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Need help designing downdraft table

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Forum topic by rmoore posted 684 days ago 1543 views 0 times favorited 5 replies Add to Favorites Watch
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rmoore

312 posts in 1138 days


684 days ago

I’m building a downdraft table. I’m using a blower from an electric stove top with a downdraft smoke filter. I will be using peg board for the table top. The peg board measures 36×36, which is about 1156 1/4 inch holes. The intake for the blower is 17 1/2×4. My question is; how much area should there be for air to be pulled in? Is there a formula to figure this out?

-- The more I learn, the more I realize I don't know. Ron, Crossville Tn


5 replies so far

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StumpyNubs

5971 posts in 1303 days


#1 posted 684 days ago

I am doing an episode of Blue Collar Woodworking on downdraft tables. But mine are based on using a dust collector or shop vac for suction.

One thing to keep in mind is the more holes in the table top, the less efficient the suction will be. Sanding creates dust in the air around you and that is what you really want the downdraft table to eliminate. So you want to maximize the power of your airflow and that means not dividing it up between so many holes. I suggest using a piece of 1/2” MDF for the top and drilling some 1/2” holes about 4 to 6 inches apart. You can always add more holes if you want later. But it’s awful hard to remove holes if you put in too many to start with.

I’m looking forward to seeing what you come up with!

-- It's the best woodworking show since the invention of wood... New episodes at: http://www.stumpynubs.com

View hairy's profile

hairy

1894 posts in 2035 days


#2 posted 683 days ago

I wonder if the motor and fan that you want to use will be enough. You might want to consider a furnace blower.
I have made 2 air cleaners using them. http://lumberjocks.com/projects/15760

If you have a dust collector you could connect to that.

-- brown shoes don't make it...

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dhazelton

1103 posts in 799 days


#3 posted 683 days ago

A stove hood is intended to collect smoke and odors, so I think you would have to have a dust and particulate filter in line which would really cut down on the CFMs. I second the idea of just putting a hole in the table for a shop vac or dust collector. But by all means, try it and let us know how it works.

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crank49

3245 posts in 1474 days


#4 posted 683 days ago

Pie Are Squared is the formula for the area of a circle.
or

3.14159 X ((1/8”)squared) = area each hole = .04908 square inches

And if your sheet is 36” x 36” and holes are on 1” centers there are 34 X 34 = 1156 holes.

The area of each hole is .04908 square inches X 1156 holes = 56 square inches.

Intake is 4” x 17 1/2” = 70 square inches.

Your pegboard will restrict the air flow which will increase its velocity (FPM) and reduce its volume (CFM).

The velocity number you want for good capture is 50 to 100 FPM across the 9 sq ft area.

To figure that out you have to know the CFM of the fan and the static pressure the fan is capable of which gets more complicated than I can cover here.

To simplify, your effective area is 56 sq in. and that’s about .38 sq ft. (56 / 144)
You want 75 FPM across 9 sq ft. = 675 CFM
And you have to suck that through .38 sq ft.
That would require 1775 FPM velovity through those little holes and I doubt your fan can generate enough pressure to do that. You would probably need about a 1 hp fan generating about 5” static; depending on the filter used.

-- Michael :-{| Diapers and politicians both need to be changed often; and for the same reason.

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rmoore

312 posts in 1138 days


#5 posted 683 days ago

Thanks to all for the comments. I will look and see if there is a rating for CFMs. I plan to make a box to surround the blower and put filters after the blower. I will make it the same height as my table saw so it can serve as an outfeed table, also.

-- The more I learn, the more I realize I don't know. Ron, Crossville Tn

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