I was reading recently about the idea of using 4" ducts for vertical
portions of your dust system when you otherwise use 6". What if you have a
significant amount of duct work between the vertical run and the machine?
After my main drops straight down 3 feet to the floor, I must run another 6
to 8 feet to my table saw. Should I run that portion in 4" as well or
should I go back up to 6" inch?
Actually, what you should for the secondary horizontal run is run the 4"
*inside* the 6". You might think that would add up to 10", but if you work
the numbers out, the area is equivalent to about 7.2" diameter. Or you
could just run everything from the main branch as 4".
The idea behind this is that for some combinations of duct length and
dust collector power, the airflow is enough to suspend the dust in 6"
horizontal sections, but the airspeed in the vertical sections was just
not fast enough to keep all the dust suspended.
By going to 4" duct for the vertical sections, the airspeed is increased
and the dust is kept suspended.
If this is similar to your particular scenario, then you would do best
to go back up to 6" for the horizontal run.
It all depends on your specific setup, how long the runs are, how much
airflow the dust collector gives you, etc.
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