I'm about to buy a dust collector that has a 4" port. I've read some
posts that suggest not using 4" ducting in the shop but to use 5" or 6"
ducting and then reduce to 4" at the dust collector and at the
What is the point of going from a 4" port on the dust collector to 6"
pipeing? Is this more efficient?
It's all about friction. The friction of the air moving along the pipe
walls slows the air down. The air has to move faster in the smaller
pipe than it does in the larger pipe, so friction losses are greater.
The air can speed up to pass through a short section of smaller pipe
without much loss. The longer the section of small pipe, the greater
the friction loss.
However, you must be careful not to use too large a pipe, or the air
will slow down too much in it and allow the chips to fall out. See
Bill Pentz's very informative web site
"If your only tool is a hammer, you tend to see every problem as a
nail." - Abraham Maslow
Doesn't it also help to prevent clogs where you won't be able to get to
them? In that if it's small enough to fit through the 4" port, then it
should be able to make it through the rest of the system without too
Yes, but...(there's always a "but" :) )
You have to be sure you have sufficent volume capacity in the collector
to retain a minimum linear velocity, not just cfm. For fine dust and
chips, 3000 fpm may be adequate but for a planer or jointer w/ larger
chips, 4000 fpm is nearer a minimum for adequate performance. Remember
area increases by square of diameter, so the linear velocity drops
The basic ideas of wall friction and all in another response are ok,
but note that a single reducer or elbow or wye or tee as the effective
pressure drop of several feet of pipe. If this is a single-stage
collector w/ a single 4" duct, I'd wager anything greater than 5" will
definitely be too big.
The last issue of "Shops and Tools" from Fine Woodworking has a nice
overview where they evaluated volume and linear flow rates from a
variety of collectors and prepared some comparative figures showing
what you can expect from various-sized single- and two-stage collectors
from 1-1/2 to 3+ hp. I assume you may be able to find the article
online at www.taunton.com, perhaps. If not, it would definitely be
worth a trip to the library and if you're serious, probably the few
bucks at the newstand.
I got the specs for the dust collector I'm going to buy. It will only
be running one machine at a time and I have a pretty small shop. The
longest stretch of ducting will be 15'.
It is a 1 HP 600 cfm machine. From that is there a formula I would use
to figure out the correct size of ducting?
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