I have used a single 50 flex 4" diameter flex dust collection tubing
(with an internal steel grounded wire) for all my machines for the
past 15 years but now it is wearing out with many small holes. I have
a 1.5 HP DC with a trash can cyclone. Are there any recommendations
for replacement? I was looking at T-7 Santoprene tubing which is
supposed to be good at abrasion and contains a steel wire which I can
ground. It is about $4.50 a foot. Is this a good choice/value?
"Edwin Pawlowski" <[email protected]> wrote in message
news:[email protected]...
>
> >
> Where I work, we move 20,000 pounds of material a day through 4" pvc.
> Probably half of our industry does. I use pvc in my shop with good
> results. Just emptied the DC last night, in fact.
> Ed
> [email protected]
> http://pages.cthome.net/edhome
>
>
You are still alive?You should have blown up long ago!! ;-)
Greg
This may be of interest,check it out, I have it and like it a lot!
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=47911
Good luck
Jim
"Edwin Pawlowski" <[email protected]> wrote in message
news:wdjIc.3209$m%[email protected]...
>
> "Phisherman" <[email protected]> wrote in message
> news:[email protected]...
> > I was looking at T-7 Santoprene tubing which is
> > supposed to be good at abrasion and contains a steel wire which I can
> > ground. It is about $4.50 a foot. Is this a good choice/value?
>
> Rigid PVC for the long runs is a hell of a lot cheaper. Use the flex for
> connections and drops to the machines. .
>
>
No, the system is not limited by the narrowest pipe. It's not a "weakest
link" analogy. The reason is that the speed of the air will vary
inversely to the area of the duct. So the 4 inch restriction will just
accelerate the air. There is some (small) loss with restrictions, mostly
because it is easy to accelerate flow efficiently but hard to slow it
down.
Others have properly noted the fact that while large ducts can pass a
lot of air, the speed of the air drops so that dust can settle out in
the pipe.
Greg
Fly-by-Night CC wrote:
> In article <[email protected]>, "George" <george@least>
> wrote:
>
>
>>Not an engineer, but imagine the optimum transport pipe is probably ~5".
>>Force/unit area calcs show 6" less than half the four.
>
>
> 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.
>
> Wadya think?
>
On Tue, 13 Jul 2004 07:06:04 -0400, "George" <george@least> wrote:
>After the zinc is gone, perhaps.
>
>One more reason for dropping the shavings before stuffing dust through the
>impeller.
>
>Question, isn't duct tape good enough, that folks use sheet metal screws?
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
On Mon, 12 Jul 2004 01:30:11 GMT, "Mark L." <[email protected]>
wrote:
>Edwin,
> One of the recent wood mags (not sure which one) says PVC is not good
>for DC. Just can't remember why not. I know it's easy to just glue up
>and you're ready, but I would like to know why PVC won't work.
> Mark L.
>
it will explode....
<GD&R>
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.
GerryG
On Mon, 12 Jul 2004 10:38:00 -0700, Wes <n7ws@_yahoo.com> wrote:
>On Mon, 12 Jul 2004 15:21:40 GMT, "patrick conroy"
><[email protected]> wrote:
>
>|
>|"Mark L." <[email protected]> wrote in message
>|news:DQlIc.3236$m%[email protected]...
>|
>|> and you're ready, but I would like to know why PVC won't work.
>|
>|
>|Remember that guy who strapped a JATO to his Chevy Impala out in AZ? He was
>|a woodworker too, had ungrounded PVC dust collection in shop. It blew up one
>|day.
>|
>
>Folks, PVC is an insulator. You CAN NOT GROUND an insulator. If you
>could, you would not be reading this message because none of our
>computers (or anything else electronic) would be working.
>
>Repeat after me:
>
>"I cannot ground PVC"
>"I cannot ground PVC"
>"I cannot ground PVC"
>"I cannot ground PVC"
>"I cannot ground PVC"
>"I cannot ground PVC"
>"I cannot ground PVC"
>"I cannot ground PVC"
>.....
After the zinc is gone, perhaps.
One more reason for dropping the shavings before stuffing dust through the
impeller.
Question, isn't duct tape good enough, that folks use sheet metal screws?
<[email protected]> wrote in message
news:[email protected]...
> On 12 Jul 2004 21:50:11 -0700, [email protected] (Joe Emenaker) wrote:
>
> 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....
How many places per foot do you have to anchor the ground wire to the PVC to
dissipate the charge?
Hint. This is a trick question.
"Bob Brogan" <[email protected]> wrote in message
news:[email protected]...
