Ummm...yes, you can ground an insulator. Many people working in dry climates
have been zap'ed from the generated static, and they've used wire or other
methods to ground it out. Try using a comb or glass rod to generate static,
then discharge one end.. You'll find there's little if any static charge left
on the rest of it. Static charges do not propagate with the same mechanism as
an electric current in a conductor.
For static buildup on pvc you can:
1- Move it out of reach.
2- Wrap a wire (very small; current is low) around and ground it.
3- Spray lightly with conductive paint and ground one end.
Well, some believe that the PVC can accumulate static electricity,
then discharge (by means of a little electrical arc) and ignite the
dust, thereby causing your own rendition of a grain silo explosion.
These are probably the same people who turn off their cell phones when
the fill up their cars with gas to prevent the deaded "cell-phone gas
station explosion"; because... well... gosh... even though we've never
tracked down a documented/verified case of it happening, it *must*
happen a lot because everyone talks about it, right?
Ugh. That's it! I'm submitting this one to MythBusters
To answer your question, the May 2003 issue of American Woodworker
(page 54) has a section on setting up DC in your shop. The guy
suggests using steel instead of PVC for a variety of reasons. *One* of
the reasons is static buildup... but even then, the guy doesn't say
that there will be an explosion. He merely talks about the zap you can
get *yourself*. The other reasons are that steel comes in larger
diameters than the 4" where PVC generally stops at.
Strangely, he didn't mention how great steel looks when you
accidentally bump a tool into and dent it... or how yummy it feels
when you slice your hand on the sharp steel ends... or how the
sheetmetal screws protruding into the tube help the airflow!
He does point out though that, at large diameters, the price
difference between PVC and steel starts to become a non-factor in the
Issue 13 of ShopNotes has an article about building your own cyclone
and collector. The seem to use all steel.
I recall reading some article about a week ago where the author went
into 4 more-likely fire hazards in your DC system than static
electricity. One of them was hitting a nail (or other metal) which can
cause red-hot sparks to go wafting through your DC system to the
collection bag. There were a couple of others. If I track down that
article, I'll come back and post it here.
On 12 Jul 2004 21:50:11 -0700, email@example.com (Joe Emenaker) wrote:
snip post about how pvc isn't a static explosion hazard, but metal
duct is better anyway.
wouldn't metal duct (or more likely the sheet metal screws) present a
lot more opportunities for bits of hard stuff like chips of metal or
pebbles from the shop floor to strike sparks? of course the big one is
still gonna be the impeller blades themselves....
After the zinc is gone, perhaps.
One more reason for dropping the shavings before stuffing dust through the
Question, isn't duct tape good enough, that folks use sheet metal screws?
(Joe Emenaker) wrote:
FWIW, I highly recommend metal tape versus duct tape - at least from the
standpoints of being a much better seal and the tape not delaminating from
the adhesive layer after some time. The metal tape is more expensive. But
a regular HVAC duct with metal tape at the seams and the joints, if you
want, is darn "airtight" in this context, IMO. The metal tape won't handle
bumps and such as well, so in some places you could use duct tape over the
metal tape. -- Igor
I found it!
Fine Woodworking's "Tools & Shops" issue Winter 2001/2002.
On page 48, Rod Cole (a MIT professor who's office is next an MIT prof
who happens to be an expert in the physics of lightning), wrote an
article called "PVC Pipe Dangers Debunked". He makes reference to an
even more exhaustive report on the web. I searched Google for "Rod
Cole PVC" and found this:
Yeah. I'm expecting a Delta 1.5hp DC for my birthday next month, and
so I'm doing a lot of reading on how to "duct-up" the shop. Almost all
of the articles seem to be in agreement on the following principles:
o 4" is good. 5" is gooder. 6" is gooderer.
o Try to avoid any hose/tubing that's not smooth inside.
o Try to avoid tight turns (ie, use 45-degree bends instead of
o For the same reason to avoid tight turns, avoid right-angle "T"
joints and opt for the "fork-in-the-road" style "Y"s.
I'm taking so much of this as religion that my current dillema is
this. I'm also planning on building the home-made cyclone in
ShopNotes, but its outlet is at the top and the DC that it feeds into
is going to have its inlet either waist-high or at the floor. So, I'm
debating either raising the DC onto a platform or modifying the
cyclone design so that I can get a more-or-less straight shot across
from the cyclone exhaust to the DC intake.
On 15 Jul 2004 15:19:48 -0700, firstname.lastname@example.org (Joe Emenaker) wrote:
This comment is not to you so much as 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? And, doesn't velocity have a role to play in an
effective DC system? After all, we are not just talking about air -- also
talking about moving solid waste. Or, as is sometimes the case, am I
missing something here? -- Igor
You are actually right on target. Most of the various schemes I've
found for plumbing a single user home shop seem to optimize around a 1
1/2 to 2 HP collector and five inch pipe. Four inch pipe is too
restrictive, six inch pipe slows the velocity too much. So I tend to
get frustrated with all the woodworking suppliers that stock only four
inch pipe, hose and fittings.
