Once again I'm fitting another device for dust collection. And once
again I'm irritated by the fact that nothing fits without an adaptor.
Seems to me all that should be needed is:
4" male connection
4" female connection
2.5" male connection
2.5" female connection
All hoses should come with female on one end and male on the other.
All dust collection devices should accept a male connector.
All dust producing devices should accept a female connector.
Add the following adapters:
2.5" male to 4" male adapter
2.5" male to 4" female adapter
2.5" female to 4" male adapter
2.5" female to 4" female adapter
4 more adapters, but elbows instead of straight.
Wouldn't that make life a lot simpler?
Maybe I'll send this post to a couple of the woodworking magazines :-).
When fascism comes to America, it will be wrapped in the flag and
carrying a cross.
Hadn't seen the reply, sorry...
Not huge altho my experience has been that the static buildup tends to
cause small stuff to stick to the surface which can't help.
The static buildup in small shop settings at least is mostly just a
nuisance factor that I dislike simply for the effect. It is virtually
impossible to ignite a wood dust cloud with it as the ignition source;
while pretty high voltage and zap when you're the target, it has very
low actual energy. After the other thread I did a little looking but
wasn't able to find in a short period the paper I'd seen a number of
years ago where a guy at T A&M did some discharge measurements,
I was wondering more about the airflow resistance increase, though I
can see where dust sticking to joints wouldn't help. Is there a
significant air resistance difference between "clean" metal and
Yeah, though others may disagree, I think this horse has been beaten
near enough to death.
For straight pipe, no. Where I was headed was that the fittings (el's,
wye's, etc.) associated with PVC are much tighter radius (even "long"
el's) than those used in "proper" dust collection and handling systems
and a single elbow is some 5X the dP of 100 ft of straight pipe whereas
for the metal ducting with 1.5 radius it's more like 1.5-2.0X. That
differential will more than make up for whatever minor improvement the
straight PVC might have over the galvanized.
I've got quite a bit of data and design stuff from various sources but
haven't ever actually put it all together as a easy-to-use pattern w/o
being intimate w/ the details of what the various pieces mean -- stuff
like which friction-factor formulation goes with which pressure drop
correlation, etc., etc., ...
Since I'm in the process of setting up a new area in the barn and will
have to do at least a modicum of this shortly, maybe this will be the
impetus to try to consolidate some of that to a more "handbooky" kind of
The upshot is, however, that you need to keep the sizes such that for
whatever CFM you have the linear velocity for branch lines of roughly
4000 fpm and 3500 fpm or so for main ducts. This will ensure won't have
issues w/ drop out and so on. If the chip sizes are small or dust like
a sander, you can get by w/ somewhat less; the above are targets for
that include planers, shapers, routers, etc., that make a fair-sized chip.
Just for info, though, the difference in cfm required for 3500 fpm is
As you note, it goes up by the ratio of cross-section areas, obviously.
The fortunate thing is, of course that the percentage increase in area
decreases from step to step in uniform diameter increments so the
biggest jump is from 3" to 4"; 4" to 5" isn't quite as painful.
Hopefully that helps..
This link might save you some time and trouble.
One thing you did not mention was the inches of water column necessary
for the flow. This will help you with the sizing of the DC needed, or
if done in reverse what you can due with what you have.
Just didn't seem pertinent to the question asked...
Static Head ("in H20) for given diameter at linear flow velocity...
Diam 3500 4500
3″ 7.5 10.0
4″ 5.5 7.0
5″ 4.2 5.5
6″ 3.5 4.5
7″ 2.8 3.8
8″ 2.4 3.2
To me it does, for how else can you determine the flow in a given size
of pipe? If the DC is undersized then the negative static pressure
will be less as will the CFM and FPM. I was hoping that the pdf I
linked to would help you both. It also confirmed some of what you were
Of course, but KRW wasn't trying to do a size computation per se, wo was
simply pointing out that he does, for the DC he has, need to look at
what the effect of his thought of using a given size of ductwork would
do to the resulting linear velocity. Have insufficient data available
to do anything more.
Oh, I see that's the technical data from Air Handling Systems; that's
one of the many pieces and does walk thru a process...being an engineer,
I'd intended to automate it and use the actual correlations behind their
tables instead...I get fixated on the details _behind_ the big picture. :)
That helps a lot. Thanks guys. I'll have to digest this some more.
Maybe I don't have the problem I thought I did and can use a longer
run (with blast gates). Do you happen to know of an easy way to
measure the flow rate?
You can make a U-Tube Manometer. It is the cheapest way I know of,
otherwise you can buy one, at an HVAC supply house or WW Graingers.
Measure the inches of water negative pressure, take the inside
diameter of the pipe you measured and compare it to a FPM chart on one
of the links I provided. It will require that you drill a hole to
connect the U-tube to, with no leakage while connected. I can find you
a how to on it if desired.
Easy in theory, the effort to measure it well enough to be meaningful
isn't trivial...doable, yes, but must be done w/ care. If you're
serious, read Bill Pentz's stuff...
Generally you can get satisfactory results by the methods outlined
earlier to estimate pressure losses _providing_ the manufacturer of the
DC gives realistic data. That may or may not be a reasonable assumption
That is, computing pressure drops is pretty basic and reasonably
accurate from which the requirements can be estimated. Then you're
relying on the stated capacity of the chosen DC to actually match what
it performs at--if they'll supply an actual head curve other than just
total no resistance CFM you've got better chances the data are valid.
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