explanation to go into all the factors and formulas needed to properly
design a dust-collection system, here’s a basic rundown on the rela-
tionship of duct diameter to air volume, velocity, and friction losses
that occur as air travels through a duct.
Dust collectors are rated by the volume of air they can move (mea-
sured in cubic feet per minute, or cfm) while overcoming the friction
of the airflow in the duct work and in the blower itself (stated as static
pressure, or sp, measured in inches of water). Therefore, a collector that
is rated to move 1,200 cfm with no friction (0 sp loss) might only
manage 100 cfm or so when the friction’s up to, say, 8 in. of sp loss.
Now, given the same collector hooked up to two different-diameter
ducts, the air would flow faster through the smaller one. This is
because air velocity increases as duct diameter decreases—think of
how much faster a stream of water flows as it passes through a narrow
garden-hose nozzle. If you want to move a large volume of air
through a small-diameter duct, the air would have to travel at a very
high speed. In your situation, to move the amount of air required to
collect the dust from an average tablesaw (350 cfm is typical), the air
would have to travel at around 9,000 feet per minute, or fpm. This
is more than double what is normally recommended for wood-
working dust-collection systems, which is 3,500 fpm for main ducts
and 4,000 fpm for branch ducts.
The reason that the air would have to travel so fast is that it would
have to overcome a huge static pressure loss (lots of friction). Static-
pressure losses increase as the diameter of duct decreases. And sp losses
are cumulative over distance: the longer the duct (and the more bends
and junctures it has), the greater the friction and, therefore, the higher
the sp losses.
SHOP SETUP