So until I can find a big enough crowbar for my wallet, I am still
dragging around the forty feet of four inch flex that came with my
Penn State collector.
But if your running a one man shop, and running only one machine at a
time, does it really matter if the main runs are 4" or 5"? I've got a
delta 1.5 HP DC in my garage shop. I use 4" PVC sewer and drain pipe for
the main runs and flex pipe to the machines. The delta has 2 4" mains
coming off it so I ran 2 runs on each side of the shop. I don't have any
problems with airflow even to the farthest machine, which is a planer.
The PVC was cheap and easy to work. I also didn't bother to ground it
although is does develop some static when I run the planer or jointer.
But I look at it as a dust filter. The suspended dust in the shop sticks
to the pipe. I just vacuum it once in a while.
Just my 2cents.
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%!!!
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.
Perhaps someone can point out the error of my thinking on this subject...
The system can only flow as much as the smallest port in the factory
design. Take my Jet 1.5hp for example, what I'm getting at is that the
port and hose from the blower housing to the bag hanging ring is, I
believe, 5" diameter. To my thinking whatever size of the system outside
of the factory setup is limited by this 5" - in other words, one can't
fully draw 6" of main trunk air before the blower through a 5" hose
after the blower - therefore the appropriate size of the main trunk
should be no larger than 5" - or whatever the size of the smallest port
in the manufactured assembly.
Has to do with velocity. I'm sure others can explain it better. Let's say
your blow has a true capacity of 1000 cfm. It will take in and blow out
1000 cubic feet every minute. If there is no duct at the entrance it will
suck air from any place in the room it can. As you get closer to the blower
you will feel the air moving. The more you restrict the opening, you will
feel the air moving faster. The blower sucks in the air and puts it into a
smaller outlet space and thus adds more force to the air and it moves faster
on the way out. Ducting allows you to concentrate and "aim" the point of
The blower can move 1000 cfm, but your compressor can make only 4 or 6 cfm,
but when you glow that amount of air through a small nozzle, it feels like a
lot more pressure than the outside of a fan or blower moving much more air.
If the blower did not have enough velocity, the dust would just fall on the
other side and not get moved into the bag. A window fan of the same
capacity with not ducting will move little dust by comparison because it h
as the cfm capacity, but at a much lower velocity.
Italian fellow name of Bernoulli, I believe, has some good words to say on
Consider the original force per unit area I mentioned. That's where the
term PSI comes in. You can haul more air through a larger pipe, but the
pressure drops, because you're not capable of real compression through the
open sides of the impeller. This means that what's being carried along with
the air will also drop. Reverse is also pretty true. Take your 4" hose,
as I often do, and use a standard shop-vac adapter to 2", and notice you can
pick up pencils, chunks of scrap, and even the bolt you dropped, and were
looking for. Don't be frustrated and think you'll have to rummage through
the cyclone, those things are just upstream of the adapter, if they made it
that far, where there is no longer enough force/unit to carry them into the
bin. I rely on this when looking for dropped objects in my shop.
As mentioned, the "standard" unit now moves 1200/CFM at (some PSI) or in
reality, at some vacuum, measured in feet of water, inches/millimeters of
mercury or furlongs per fortnight. Now since the old 650 CFM @ 8 types were
the standard which spawned the 4" hose, I'm speculating that a 5" hose may
be best for the 1200, because the impellers are still pretty leaky, if you
read the mfrs specs. A 6" hose, as mentioned, would be 2 1/3 or so times
the area of a 4, negating the additional chip-carrying power.
Oh yes, don't ask about 2" hose and 2" sanding discs for the lathe. Makes
me veeery angry.
This is, well, to be blunt, kind of gobbletygook. Well, the conclusions
are more or less valid (big pipe => high flow, low speed), but the
physical explanation is not correct.
In short, larger pipes can pass more air because the wall friction per
unit length of pipe is less (because the airspeed is lower). Less
friction means the pressure losses in the pipe are less and the impeller
has less head to work against. Since it's working against less head, it
can pull more air.
The lower airspeed leads to less friction (on the dust particles) and
less turbulence, which allows dust to settle out.
(Of course, there are a few caveats involved above, but for practical
purposes, this is the basic principle).
For those who care, there are many references to explain fluid flow in
pipes and DC's; I think the FAQ has some decent references. I wrote a
primer once, and if I ever get some web space again, I'll gladly post it.
By the way, Bernoulli wasn't Italian. He was Swiss. And Bernoulli's
principles aren't really valid in this context (duct flow) because the
viscous forces are too large.
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