Brother, can you spare a flattened truncated conical cut plan?

[Bruce Seidner]

I have been all over sheet metal forums and apparently it is a piece of cake to draw a truncated cone that will serve as a duct transition. I need to make a transition from an 18" diameter fan to a 27.5" filter bag. I have an 18" exhaust blower and a 27.6" diameter x 4 yard tube bag of 16oz filter material that will become my ambient shop air filter. It is the blower for those sky dancer 60' tall advertising inflatables that you see at car dealerships. Those little squirrel cage HVAC filter boxes have always seemed a bit anemic to me.

I plan to take a stack of four 30" x 30" x .75" plywood sheets and cut two with an 18" hole to support the round blower unit. I will use the other two sheets with 27.5" holes to sandwich the filter bag between the two 27.5" pieces and screw them together. I will use 2x4's to create the cube unit at the 4 corners. There will have to be some kind of stand that will go from the floor to the floor joists above but that is something that happens as I build it.

I need to make a cone to transition the diameters and see that this is simple with a cad program. It has not been a piece of pi for me as I try to figure this out with a compass and protractor. I read where a fellow recommended no more than a 15% increase in transition or 4" long for every 1" in diameter transition. But this is for HVAC. I found another transition page that suggested 24" as standard for an 18" to 28" round duct transition. Two feet or three feet I can handle. My plan is to use hardboard and to steam it should it resist or start to crack. But I would appreciate a cut diagram from some kind soul with the requisite program. I believe that the bag, whose circumference is 87" becomes a 27.6" diameter circle. So I need to make a cone that is 18" diameter on one end and 27.6" diameter on the other end and 24" long to 36" long given that hard board is less flexible than sheet metal. I found this picture in a sheet metal discussion group.

[Wes Grass]

Since you're going from small to large, I'd think a simple box would probably flow OK. But if you want the cone cuz it looks better, I'd probably make it up out of 12 or more compound mitered segments.

[Thomas Hotchkin]

Bruce
Have you looked at Bill Pentz web site? Lot of info on how to build your own cyclone, I think he has a spreadsheet you can download and change sizes for your fan fitting. Tom http://www.billpentz.com/woodworking...yCycloneSizing

[Bruce Seidner]

Since you're going from small to large, I'd think a simple box would probably flow OK. But if you want the cone cuz it looks better, I'd probably make it up out of 12 or more compound mitered segments.

If you were to walk into my garage shop you would be certain that there is very little impression management going on. But to read these engineering tables for HVAC and ducting, one feels like it is a Class A Felony to "rob" efficiency from the system with each transition costing out in yards of added duct. I realize that this is additive and in the design of a large commercial building of 200,000 square feet it adds up dramatically while in the small size of a shop such hand wringing is over the top. But it is not that difficult to cut big holes in plywood and according to the tables a conical transition is the way to reduce diameter round to round. I was playing with some thin hard bard I picked up and I think it would be pretty simple to roll it into a cone and would save a lot of time if I had cut it to size ahead of time. I will use some brown paper to make a pattern that works. But a box would work as you point out. My first plan was to fit the 4 sides to the unequal squares, which I may still do if I can't figure this cone thing out.

[Bruce Seidner]

I was looking for cones in all the wrong places. This would be it.

Learn how to layout a cone and the formula, these numbers can be replaced with your dimensions.

The formula for the cone is...

• First you must find the difference between the large and the small Dia.
• Multiply the large dia. by the vertical height,
• Divide this product by the difference first obtained large dia and small dia)

This will give you the length of the center line you need from the top of the cone to the intersecting point (A)
Large Dia - Small Dia = 20" - 8" = 12"
Vertical height X Large Dia. 22" x 20"= 440
440 divided by the product of the difference in dia.
440 / 12 = 36.66 and this 36.66 is the length

[Larry Edgerton]

I have a small shear/break/roller that will roll the cone by adjusting the pressure on the rollers more on one side than the other. They are fairly common so if you find a local fab shop they will probably have one. With mine I mark out the cone and cut the top and bottom curves but leave the welded cut long by a couple of inches as the roller screws with the curve coming out of the rollers. I then trim that couple of inches and weld the cone.

Larry

[Mark Duksta]

This is what you need:

http://www.i-logic.com/conecalc.htm