Down-Draft Dust Collection Table
Tips and advice on rigging an old squirrel-cage fan up to power a down-draft table. October 3, 2007
Question
I have an old furnace squirrel cage blower that I was thinking of sticking under a table to create a downdraft sanding table. It's quiet enough that it seems it would be almost non-existent when surrounded by some filters, and since it's primarily collecting fine dust, I thought... This is a small one man shop right now (hopefully to add a few people soon, though). I found an old Goodwill sorting table for free and cut it into a couple of work tables; it has nice tall sides, so lots of room to lose tools and such. Any thoughts?
Forum Responses
(Dust Collection and Safety Equipment Forum)
From contributor B:
The squirrel-cage blower should work okay, but you'd have to ensure that the blower's incoming air is pre-filtered. Enclose your blower in a clean-air chamber. You may want to use some cartridge filters, or an array of pleated filters, prior to the blower chamber.
You'll also want to balance the suction at the work-surface by sizing the openings in the bench-top, at an appropriate size. If you have too large of a combined surface opening, your suction will be too weak. You don't need much suction with a downdraft table; you want volume (cfm). A small commercial downdraft table may flow 1500 cfm, but at a very low 1" static pressure.
You can play with the blower's rotation speed as well by changing the pulley diameters accordingly. This may quickly overwhelm a motor designed to deal with a certain volume at a certain pressure. Monitor your amp-draw, and replace the motor if need be.
If you have the room, use two or more blowers. Small residential furnace blowers often have very small motors, rated at 1/4 or 1/3 hp.
From contributor T:
I built mine with a large 1 hp blower. 2400 cfm. Contributor B is right; you will need excellent filters. I use 8 - 1 micron small DC bags. My top is 7' x 4'. Had a smaller one before but it was a failed experiment. Not enough cfm. Don't use furnace filters unless you want to create a dust pump... very unhealthy and will plug up the motor/fan. 1 micron is an absolute must.
From the original questioner:
Thanks for the input. Contributor B, you're right - this one actually has a 1/6 hp motor; I'm not sure what the cfm is (and don't know how to figure that out). Contributor T, I'm only building a 3'x4' right now, test run on a small one. Wish I had a 1 hp; if this works (or sort of works), I'll have to find something bigger. How does one figure out cfm? Is there a basic formula for size of cage/motor/speed, or does one have to get into number of blades, angle, etc.?
From contributor B:
I wonder if there wouldn't be a sticker somewhere on the blower that states cfm rating. There may be several cfm ratings, depending on what speed your blower's working at (low-med-high).
From contributor T:
Any HVAC supplier will have the statistics for any blower size/motor/rpm configuration. I would ask them to fax you a copy and tell them you will need prices later. Let them know what you're up to, as they may be helpful. The big blowers draw 6000+ cfm on 3 hp! Exhausted air becomes a real problem. Ducting to across the shop is the solution, as this creates a circular air circuit and turns the downdraft table into a shop air filter system. Mine is on all day and my shop is much cleaner than without it. Again, I must stress that really good filters are a must, otherwise you will pump the most dangerous fine dust into your breathing air.
From the original questioner:
What are your favorite filters? I have been perusing the site, and find a lot of back and forth about the pleated filters (i.e. Dust Dog, etc. for single stage) and they don't seem favored by the group. Seems they would work well for this application, although not necessarily in the round. Actually, I bet I could just put the blower and motor inside a round... I am thinking about using replacement air filters for the air cleaners from Delta, etc.
From contributor B:
On a single-stage dc, I find that cartridge filters just are not suitable. For a downdraft table, that's a whole different story. They're just about ideal. You'd be flowing from the outside-in, and at very low velocity. Be careful, though. The cartridge filter's large surface area is one thing; the smaller port in the base of that cartridge is quite another. Go with a very large diameter opening there, unless your blower has enough suction to deal with that. With a single small capacity furnace blower, a 4" or 5" filter port would most likely be too small. 8" or larger would be best. If you're running multiple blowers (3 or more), use two cartridges.
From contributor M:
I currently have a 5 horsepower dust collection system... about 35 feet away from where I'd like to position a down draft table. I am thinking about building a 3'X4' table with a 1 horsepower motor. Can I use as an exhaust port the 4" dust collection line rather than install furnace filters? Also any recommendations for the surface top, and what is the optimal size of the holes in the top?
From contributor T:
I don't know about using a dc to draw air through a downdraft table, but I don't see why not. Especially a small top of 3 x 4. My top has rows of 1" holes about 1" apart. The holes are chamfered on both sides. I have matching bench dogs and use mine for a work/assembly table as well as sanding.
From contributor I:
You don't need a larger motor. I have a furnace blower that I use as an air filter for shop. You simply need to change the pulley on the motor for a larger one and change the pulley on the squirrel cage for a smaller one. I have a 6" pulley on the motor and a 2" pulley on the cage. Works wonders.
By the way, you absolutely need to use at least 2 furnace filters rated at a minimum of 1 micron, otherwise you're doing more bad than good.
From the original questioner:
Thanks, I'll try that.
Contributor M, the fellow in the shop next to mine has his dust collector hooked up to his downdraft table. Granted, it's small, as he uses it to sand planes, but it does work very well. Has about a half mile of duct work to get to it from a 2 hp cyclone, so...