> In article <[email protected]>,
>
> Yes, PVC is more a dielectric. That said, one puts wire through the
> ducts so that it "collects" the charge that is generated by the movement
> of the air/wood through the duct. You ground the wire to dissipate the
> "static charge".
>
> So while PVC doesn't "conduct", it can generate "Static electricity"
> under the right circumstances. That is what you're collecting and
> grounding.
> --
Precisely. Which, I suppose, is tantamount to saying the wire is
ineffective.
My 4" hose has a wire helix inside, and it grows dust in the winter.
Check the tables here for some good info.
http://www.esdsystems.com/training/staticgeneration.htm
<[email protected]> wrote in message
news:[email protected]...
> On Tue, 13 Jul 2004 14:39:24 -0400, "George" <george@least> wrote:
>
> >How many places per foot do you have to anchor the ground wire to the PVC
to
> >dissipate the charge?
> >
> >Hint. This is a trick question.
>
>
> an infinite number.......
I wouldn't worry. After making the way clear (and, as noted, maintaining
pressure) for dust and chips on the way to the cyclone, it's only air you'll
want to move to the impeller. It's a fluid, not a solid.
"Joe Emenaker" <[email protected]> wrote in message
news:[email protected]...
>
> 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.
>
> - Joe
Not an engineer, but imagine the optimum transport pipe is probably ~5".
Force/unit area calcs show 6" less than half the four.
"Joe Emenaker" <[email protected]> wrote in message
news:[email protected]...
> igor <[email protected]> wrote:
> >
> > .... 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?
>
> 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.
>
> - Joe
Italian fellow name of Bernoulli, I believe, has some good words to say on
the subject.
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.
"Fly-by-Night CC" <[email protected]> wrote in message
news:[email protected]...
> In article <[email protected]>, "George" <george@least>
> wrote:
>
> > Not an engineer, but imagine the optimum transport pipe is probably ~5".
> > Force/unit area calcs show 6" less than half the four.
>
> 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.
>
> Wadya think?
Well, fluid dynamics was not my prime concern. The concern was with the
"carry", which of course is related to the flow rate. You are concerned
with the fluid, I with the solid, which, at least to me, is the reason for
having a collector, not to move air around.
"Bernoulli's principle can be explained in terms of the law of conservation
of energy (see conservation laws, in physics). As a fluid moves from a wider
pipe into a narrower pipe or a constriction, a corresponding volume must
move a greater distance forward in the narrower pipe and thus have a greater
speed. At the same time, the work done by corresponding volumes in the wider
and narrower pipes will be expressed by the product of the pressure and the
volume. Since the speed is greater in the narrower pipe, the kinetic energy
of that volume is greater. Then, by the law of conservation of energy, this
increase in kinetic energy must be balanced by a decrease in the
pressure-volume product, or, since the volumes are equal, by a decrease in
pressure."
Will you go this? Lower vacuum (large pipe), pieces drop - higher vacuum
(narrower pipe) , pieces move.
"G. Lewin" <[email protected]> wrote in message
news:[email protected]...
> 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.
Kinetic energy, as stated. Seems that demands some consideration of mass
or force.
I think Owen already realizes that air through a tube is not the same as
trying to put 3# of the proverbial solid into a 2# bag, which answers his
question. So here's my question. If I've a 4" flex hose (standard), and
the current "standard" 1200CFM @ 11 ft of water static pressure impeller,
what percentage of my potential chip-carrying energy will I lose between
equal lengths of 6,5, or 4" inside diameter transport pipe? I figured it
would be in approximate proportion to the difference in cross-section. So
or not?
"G. Lewin" <[email protected]> wrote in message
news:[email protected]...
> Yes, the pressure and flow rate do change (and you can use Bernoulli's
> principle on a limited basis at the junction) when you change duct size.
> But pressure is just a means to an end (in that pressure differences are
> what move the air, of course). It is air speed that is responsible for
> carrying the particles (turbulence and particle friction, in
> particular). So when you say "Lower vacuum (large pipe), pieces drop" it
> should really be "Lower speed...".
OK, pretty much as advertised. Lower velocity (sqroot) lower the pull, I
guess.
"G. Lewin" <[email protected]> wrote in message
news:[email protected]...
