Joe Emenaker wrote:

igor wrote:

.... to a number of people here who say
that bigger is better with ducts. I can see that as a general
proposition, but at some point, relative to the size of the fan, won't
velocity suffer in a significant way?

I was thinking the same thing myself. In one of the downloadable
reviews of dust collectors that are available at WoodStore, they
mention that you need a certain number of feet-per-second of air
movement in order to keep the sawdust suspended. They use that as
justification for why you need "xyz" amount of cubic feet per minute
from your DC.

The first question that ran through my mind was "Why don't you just
decrease the diameter of your hose?". If you just went from 4" to 3",
the velocity of the air inside the hose (provided that your CFM didn't
suffer too much because of it) would go up by 60%!!!

But you'd also get more clogs. 3" is getting down to the shop-vac hose
territory.

Of course, there's also the other extreme. You don't want to hook up
half-inch pipe as your ducting, either, because the CFM will suffer so
much at that point that, even the small diameter probably won't help
the linear velocity of the air (and you'd have the new problem of not
drawing enough CFM from the *tool* itself ...).

Where the magic cross-over point is depends upon the CFM of your DC,
how long and curvy your ducting is, and how resistant to airflow the
inside surface of your ducting is.

- Joe

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--John
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