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Art Greenberg
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Default 4" vs 6" ducting for effective dust collecting

On Sun, 22 Jan 2006 20:07:12 -0700, mywebaccts (at) wrote:
I'm reading "Bill's Place" (Bill Pentz) and his writeups on what does or
doesn't constitute an effective dust collection system. He insists that
4" ducting is woefully inadequate for safe dust collection. He writes
that 6" is what is needed.

His writeup gets fairly technical and sounds very convincing. But of
course, using 6" ducting greatly increases the cost of the ducting for
my workshop.

Are there other opinions on this or do people pretty much agree that 4"
ducting is inadequate?

It depends. The article on Bill's site tries to explain why and when to choose
a particular size. You can't ignore the physics behind what's going on without
reducing efficiency.

Look at the velocity needed to keep the dust suspended in the airstream. If
you don't have that velocity, the dust will settle out in the pipe. Velocity
is a function of CFM and pipe cross-section. FOr a given CFM, making the pipe
larger (larger cross section) will result in lower velocity. At some point,
you end up with a situation where the dust never makes it to the blower.

In vertical runs, gravity is working to reduce velocity as well. So you might
need a smaller pipe in vertical runs to keep the dust moving.

Too small a pipe, on the other hand, introduces a lot of frictional loss, so
the velocity will drop. The result is the same; the dust does not move through
the system.

For a small shop, it is probably not necessary to go through a bunch of
precise computations. Rule of thumb ar a good thing. I used one in designing
my DC system, and its worked very well for me. I think I found this on Bill's
site, but I don't remember so well, and I'm not looking at that right now. But
it said that, for a 2HP/1200CFM blower, 6 inch pipe is about right for
horizontal runs, but a bit too big for vertical runs. So I used 6 inch pipe
everywhere, except for the drops to my machines, where I used 4 inch pipe. I
built my system using S&D PVC, and used 45 degree bends exclusively (2 in a
row where I needed a 90 degree bend) and 45 degree Ys for take-offs.