Got one of those high capacity air fans. This is the output.
CFM: H 6000 /M 4350/ L 3480
If I build a wood frame and put furnace filters on front and back, what would the CFM be reduced to ??
[OP]
Member
Got one of those high capacity air fans. This is the output.
CFM: H 6000 /M 4350/ L 3480
If I build a wood frame and put furnace filters on front and back, what would the CFM be reduced to ??
Depends on filter media but, that should be fine for an ambient cleaner with almost any filter. I used a 2350CFM gable fan and shot too high. I had two filters on intake and one at the exit. IT worked but, only so-so. I removed the exit filter and it does great. It is pretty surprising what these things gather in just a few days. Shop your filter sizes. Like ammo, some are more reasonably priced than others; a common size will be cheap to replace.
"A hen is only an egg's way of making another egg".
– Samuel Butler
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I looked at paint booth filter rolls. is pretty cheap to buy only 10 micron filtering range. 1 micron or less is recommended for wood dust. I have no idea what home furnace filter micron range is.
Bil lD.,
My experience with trying to adapt a box fan as a filtering device was less than spectacular. Put any sort of restriction like a high efficiency filter ahead of it and the flow drops to the point it almost isn't worth it. The fan blade design is just not up to any sort of pressure drop. You are better off to adapt an old furnace squirrel cage style fan/blower. But of you are starting with 6000 cfm, you might be able to salvage enough airflow to be worthwhile.
NOW you tell me...
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I tried that with a cheap box fan. With no filters it was roughly 800CFM. With one good filter it dropped to something around 100CFM. When I added a coarse prefilter I couldn't get a reading on the (cheap) anemometer; it wouldn't start spinning. It does work though. It just takes a while to clear the air.
Edit: I almost forgot, I had to add a shroud around the fan blade to get that 100CFM. Without it, most of the air was just recirculating around the fan in the corners of the box.
Last edited by David L Morse; 06-02-2020 at 9:16 AM.
Beranek's Law:
It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.
Member
Here's a chart I found, that shows MERV rating versus micron size. The last ones I put in our house HVAC were MERV-14, which are pricey compared to the lower ratings.Originally Posted by Bill Dufour
Contributor
Dave:
Thanks for the chart. I always wondered about that.
I usually go for MERV 14 filters in the house also, if I can find them. Not cheap.
Of course, finding out the MERV rating on 3M Filtrete filters isn't easy sometimes.
Their top-of-the-line 1900MPR Filtrete filter (Purple packaging) is only MERV 12, so not awesome for wood dust (0.3-1.0 particle size).
Their common MPR 1085 filters (Red packaging) have a MERV of 11.
And their Filtrete Basic filter (White packaging) has a MERV of only 5, so I would say pretty useless for removing wood dust.
- After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
- It's above my pay grade. Mongo only pawn in game of life.
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Izzy Swan did a comparison on You tube, in 2018. Even though a filter company sponsored him, it was straight forward. https://www.youtube.com/watch?v=fQCEPNFFpy8 Dan
Contributor
I took a cheap box fan and put a furnace filter on the input. When freshly cleaned, it blows fresh air on me as a wood turn and sand. When it plugs up, I can still feel a little breeze, but it is really just air recirculating around ends of the fan blades.
I saw a video on youtube by Mathias Wandel where he put a sheet of plywood just outside of the "throw" of the fan blades to reduce the re-circulation. It seemed to work for him (he did measurements). One of these days I might do the same.
I'm thinking of making a new dust filter using a squirrel cage fan (or if I can get it, a furnace fan). What I'm wondering if it is better to BLOW air into the filter or SUCK the air in?
BTW, the window fan actually works. When the throughput slows, I take it outside and I blow out a load of dust.So, it's efficiency may be questionable but it is doing SOMETHING.
I'm hoping that others can recommend the pros and cons of blowing air into the filter versus sucking air through the filter.
Contributor
Even crappy filters are decent at filtering large particles (see chart above, and Brice's comments about blowing dust out of his). But it's the small particles that are so dangerous to breathe, as they get farther into your pulmonary tree perhaps to the alveolar level. Not a wonderful thing.
- After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
- It's above my pay grade. Mongo only pawn in game of life.
Contributor
If by comparison, you mean he thinks a filtration unit with 1-micron bag filter and 5-micron pre-filter is the same as a box fan with a MERV-11 filter on it. Not even close.
Member
Why do you say that? What's the efficiency of the bag filter at 1 micron?
Beranek's Law:
It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.
Contributor
Jet is the only brand I've seen numbers from, but on their 1 micron bag filter, they market is as 85% efficient at 1 micron particles and 98% efficient for 5 micron particles.
That's around MERV-15 level. Strap one of those onto a box fan and see if the output compares.
Also, I'd refrain from putting high MERV rated filters on a home AC unit unless it's adequately designed for them. Most are not and will put excessive strain on your unit.
Member
Right, they are close in efficiency. The difference is not large. As you point out though, there is a big difference in flow rate. Box fans start out with some fairly impressive numbers but axial fans can't generate much pressure. The flow drops drastically with the addition of a filter. It's the effective flow rate, efficiency times CFM, that determines how fast the dust concentration is reduced.
Beranek's Law:
It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.
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