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Downdraft Table #1: Draft/Concept

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Blog entry by Peter Oxley posted 10-23-2007 07:43 AM 16859 reads 2 times favorited 13 comments Add to Favorites Watch
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I’ve been planing to build a down-draft table for quite awhile. It’s probably as important as a dust collector for staying ahead of dust collection. Someone gave me a furnace blower that puts out a lot of air, and I’ve been planning and plotting. My big hangup has been that the filters will plug up fast and I’ll be cleaning them all the time.
SketchUp downdraft 1
This weekend I cut up a bunch of MDF and was surprised to see quite a pile of MDF dust in my dust seperator. When the air and dust enter the seperator, the velocity drops allowing the heavier dust particles to fall from the air stream. Having seen MDF dust hang in the air in the shop, I would never have thought it would drop out in the seperator, but it did! And it got me to thinking … what if the downdraft table could capture some or most the dust in a bin instead of sucking all of it into the filters? Great idea, but then I’ll just have a bin full of dust instead of filters full of dust. How to get the dust out of the bin? Make a drawer? Hinge the top of the table and clear it out with the dust collector? What about just hooking the dust collector to the bin?
SketchUp downdraft 1
I’m thinking right under the blower could be a port for the dust collector. The dust collector could run while the blower was running, or it could be just be turned on now and then to clear the bin. In the drawings, the green thing is a filter. I’m thinking perferated hardboard for the top (not shown). The slats are on an angle to avoid blocking the holes in the hardboard, but may have the secondary effect of directing the airflow. I’m wondering if there should be an additional baffle in front of the filters so that air is only taken from the top of the bin instead of the whole side.

Any thoughts? I’d appreciate suggestions and feedback and will post design and construction progress.

-- http://www.peteroxley.com -- http://north40studios.etsy.com --



13 comments so far

View Thos. Angle's profile

Thos. Angle

4435 posts in 2620 days


#1 posted 10-23-2007 01:19 PM

Peter,
I’ll think on it and maybe someone else will come up with a good idea. LOL

-- Thos. Angle, Jordan Valley, Oregon

View Bob #2's profile

Bob #2

3808 posts in 2679 days


#2 posted 10-23-2007 01:35 PM

I’m at the same stage as you with a “store bought top and a furnace blower. I too am stalled at the box containing the blower. This one has a 4” porton the back to connect to the DC but I doubt I have enough volume of air to pull dust down from such a large area the top is 24 by 39”. This why I have opted for the furnace blower.
I keep seeing adverts on TV where a guy invented a mini cyclone for a household vacuum and wondering why I could not build a small version that could dump to a pan on the base and the balance of the dust perhaps caught on furnace filters.

-- A mind, like a home, is furnished by its owner

View Brad_Nailor's profile

Brad_Nailor

2531 posts in 2615 days


#3 posted 10-23-2007 01:40 PM

Nice Sketchup model! Did you do that?

-- http://www.facebook.com/pages/DSO-Designs/297237806954248

View Peter Oxley's profile

Peter Oxley

1426 posts in 2532 days


#4 posted 10-23-2007 08:35 PM

Bob – The problem with a true cyclonic seperator is that you have to direct the air in a curve around the outside of the cannister. That requires a curved chute and a curved cannister. A small cannister might be made from a 5-gallon bucket with the bottom cut out? A larger cannister could be hardboard or laminate on curved forms. I was thinking cyclonic for awhile, but it seemed like a lot of effort. The seperator on my DC, even though it claims to be cyclonic, is really just a chamber where the velocity of air drops and it seems to do alright. Having said all that, if you want a cyclonic seperator, I would be very interested in working with you on designing/building one.

David – Yeah, I use SketchUp a lot now. I’m actually an AutoCAD guy, so SketchUp drives me nuts a lot of the time, but I really like the way Sketchup renders textures and allows you to overlay photos on the drawings, and clients really like it too. Oh, yeah, and there’s the big price difference!