>
> If you have ONE system, with both 4" and 5" ducts connected in series,
> obviously the mass flow rate is the same in each duct. Since the volume
> changes little at these pressure differences, the volumetric flow rate
> is nearly unchanged. Then the airspeed will go almost exactly like
> 1/AREA for each section of pipe. Of course, the 4" will cause greater
> pressure losses; for equal sections of pipe, the narrower pipe will be
> more "lossy." Geez, I wish I could say how much; off the top of my head
> I think pressure loss goes like 1/RADIUS^3, but don't quote me on that.
> When all my textbooks get out of "storage" (read: the moving van blew
> its transmission), I can look it up.
>
> When it comes to "chip carrying energy," if you mean kinetic energy,
> well, you know how to find that. If you mean "chip carrying _ability_,"
> we'll have to define ability first. Good luck on that one. The best I've
> seen is a relationship between airspeed and maximum particle size, but I
> can't remember where I saw it. I seem to remember 3000 ft/min. is a good
> rule of thumb for wood dust, chips, and fingers.
>
> Greg
George wrote:
> So here's my question. If I've a 4" flex hose (standard), and
> the current "standard" 1200CFM @ 11 ft of water static pressure impeller,
> what percentage of my potential chip-carrying energy will I lose between
> equal lengths of 6,5, or 4" inside diameter transport pipe? I figured it
> would be in approximate proportion to the difference in cross-section. So
> or not?
Hmmm...not quite sure on your question, so I'll answer it two ways:
If you have two otherwise identical systems, one with say 4" ducts and
one with 5" ducts, the airspeed will go [to a rather gross first order]
like 1/AREA. With the reduced resistance of the 5" duct, however, that
system will have a higher flow rate, and so the airspeed will be higher
than said 1/AREA back of the envelope analysis. By how much depends on
many factors, as you well can guess, including the impeller design,
roughness, duct layout, etc.
If you have ONE system, with both 4" and 5" ducts connected in series,
obviously the mass flow rate is the same in each duct. Since the volume
changes little at these pressure differences, the volumetric flow rate
is nearly unchanged. Then the airspeed will go almost exactly like
1/AREA for each section of pipe. Of course, the 4" will cause greater
pressure losses; for equal sections of pipe, the narrower pipe will be
more "lossy." Geez, I wish I could say how much; off the top of my head
I think pressure loss goes like 1/RADIUS^3, but don't quote me on that.
When all my textbooks get out of "storage" (read: the moving van blew
its transmission), I can look it up.
When it comes to "chip carrying energy," if you mean kinetic energy,
well, you know how to find that. If you mean "chip carrying _ability_,"
we'll have to define ability first. Good luck on that one. The best I've
seen is a relationship between airspeed and maximum particle size, but I
can't remember where I saw it. I seem to remember 3000 ft/min. is a good
rule of thumb for wood dust, chips, and fingers.
Greg
"Mark L." <[email protected]> wrote in message
news:DQlIc.3236$m%[email protected]...
> Edwin,
> One of the recent wood mags (not sure which one) says PVC is not good
> for DC. Just can't remember why not. I know it's easy to just glue up
> and you're ready, but I would like to know why PVC won't work.
> Mark L.
Because people will lead you to believe that if you use non-grounded plastic
there will be an explosion bigger than the H-bomb.
Where I work, we move 20,000 pounds of material a day through 4" pvc.
Probably half of our industry does. I use pvc in my shop with good
results. Just emptied the DC last night, in fact.
Ed
[email protected]
http://pages.cthome.net/edhome
When I'm lucky (wealthy?) enough to have a two-bag DC, I'll let you
know. OK, really, there are a lot of caveats that are important in
practice, and not quite knowing what you're describing, I'll just chalk
it up as "it's quite possible."
There is one thing I'd like to point out and that is that the reason a
shopp-vac has much higher static pressure is that the impeller speed is
much higher. Pressure rise at zero flow goes something like [rotation
speed * radius]^2 (I think--again, don't quote me). Despite the larger
diameter of DC's, the high speed of the shop-vac is more than enough to
compensate. Obviously, when there is airflow, things change, but you get
the idea.
Greg
GerryG wrote:
> Greg, while I agree with your statements, per se, I'd like to toss in one more
> item. Specifically, the intake bypass in a 2-bag DC. We have a single fan
> (impeller), and if the air line to that was fully (or even mostly) blocked for
> some reason, the upper bag would collapse. To avoid this, there appears to be
> a partial intake bypass. The air movement would then split between the main
> duct and the bypass by the relative resistance of the two paths.
>
> Now, I imagine a pressure limit valve could be used in the bypass, but I doubt
> they do this.
