W Canaday wrote:
spend the extra few bux and go 6" - it will make a world
of difference.


How? The impellers can only move 'x' amount of air in a given time. This
is a volume measurement. A larger pipe 'consumes' more of that volume per
linear unit and this would reduce velocity all by itself. But there would
seem to be a more important matter ... that of the neck and impeller
dimensions. Even if I make the main run 6", I am still constrained by
that, aren't I?

Or am I?

Bill

Bill, you are making some incorrect assumptions. You are assuming that
the impeller is moving all the air it can move, but that's almost never
true. Friction of the air moving against the walls of the pipe slows
down the air so that the impeller is spinning its wheels so to speak.
But you are right that it is possible to size your pipe so large that
the velocity falls so low that the heavier chips fall out and aren't
moved.

The air can speed up to pass through short sections of a smaller
diameter pipe like the throat of the DC.. But if forced to travel
through long sections of that same smaller diameter pipe, the friction
losses against the sides of the pipe rob precious static pressure. And
with vacuum systems, we only have a little static pressure to work
with, so it's important not to throw it away.

Follow this link to Bill Pentz's place. I don't buy quite everything
he says about the grave danger of airborn dust, but he certainly seems
to understand and communicate the science (art?) of collecting dust.
http://billpentz.com/woodworking/cyclone/Ducting.cfm

DonkeyHody