-- http://www.peteroxley.com -- http://north40studios.etsy.com --

View Bob #2's profile

Bob #2

3808 posts in 2679 days


#5 posted 10-23-2007 10:06 PM

Hi Peter.
I thank you nailed it with the proper terminology for cyclonic vs cyclone separators. One type relies on High Volume low sp and the true cyclonic relies on High sp and low volume.( different beast entirely)
I don’t think we can get the air moving fast enough with a furnace squirrel cage to get a true vortex.
Having said tht we have to work within the limitations presented most times so for this exercise I would like to attempt to drop the velocity in a chamber before moving the air mass to the furnace filter.
I had hoped a small can could rotate the air once to let more particles drop out before leaving for the filters.

what are your thoughts?

bob

-- A mind, like a home, is furnished by its owner

View Peter Oxley's profile

Peter Oxley

1426 posts in 2532 days


#6 posted 10-24-2007 06:33 AM

Okay, so we’re going to at least get get a swirl if not a vortex! How small a can were you thinking? 5-gallon bucket size? A furnace blower moves a lot of air at fairly low pressure. I think if the can is too small it just becomes part of the duct instead of a large enough chamber for the pressure to drop. On the other hand, if it’s too big, we’ll loose the swirling motion Any thoughts on ducting into and out of the can? I mean what to use for duct and how to attach it? The can creates the air rotation, but could have a square bin below it for the dust to drop into. Also if the inlet was lower than the outlet, it might cause even more dust to drop out. Here’s a rough sketch of what I’m thinking.
Cyclone Concept
Another idea would be to have the air-out duct in the top center of the can instead of on the side. I wonder which would make the dust drop out better? I’ll be thinking about this and about a way to test it. Let me know your thoughts.

-- http://www.peteroxley.com -- http://north40studios.etsy.com --

View Bob #2's profile

Bob #2

3808 posts in 2679 days


#7 posted 10-24-2007 04:06 PM

Great sketchup diagram Peter.
If I were to go with this idea I thnk I would try to incorporte a ramp in the cylinder to prevent the air from recirculating on itself and holding the fines in suspension.

We might also consider an innner pipe that picks up the air after it leaves the ramp.
Sort of a pipe within a pipe with the inner pipe the exit to the blower filter.

Bob

-- A mind, like a home, is furnished by its owner

View Peter Oxley's profile

Peter Oxley

1426 posts in 2532 days


#8 posted 10-24-2007 05:10 PM

Yeah, I wondered if this would just create a current between the “in” and “out” and carry dust right through the chamber. I think a center “out” should force the airstream to go through a pressure drop instead of following a current. On the other hand, it might create turbulence which would stir up the dust. Also, I think if we are trying to get dust to “drop”, the mouth of the exit should be higher than the end of the ramp, otherwise settling dust could just be picked up again as it drifted past the exit.
Seperator with internal air ramp
Here’s a sketch – I couldn’t get a true spiral from sketchup, but I think it shows the concept. We could cut the ramp from 1/4 BB ply and anchor it to the inside of the cylinder with cleats?? Glue??

-- http://www.peteroxley.com -- http://north40studios.etsy.com --

View Bob #2's profile

Bob #2

3808 posts in 2679 days


#9 posted 10-24-2007 07:01 PM

Peter:
I was thinking of say 22 ga galvanized for the ramp with a couple of tags that we can pop rivet to the outer wall.
I little silicone and “bob’s yer uncle!”
The ramp is just a circle with the center cut out.
If our cylinder is sitting on a box/drawer. the fines should drop pretty well???

Bob

-- A mind, like a home, is furnished by its owner

View Peter Oxley's profile

Peter Oxley

1426 posts in 2532 days


#10 posted 10-25-2007 09:56 PM

Bob – I’ve been trying to think of anything else we want to do with this. I haven’t come up with anything new. I won’t have time to actually build one for testing for the next several days at least. Do you want to have a go at it, or are you okay waiting a bit?

-- http://www.peteroxley.com -- http://north40studios.etsy.com --

View Peter Oxley's profile

Peter Oxley

1426 posts in 2532 days


#11 posted 11-05-2007 12:54 AM

Bob – is this kind of what we were talking about?
Cyclone Seperator
Looks like you’ve already given it a try! How does it work?

-- http://www.peteroxley.com -- http://north40studios.etsy.com --

View Bob #2's profile

Bob #2

3808 posts in 2679 days


#12 posted 11-05-2007 01:45 AM

Tomorrw I will give you a pic of the lower bag on the DC after roughly 10-12 loads through the separator.

I can tell you right now that ther is less than a teacup of dust in that bag.

Bob

-- A mind, like a home, is furnished by its owner

View Peter Oxley's profile

Peter Oxley

1426 posts in 2532 days


#13 posted 11-05-2007 02:40 AM

Sounds like it works good! The issue is getting it into the bottom section of a downdraft table!

-- http://www.peteroxley.com -- http://north40studios.etsy.com --

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