Flattening DC DUCTING

[Victor Stearns]

Greetings to All,
I am finally ordering the rest of the components to install my DC permanently to the outside of the workshop. I have a good layout that I like.
So I do have one area where space is somewhat of an issue. I am using 6" ducting for most of the system. The branch that will serve the lathe is going to be 5". And this is what I am thinking. A round 5" duct mounted to the wall will stick out more that I like. I am thinking of using a press to flatten the 5" duct such that it will become an oval type shape. This will not change the volume of the pipe, only the shape.
Any Thoughts?
Thanks
Victor

[Jim O'Dell]

Only thing I would worry about is the guage of the pipe. Being in an oval shape will make it easier to collapse on itself. Thiner snap lock would probably have a problem. My gut tells me spiral would probably be ok, but I have nothing to base that on. Have you thought about getting an HVAC transition from 5" to a corresponding rectangle that angles down? Might help your clearance problem, might not. But worth a thought if it hasn't crossed your mind yet. I used one that was 4X10 to 6" for my router table. You might be able to keep the part that sticks out down to about 4 1/2", maybe a little less. Jim.

[Rob Bodenschatz]

I asked a similar question here:

http://www.sawmillcreek.org/showthread.php?t=53644&highlight=floor+duct

There may some info in there that helps.

[Joe Chritz]

I don't know what you have for a DC but most of the manufactures are really good about answering questions about set up.

I can't see why it wouldn't work but since all the pipe sold for DC is round there may be a reason. Resistance to collapse is my gut thought on it.

If you get an answer post it up, I have a place a 10" rectangle would work good for DC. (Behind a SCMS).

Joe

[Rick Christopherson]

There is not very much static pressure for a home DC so collapsing even thin walled ducting is not really an issue.

[Ben Grunow]

You can buy oval pipe at any good hvac supplier. It comes in heavier "smoke pipe" as well. My Dust Gorilla is all smoke pipe and it is fine.

[Bob Feeser]

My concern about flattening a pipe would be twofold. One is that getting the collapsing job to be even, and not unsightly would be a challenge. Secondly, if you are talking about a metal 5" pipe, with seams, collapsing could cause faulty seals at the joints, causing leaks, and therefore performance issues. I am with Jim, get square ducts, designed from the gitko to lay flat, and seal properly.
In high school they had what we used to referred to as the Super Suck room, and it was fed by flat duct work. The rectangular ducts went all the way to the floor, with a sweep. You could sweep sawdust from the floor, and just broom it into the collector opening. We were warned not to get nails and other kinds of debris in there though.

[Thom Sturgill]

Actually, it does change the volume of the pipe. For example, if the pipe is round the area of crossection is pi x radius x radius or over 19 sq. in. If flattened to 1" then it is less than 7 sq in.

[Tom Veatch]

... This will not change the volume of the pipe, only the shape.


That's not quite correct. It won't change the perimeter of the pipe, but it will definitely change the cross sectional area of the pipe. A circle has the maximum area for a fixed perimeter. Any other geometric figure with the same perimeter has a smaller cross section area.

Think of it this way. What would be the area of the pipe if it were flattened to the point that the opposite sides touched?

The reduction in area might not be significant for a small amount of flattening, but could become very significant for moderate or severe flattening.

[Wade Gehlhoff]

There is not very much static pressure for a home DC so collapsing even thin walled ducting is not really an issue.

If you consider 26 gauge thin wall then I would agree but,

I installed the newer cv1800 Clearvue with 7" hvac mains and 6" drops. I have to keep a 4" port open along with a 6" vacuum relief to keep the duct from collapsing.(think throttle plate with offset pivot and weighted arm to hold it shut until the vacuum gets too high) I’m pretty sure the shelves were not labeled correctly at the box where I boaught the snap lock. it is definitely thinner than the 26 gauge sheet I have, I’m sure it is 30 gauge. If I start to close the 4" gate the 7" duct starts to go oval everywhere there is a minor defect. based on the current draw I must be well over 1000cfm all the time just to keep the duct from collapsing.

[Rob Blaustein]

Actually, it does change the volume of the pipe. For example, if the pipe is round the area of crossection is pi x radius x radius or over 19 sq. in. If flattened to 1" then it is less than 7 sq in.


That's not quite correct. It won't change the perimeter of the pipe, but it will definitely change the cross sectional area of the pipe. A circle has the maximum area for a fixed perimeter. Any other geometric figure with the same perimeter has a smaller cross section area.

Think of it this way. What would be the area of the pipe if it were flattened to the point that the opposite sides touched?

The reduction in area might not be significant for a small amount of flattening, but could become very significant for moderate or severe flattening.

Tom and Thom beat me to the punch. It is often instructive, as Tom points out, and as the Eagles said, to "Take it to the limit..." Not that it probably matters here though.

[Victor Stearns]

Ok Gentlemen, I stand corrected. The area will be different.
Here's the math.
Area of a 5" pipe = 19.635in^2, (3.1416*2.5^2)
If I want to press the pipe such that the height of the section is 2.5" high, then the area of the new shape is now a composite of three shapes, the first is a 2.5" semi circle then a rectangle and 3rd another 2.5" semi circle. Doing the math reveals that the area is now 14.7312in^2. This is a reduction of 25%. (14.7312/19.635=0.75, where 0.75 = 75%, 100-25=25)
I will have to finish a velocity calculation prior to determining if I can do this.
Thanks
Victor

[John Newell]

FWIW, you can buy rectangular HVAC ducting.