FAQs – Dust Collection System Design and Operation
Question: What is “normal” differential pressure in a baghouse?
Answer: In most applications a baghouse dust collector should run between between 3″ to 6″ w.g. under normal use. Once levels rise above 6″ (roughly) and the cleaning system cannot return it any lower (even when turned to continuous cleaning or “Test” mode) it is a sign that the filters are beginning to be blinded and likely need to be changed. It is not advisable to run a baghouse with a DP higher than 6″ for any length of time as this will have an impact on the function of the entire system. Running at such a high DP will lead to a number of problems including vacuum loss at the pickup points of the system (loss of suction), lower air speeds in the ductwork, higher emissions, and higher energy usage.
If you are seeing levels below 3″ after having run the baghouse for sometime you liking are getting false DP readings. When brand new bags are installed in a dust collector they should provide approximately 1″ of resistance alone. Once they begin to load dust that number will rise to between 2″ – 3″ no matter how much you clean them.
A clean on demand baghouse controller (i.e. clean on pressure) is the best way to keep a dust collector running in the recommended DP range. (see article: 3 Cheap Ways to Increase Efficiency in Dust Collection Systems)
Question: What is minimum conveying velocity in my baghouse and why is it important
Answer: The minimum airspeed required to keep dust particles suspended in the conveying system (i.e. dust collection system). When the air in any part of the dust collection system slows below the minimum conveying velocity the dust will begin to drop out of airstream and settle to the bottom of the ductwork (known as product drop out).
Maintaining the airspeed throughout the system above the minimum conveying velocity is required to prevent the accumulation of dust in the ductwork. Overtime, dust can accumulate into large piles, eventually blocking off part of the ductwork and reducing suction downstream in the system, further compounding the problem. Blockages can also cause the passing airstream to accelerate (forcing same air through a smaller space) that can lead to abrasion issues and eventually wear holes into the ductwork. Large accumulations of dust can eventually collapse sections of the ductwork due to the added weight.
Preventing product drop out is even more serious in applications involving combustible dust. Any accumulations of dust within the ductwork provide a potential fuel source for any ignition source that may find its way into the ductwork such as sparks. Additionally, if a fire starts in one part of the system it could continue to propagate throughout the rest of the system being fed by the accumulations in the ductwork. Further, if the system is operating below capacity due to blockages, dust may accumulate elsewhere in the facility including on elevates spaces that can then become fuel for both primary and secondary dust fires and explosions.
Where can I download a copy of your e-book, “Guide to Sizing Your Baghouse Dust Collection System”
Thanks for your comment. We were having trouble with our email signup system and the guides were not being sent out. We have fixed the problem. I will send you a personal email with a copy of the guide.
In what applications would a baghouse dust collector NOT run between between 3″ to 6″ w.g. under normal use? Even from a complete new set of filter bags, our dp is regularly over 7″ w.g. We burn municipal waste. Is this an issue with our induced draft fan? Or a design flaw elsewhere?
Good question. I would say the only times higher than 6″ is permissible is when its dealing with an imperfect situation. For example, an older plant that is trying to avoid investing in a new collector for a few more years might be able to get away with the higher DP if the filter costs are not too great.
In response to your situation it could be many things causing the high DP. But new bags should not pop right up to 7″ if everything is working, including the cleaning system…unless…the system is undersized for the application or you have the wrong kind of bags (fabric and/or treatment).
If 7″ is the absolute maximum you have then you can live with that…but if you start up at that then I would imagine you either have to crank up the fan significantly or you see a huge loss of suction in your system when the bags fluctuate between clean and dirty (whether you realize it or not). What does your DP rise too before you initiate the cleaning cycle? How long do your bags last? Do you meet the emissions limits set in your air permit?
I am currently reducing my dust collection points(obsolete machines). Should I just seal the end point of the branch? OR should I remove the branch up until the point where it meets the main duct?
In a perfect world you would seal it up on the trunk and remove the old branch down. But if that involves a lot of work it won’t be too much of a difference if you seal the bottom of the branch instead. However, it leaves open more space for potential leaks to form over time. So if you can afford to (time and money) I would say seal it at the trunk.