Shopnotes Airfiltration box

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Project by rwyoung posted 01-11-2009 09:44 PM 5392 views 16 times favorited 8 comments Add to Favorites Watch

I was browsing through the projects here and I came across a couple of other home-built air filter units. I made this one based on a plan from ShopNotes (at least I think it was ShopNotes, I can’t find it now).

The idea is to use three filters, two pre-filter and one post-filter. As the first filter gets clogged, the filters are marched “forward” with the post filter being replaced by a new one. This is sized for 12×24 furnace filters. So far it I have noticed quite a difference, at least when I remember to turn it on.

I made a few small changes to their plan in the handles and in the way the blower is wired, mine has a fuse on the hot lead as well as a pull cord switch.

The fun part was hanging the darn thing, it weights a a lot being made from mostly 3/4” ply. It hangs in the garage and when the door goes up, it just clears the bottom of the box by a few inches.

I haven’t bothered with any precise calculations to see what the flow rate. This was a “something is better than nothing” situation. Combined with a mask when I’m making fine dust and chips and left to run for a while afterwards it does a great job. At least the intake filter cloggs up in short order! :)

-- Don't sweat the petty things and don't pet the sweaty things.

8 comments so far

View ChuckM's profile


606 posts in 3629 days

#1 posted 01-11-2009 10:48 PM

Good work.

Although you stated that “I haven’t bothered with any calculations to see what the flow rate. This was a “something is better than nothing” situation,” a word of caution is desired. It’s what you can’t see or feel that may hurt your lungs most. Unless the mask you’re wearing does the job, you may have a false sense of safety after turning on your shop-made filtration system. Part of your workshop may have filtered air but not the whole shop because there’s not enough circulation of filtered air. From your pix (but not knowing the size of your shop), the blower looks really small given the size of the box. If you built the box according to the spec. by the article, you could tell the blower is too small for such a box (the size of the box has a reason to be that big – to house a bigger blower). Another suggestion: go for the more expensive 3M filters instead of the cheapo $2 or $3 filters. They don’t really filter the fine dust that even your mask may not capture. I use the second most expensive 3M filters (they have different ratings) since my lungs are more important than a saving of $15 or so. Of course, I also wear a respirator or N99 or N 95 compliant masks or plus dust collection system that also has very fine dust filtration capability in terms of micros. We know if our fingers are cut but what’s inside our body can’t complain until it may be too late. P.S. Like stopping to smoke, it’s never too late to improve our dust collection tools.

Here has the info. on determining how big your blower should be vs the shop space.,42401

-- The time I enjoy wasting is not time wasted

View woody57's profile


650 posts in 3389 days

#2 posted 01-11-2009 10:51 PM

What kind of fan did you use? I have and old attic fan that still runs well that I’ve thought about using but the box would be real big.

-- Emmett, from Georgia

View rwyoung's profile


409 posts in 3434 days

#3 posted 01-11-2009 11:04 PM

The mask I wear is one of the high end 3M ones. Don’t have the part number handy but it is the last one before they switch over to the cartridge style.

The filters are not $2 ones, they are PureAir and have a pretty fine particlate size, better than 30micron, might be 10 but again, I don’t feel like getting off my butt to look. ;)

The blower may look small but it is a high volume unit (a function of RPM and impeller shape), not a household furnace cast off. Its original task was part of an air handling unit for a high-efficiency corn burning stove. These stoves were designed for between 40,000 and 60,000 BTU/hr and so they had to push a LOT of air through the heat exchanger otherwise they tended to dangerously overheat. A long story…

I think I have another one of these blower units around, might be a good canditate for a self contained down-draft table.

You are right about the false sense of security that may arrise and my comments were a bit flip. When running, the unit has no problem pulling air through. Most of the fines are trapping in the front filter which I have been changing once a month, more or less. I just really don’t feel like building a hood and hooking up a manometer to check flow, or putting a known concentration into the air and weighing filters before and after. It can suck out a match held a few inches from the inlet filter and blow one out quite easily on the outlet side. While nothing more than anecdotal evidence of flow, it doesn’t take much total air flow to get a circulation running and pull the fines from the shop air. The key is to start the unit early and leave it running after you are finished with the dust making task. For immediate protection, with any filtration unit, a mask is necessary as you noted.

FYI, a very rough calculation based on the spec I remember for the blower unit, I should be getting about 8 exchanges per hour. This might be a bit optimisic because the simple volume/cfm method doesn’t take into account any “leakage” of fresh air into the room. If I’m getting 10 exchanges per hour, that is doing pretty well.

-- Don't sweat the petty things and don't pet the sweaty things.

View ChuckM's profile


606 posts in 3629 days

#4 posted 01-11-2009 11:32 PM

rwyoung – if yours can do 10 exchanges, it’s fine. The small blower looks deceptive. You’re fine also as long as you’re aware of the limitation of shop-made filtration systems. Common store bought systems trap up to 98% of all dust particles as minute as 5 microns and up to 94% of particles as small as 1 micron (a human hair is about 40 microns in diameter – which can easily be filtered even with an ordinary paper mask).

-- The time I enjoy wasting is not time wasted

View rwyoung's profile


409 posts in 3434 days

#5 posted 01-11-2009 11:37 PM

A small blower is nice from a weight standpoint but these are pretty noisy little buggers. Mr. Venturi having his revenge.

I believe the multiple filters add in an RMS fashion as far as their ability to filter particles. That is to say two filters aren’t twice as good, but more like a 70% improvement. Three filters would only be about 86% better than a single. And so on.

-- Don't sweat the petty things and don't pet the sweaty things.

View BarryW's profile


1015 posts in 3869 days

#6 posted 01-12-2009 12:32 AM

Filtration systems…masks…real coughing is a man’s prerogative. Then there’s the pain and dying part…better use the filtration and masks.

-- /\/\/\ BarryW /\/\/\ Stay so busy you don't have time to die.

View dbhost's profile


5704 posts in 3194 days

#7 posted 02-10-2011 05:30 PM

That air filter looks great. If you haven’t spent the time or money yet, I would suggest that you add a proper dust collector to your workshop aresenal as soon as you can. Those ambient filters are just meant to clean the air AFTER you leave the shop… Think about it, they have to pull the air up, past your head, where it has a chance to get to your lungs, before it can clean it…

Add a dust collector, upgrade your tool dust hoods as best you can something I am working on myself… and clean the air of anything the dust collector misses which is what this is supposed to do…

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View bigike's profile


4050 posts in 3251 days

#8 posted 03-14-2011 05:43 PM

cool looken system, thanks for the info I got from this post.

-- Ike, Big Daddies Woodshop,

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