# 4" vs 6" ducting for effective dust collecting

On Sun, 22 Jan 2006 20:07:12 -0700, mywebaccts (at) PLUGcomcast.net wrote:

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.
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Art

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Art Greenberg wrote:

Yeah, I was about to point out: it's very possible to have pipes that are too big for your blower!
If the blower is pulling the same volume of air, the airspeed in a 6" pipe will be half that of the airspeed in a 4". If you don't have a big enough blower, then that can be a problem.
- Brooks
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The "bmoses-nospam" address is valid; no unmunging needed.

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mywebaccts (at) PLUGcomcast.net wrote:

What about fine dust escaping the bags or filtration material? Seems that a 20" hose wouldn't help that issue. There's gonna be SOME fine material floating around. Wearing a mask is the prudent and least expensive solution. I'm not about to over engineer some esoteric DC system when I've next to no problem with my conventional and affordable system using a 20' 4" hose and 1.5 HP motor equipped DC. It's "good enough". Now someone with a HUGE shop would be wise to install some gawd awful expensive cyclone system with automatic blast gates and the whole nine yards and put the unit outside the shop in an enclosure. There's a wide range of shop size and sophistication. there's hobby and there's semi-pro, and pro, and production...
Dave
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