone and the Enclosureless Dust Collector?
“No, that’s not required,” Niels says. “All that the standard calls for is just a little different configuration of the normal enclosureless dust collector. They may have some easier way that they want to get the material to the outside so it’s easier for them to dump. And it may also be a little bit heavier material that separates out, and therefore, it’s a practical solution. There are no other requirements for this particular application like a rotary valve below the cyclone because, again, the danger zone is still the collector because that’s where you have the dust concentration.”
Conclusion
Niels concluded the interview by stating that NFPA 654 has a few more restrictions on the use of enclosureless dust collectors.
“One of them is that the MIE of what you are using should be 500 millijoules or more. It says in the Annex that it is tested as received. So that means if you have a bigger particle size and they get mixed with finer particles, you are actually increasing your MIE on that. The main thing for 654- they talk about a fan suitable for Class 2 Division 2. I’ve researched that a lot, and honestly, it’s the same thing as a TEFC motor in the 664 requirement. You’ll have to even find this kind of motor out there, but they work perfectly well.”
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Niels Pederson directly:
LinkedIn: https://www.linkedin.com/in/niels-pedersen-1708664
Website: http://www.nfpa.org/
If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.
Resources mentioned
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Dust Safety Professionals
Dust Safety Share
Companies
Nederman
Organizations
NFPA
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