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I need to install a replacement air handler in a facility that has an area where a large volume of particulates are in the air. Unfortunately, the previous designer tried to get away with the cheapest design possible and just put in pleated filters rather than the bag filters that were an option in the existing RTU.
In the new unit, I would like to put in bag filters, but realized that even if they allow more throughput for dirty air, they will still probably need to be cleaned out more frequently. Thinking about this reminded me of a baghouse on a silo that I specified awhile back and it had the functionality to effectively backwash the bag filters by running air in reverse to clean off the filter. Is anyone aware of such a design being available for HVAC where, perhaps, a damper would flip temporarily and change the flow through the filter to run backwards and it would route that airstream as exhaust that would be dumped to the outside periodically?

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Ignoring the concept that "backwashing" a filter as exhaust effectively negates the value and purpose of having filters, consider instead of a constantly clogging filter system, one that separates the particulates from the primary air stream.

I have a shop vac which has pleated filters. The filter is easily clogged by sawdust from a table saw, band saw, disk sander, etc. I purchased a high quality cyclone separator which causes the incoming laden air to travel in a circle. The particles are forced to the outside by inertia, pulled downward by gravity against a sloping inner chamber, which increases the velocity until the particulates reach the much narrower opening in the bottom, falling into the static chamber.

The air flow, being lighter, is able to travel to the center of the conic chamber and upward as part of the vacuum exhaust forces.

This diagram below courtesy of http://canvas.pantone.com/gallery/4424943/External-Cyclonic-Filter-for-Vacuum-Cleaner

cyclonic filter for vacuum cleaner

The device I purchased removes easily the fine particles of which powdercoating powder is made. It's possible that the design effectiveness is related to the force available from the vacuum cleaner motor. If a larger volume and smaller velocity is involved, it may be necessary to have a larger diameter conic section to establish a greater separation force.

Another advantage of this type of separation device is that the vacuum effectiveness is not reduced by the quantity of particulates removed, while a filter weakens the vacuum as the cells become clogged. One must stop the vacuum periodically to dump the chamber, but the same applies for changing or cleaning filters.

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  • $\begingroup$ I've never seen a cyclone filter for HVAC, likely due to the large velocities they require and resulting pressure drops/fan HP requirements. $\endgroup$ – Tiger Guy Feb 14 at 20:29
  • $\begingroup$ I'd expect that a system of this sort that worked would have a huge diameter, to accept the high volume of air without impeding the flow and to provide the necessary "moment arm" for the particles to drop out of the flow. Room-sized comes to mind. $\endgroup$ – fred_dot_u Feb 15 at 0:04
  • $\begingroup$ large cyclones I have been familiar with have banks of smaller cones. $\endgroup$ – Tiger Guy Feb 15 at 8:17
  • $\begingroup$ I'm not sure I follow your statement that backwashing a filter would negate the purpose of filters in the first place. If the filters are doing their job and you can exhaust a portion of air rather than recirculate it in order to clean the filters off in between actual filter changes, it seems like it could still be valuable. $\endgroup$ – Secundus Feb 18 at 0:19
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There's nothing stopping you from doing a reverse air flow cleaning system. I think the type of particulate and filter media type would determine how well it would work. I would think the problem is where does the backflow material go? A big plenum you have to sweep out regularly maybe?

If you are replacing the air handler entirely, pulse jet or vibrating bags might get you there. Again, type of particulate matters a lot here for how well it can be made to fall off of the bags.

As with almost everything, how well these things work are going to be proportional to the cost.

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Pardon my illustration (I didn't find an image online) but this was an innovation I heard of recently as part of a course.

As dust accumulates on the bag, it gets heavier. This causes the spring keeping the bag taut to extend, and the bag loses its tautness. This causes the dust to fall off, and spring is able to pull the bag taut again.

I think there was some mechanism to give a 'jerk' to the bag to make the dust fall off faster but it evades my memory.

enter image description here

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  • $\begingroup$ Do you have any more information beyond this? Unfortunately those illustrations are not very self-explanatory to me. $\endgroup$ – Secundus Feb 18 at 0:17
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    $\begingroup$ @Secundus I'll add the explanation. $\endgroup$ – Eashaan Godbole Feb 19 at 10:23
  • $\begingroup$ Thanks for the explanation. Do you recall how the airflow enters the bag? Is it in through the bottom and out through the sides or something? $\endgroup$ – Secundus Feb 19 at 19:34
  • $\begingroup$ @Secundus Through the sides, I believe. $\endgroup$ – Eashaan Godbole Feb 21 at 9:51

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