Dust collection ductwork in short time
I get a fair amount of questions about
dust collection ductwork some from folks
who think that the size of the ducts
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don't matter all and others who think
that the bigger the ductwork the better
both points of view though can lead to a
massive waste of money and loads of
regret I've learned that the hard way
so today I just want to offer a simple
easy to understand explanation of this
one part of the larger dust collection
subject specifically why the size of
your dust collection hoses and ducts do
matter
and what's right for your shop may not
be what you think
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I think this is best broken down into
four factors are volume velocity resistance
and pressure you must balance these four
factors between your ductwork your tool
dust ports and the capabilities of your
vacuum or your dust collector it is very
important to understand this before you
spend any money on dust collection
equipment or ductwork so let's consider
these factors quickly one at a time
beginning with volume
air may be invisible but it occupies
space a balloon for example has to
stretch to create more space as the
the volume of air inside increases a dust
collection hose or pipe can't expand
like a balloon, it can only hold so much
the air within its walls while it's true
that air can be compressed it takes a
lot more pressure to substantially
compress air than a wood dust collector
generally can generate so the only way
to really increase the volume
limitations of your duct sizes
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is to use larger ducts or larger hoses
air volume or airflow is measured in
cubic feet per minute cfm
generally a four inch duct can
accommodate four times the air volume as
a two inch duct and a six-inch duct can
accommodate about twice the volume as a
four inch duct don't forget that the air
isn't the only thing inside your ducts
and hoses
sawdust and wood chips also compete for
that space with the air but before you
go out and you buy the biggest PVC pipe
and hoses you can find
consider velocity
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while it takes a certain volume of
airflow to gather the dust and the chips
up from your tools it also takes a
certain amount of air velocity to keep
those chips suspended and moving through
the airstream on their way to the
collection bin
if the air velocity is insufficient dust
will drop out of the airstream and it
will pile up inside your ducts you might
test this principle by blowing sawdust
off your bench through a drinking straw
then try the same thing except this
time blowing through a larger cardboard
tube like you'd find in a roll of paper
towels the smaller diameter of the straw
obviously restricts the volume of
airflow that can go through it compared
to the larger tube but the velocity of
the air coming out of the smaller straw
is much greater and it moves more dust
to compensate for the larger tube you
would have to increase the volume of air
from your lungs basically blow a lot
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harder to move the same amount of dust
the key points are if the volume of air
is the same you will get more velocity
from a smaller duct
likewise, if you increase the size of the
duct you will need more volume to
maintain the same velocity now let's
talk about resistance
if you look at the specs on your vacuum
or your dust collector you're likely to
see a cubic foot per minute or cfm
a rating that indicates the volume it can
move when the blower is completely
unresisted but in the real world we had
all sorts of resistance to our dust
collection systems so focusing again
just on the ducts and the hoses every
bend and turn every rib in the hose even
the walls of the ducts themselves along
their length add resistance
resistance slows velocity and as i
explained velocity is what keeps the
chips moving
so in our first section, we learned that
volume is closely related to velocity
now we learned that velocity is closely
related to resistance
and in our final factor, we learned that
resistance is closely related to
the pressure it takes more pressure to push
an equal amount of air through a smaller
space than a larger one you might test
this by drinking water on a regular
straw as compared to those little
cocktail straws you have to suck a lot
harder on the ladder and even then you
won't drink as much volume because your
lungs are not capable of generating a
a lot of negative pressure
blower-style dust collectors such as the
big cyclones or the single-stage harbor
freight style ones they produce very
little static pressure so like your
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lungs they struggle more when you
connect them to smaller ductwork and
hoses
vacuum cleaners on the other hand
generate vastly more static pressure and
they work better when you connect them
with smaller hoses and ducts
so we've covered our four points and
clearly, you see you have to find the
the right balance between those four factors
based upon the system you have
take a shop vacuum for example shop
vacuums typically come with two and a
half-inch or smaller hoses for a reason
they're not designed to move enough air
volume to maintain sufficient velocity
through the larger ducts to keep that
dust moving
but they do generate enough static
pressure
to pull a decent amount of volume
through a small hose or a small dust
board on a tool a blower-style dust
collector on the other hand like your
harbor freight dust collectors or your
big cyclones don't generate enough
static pressure to pull enough volume
through a small hose or a dust port to
effectively collect sawdust and chips
from your tool but if you increase the
size of the ducts or the hoses or the
tool ports you remove the volume
restrictions and you dramatically
increase that type of system's
effectiveness
until you reach the system's tipping
the point where the size of the blower can
no longer move enough volume of air to
maintain the velocity required to keep
the dust moving in the ducts
I know this is sounding a little bit
complicated so let me simplify it as
much as possible
big six-inch ducts go best with big dust
collectors
medium four-inch ducts go best with
medium dust collectors
shop vacuums go best with little hoses
and all of them work better as
little added resistance as possible from
excessive duct length bends or
corrugated hoses
anytime you mix up those duct sizes as
you sometimes have to do when you build
a shop dust collection system
you have to deal with the changes in
performance that I just outlined in
those principles so let's apply that to
some real-world examples
here in my shop in the main area I have
a powerful cyclone system with a big
a blower that can move a lot of volumes
so I used large six-inch ducts to
accommodate that volume
some of my tools though have four inch
ports on them I'm obviously not going to
get my maximum airflow through those
four-inch ports
but I have to reduce down to them
still, I use the six-inch on the main
ducts because I do have other larger
connections in the shop and I also have
some tools that have dual four-inch
connections so I need the larger main
duct to handle the combined air volume
from those dual connections in fact with
With a big duct like that, I have to leave two
four-inch lines open at all times
because I need to get enough airflow
passing through the six-inch duct to
maintain the velocity I need to keep the
dust moving and the ducts clear
in the corner workshop over here I have
a harbor freight dust collector that has
two four-inch ports on the bottom but
they're not meant to be open at the same
the time it's a smaller system that moves
significantly less air not enough to
maintain the velocity I need to keep six
inch ducts clear
if I were to connect that to some duct
work it would have to be a four-inch pipe
and I couldn't add too many bends or
much flex hose that would further resist
its modest airflow
because both the harbor freight single
stage system and the big Clearview
cyclone system
are blower type dust collectors they
generate low static pressure so I avoid
reducing them down to really small hoses
and dust ports if at all possible the
the exception to that is on the router
tables where you have a four-inch port
on the bottom that tees off to a two and
a half-inch on the top I do use those
but that's out of convenience frankly a
shop vacuum would be better for
that upper fence connection and it would
certainly, be better for a small handheld
power tools like sanders because
shop vacs have high static pressure that
can overcome the volume restrictions to
a greater extent from those small ports
then the dust collectors can
of course, this just scratches the
the surface of shop dust collection there
are many more things to consider and
I've made a lot of videos on this
subject I'll link to some of them below
see you next time
we couldn't do what we do without folks
like the bushy family over at Clearview
cyclones not only do make the best
dust collection cyclones on the market
but they're woodworkers supporting
woodworkers, they're the sponsor of this
video and I really hope you'll thank
them by visiting