Large scale dust collection - Stage 2

[odulfst]

So I've got a rough outline of what I want to do, but I could still use some advice on fan type/sizes, motor power, and suction area. I plan to enclose the 4x8 table, leaving a gap on the long (x axis) sides to draw air through. I have not yet figured out how big to make this gap to maintain proper velocity. Both gaps will be ducted to a collection hose (4"?) and run through a cyclone (homemade?), possibly a muffler, and will terminate in a filter box exiting into the shop. I've found a very small amount of information concerning cyclone design to fine tune it's optimum chip size, and have concidered building 2 if I can figure out how to make it better at collecting fine dust. Also I need to design a proper sized filter box for my air flow requirements, and from what I've read, HEPA is a must if exhuasting indoors.

I have a couple of idea as far as moving the air goes. First off I have an old out of commision wind tunnel which can afford to donate it's motor and fan. 2HP 3450rpm elec. motor and the fan is 2' diam with 6 blades with about 40sq.in. area on each. I'm not sure if a blade-type fan will work (downstream of the cyclone) or if it's better to mount an impeller or blower style fan right on top of the cyclone. I also found a 3hp 1155rpm motor. Big SOB. I suppose I can gear that to whatever rpm is optimal, but I really don't know what an optimal RPM is for fans of given dimensions.

So there's alot of gray area there. I'm not sure where to start. Maybe the best place is required CFM to catch particulate dust in the 4x8 section, while only drawing from the sides. Then pick fan type, cyclone size, etc. Anyway, any advice is appreciated.

Thanks,
Tony

[thewoodcrafter]

Tony,
CFM is one thing, static pressure and velocity is another.
You will need a fan blade that can handle high static pressure 8" to 12". This is not your standard squirrel cage.
All fan blades will have an air flow chart for a given static pressure, RPM and CFM. Using a found blade is shooting in the dark. You will not have the information on the blade.
Velocity is the important part of dust collection. Designning the system for the right velocity and working backwards to get CFM and static pressure is the way to go. Then you find a fan blade with that spec. That will then give you the RPM and HP needed to get that air flow.
Your duct must be sized within this also. The duct needs to be sized for the velocity needed to carry the dust so it will continue to move and not just settle out in the duct.
A starting point maybe about 4000FPM velocity.
You are talking about a large amount of air volumn here.
Why don't you want to collect the dust at the bit?
How do you plan on closing in the top of the table with the gantry in the way?
The taller that enclosed is the more air needs to be moved across it.
Closing off the sides (long) and pulling the air from one end will be better.
If your table is 4ft wide and the top is 2ft tall you will need to move 8sq2 of air at 4000FPM. I may be a little rusty but I think your talking about 32000CFM.
Good luck.

[odulfst]

Hey Rodger,

Thanks for the input!

Collecting at the bit works well for flat machining, but I do alot of complex 3D cuts where a shroud would get in the way. Quite often the router itself gets in the way.

I was thinking of taking a velocity measurement of the fan I have and calculating from there. Then I'll see if it will support enough area at the ~4000ft/min you suggested.

I was planning on leaving the top open, hoping that the downward flow of air will keep the dust contained. I had thought about a crossflow design, which does sound more appealing, but like you saw, the flow numbers are just too high.

If I'm not mistaken, the velocity at the top of the table will be the same no matter what the area of suction (at the base) is (unless you constrict the flow) maybe one way to help that would be to add a huge constriction duct above the machine that will seal on the walls. local velocity at the bit might still be low, making it messy inside, but at least the dust would be constained.

Do you think this will work?

[thewoodcrafter]

Tony,
Either way you look at this you are going to need more air flow than is practical.
If you leave the top open your area is bigger, it would be 32ft2. If you drop the velocity to 3000FPM that is 96000CFM. Not practical at all.
If you just try to keep the really fine dust contained within your sides and then shop vac everything up after cutting, you may be able to make something work. But no way you can move enough air to get the dust to move off a 4 x 8 table.

[odulfst]

Rodger,
Ya, I understand that. My main focus was just for containment. I was shooting for 4kFMP at the suction nozzle at the base of the table, not at the open top. I really am just trying to keep the mess as contained as possible, I don't mind a small clean up.

say I had 2 2" suction grids along the length of the table having a total area of 3.2ft^2, 4kFPM would require almost 13kCFM, alot, but do-able. Problem is, that's only 400FPM on the table area. Is that enough velocity to keep dust down? Would I need to constrict the top? I think thats the most important question in my design; how fast do I have to move the air to contain dust?

Time to go find an anemometer and test that old tunnel out.

Thanks!
Tony.

[odulfst]

ok, from what I've researched this morning, 50FPM is all that's needed to contain dust, so using that as a starting point, I need a minimum of 1600CFM, and max duct size of 4.25" to maintain 4000FPM in the duct.

So I guess right now I'm shooting for is at least a 2000CFM system, and of course, the more the better.

Anyone know anything about cyclones?

Tony

[jlawren6]

Tony,

If you haven't come across this site already, http://billpentz.com/woodworking/cyclone/index.cfm its chocked full of good info on cyclone and general dust collection design. Lots of interesting reading there. Hope that helps.

Jon

[odulfst]

Thanks Jon,

I did see that site, but I couldn't find any info on cyclone tuning. He had an easy template to follow, but I'm looking for info on the effects of certain dimensions on dust collection. Though I'm reading more and finding more....