>
> Haven't seen this mentioned before in discussions. But it explains why a 2-hp
> DC cannot match the static vacuum of even a medium shop vacuum, no matter how
> much you restrict the opening. It would also impact some of your conclusions
> (by degree, not type), in that moving from a 5- to 4-inch hose would be worse
> than expected since more air would flow though the intake bypass.
>
> Does this make sense, or am I missing something?
> GerryG
>
> On Mon, 19 Jul 2004 08:28:54 -0400, "G. Lewin" <[email protected]> wrote:
>
>
>>No, the system is not limited by the narrowest pipe. It's not a "weakest
>>link" analogy. The reason is that the speed of the air will vary
>>inversely to the area of the duct. So the 4 inch restriction will just
>>accelerate the air. There is some (small) loss with restrictions, mostly
>> because it is easy to accelerate flow efficiently but hard to slow it
>>down.
>>
>>Others have properly noted the fact that while large ducts can pass a
>>lot of air, the speed of the air drops so that dust can settle out in
>>the pipe.
>>
>>Greg
>>
>>
>>Fly-by-Night CC wrote:
>>
>>>In article <[email protected]>, "George" <george@least>
>>>wrote:
>>>
>>>
>>>
>>>>Not an engineer, but imagine the optimum transport pipe is probably ~5".
>>>>Force/unit area calcs show 6" less than half the four.
>>>
>>>
>>>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.
>>>
>>>Wadya think?
>>>
>
>
>
It's not Bernoulli mainly that factors into this, it's Boyle.
Pressure dynamics is the same whether it's for gas, liquid or even traffic
patterns.
Reduce the size of the pipe, duct or road and you increase the pressure and
reduce the velocity.
So the idea that a reduction at one point (be it the smaller pickup at a saw or
a roadblock in the middle of the road), the pressure increases, the dust, car,
water, whatever slows, but then, as the pressure decreases with the increase in
the roadwork, the speed increases.
The traffic analogy was not mine, but worked out by some highway engineers.
They were surprised to learn that traffic flow basically obeys Boyle's law.
Which is why you want large main ductwork, this is your freeway. The smaller
gates are your on ramps.
The speed cannot be the same throughout. Just as traffic picks up after a
slowdown. Sometimes when you hit traffic and then it speeds up, you wonder why.
Well, there was a stoppage a while ago, and the system is simply recovering. It
does not stay slow the entire way.
On Thu, 22 Jul 2004 11:39:58 -0400, George <george@least> wrote:
> Nope, they behave as masses acted upon by outside forces.
>
> The fluid is a lube to reduce friction.
I'm not sure that that's how a suspension behaves. "lube" would indicate
that it forms a film between the thing being transported, and the plenum
it's being transported in. Seems to me you're moving both the air
_and_ the sawdust suspended in the air.
Nope, Newton.
We're moving solids, hopefully. That's Newton. Thus the concept passage
cited.
"DarylRos" <[email protected]> wrote in message
news:[email protected]...
> It's not Bernoulli mainly that factors into this, it's Boyle.
>
Nope, they behave as masses acted upon by outside forces.
The fluid is a lube to reduce friction.
"Dave Hinz" <[email protected]> wrote in message
news:[email protected]...
> On Thu, 22 Jul 2004 10:21:13 -0400, George <george@least> wrote:
> > Nope, Newton.
> >
> > We're moving solids, hopefully. That's Newton. Thus the concept
passage
> > cited.
>
> Solids suspended in air perform as a fluid, do they not?
>
Chunks, man, think chunks.
"Dave Hinz" <[email protected]> wrote in message
news:[email protected]...
> On Thu, 22 Jul 2004 11:39:58 -0400, George <george@least> wrote:
> > Nope, they behave as masses acted upon by outside forces.
> >
> > The fluid is a lube to reduce friction.
>
> I'm not sure that that's how a suspension behaves. "lube" would indicate
> that it forms a film between the thing being transported, and the plenum
> it's being transported in. Seems to me you're moving both the air
> _and_ the sawdust suspended in the air.
>
Guess we need to think about how much energy we want to waste in turbulent
flow to get things in suspension versus what we'd like to have to get them
flowing in a more laminar pattern toward the impeller.
That is what velocity is, is it not? Motion in a direction?
http://scienceworld.wolfram.com/biography/BernoulliDaniel.html says Swiss,
but
http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Bernoulli_Daniel.html
goes with Netherlands
Ethnicity of the name? Probably Italian.
Wrote in Latin, so what's the diff?
Swiss are by language German or Italian, with a bit of French.
"Todd Fatheree" <[email protected]> wrote in message
news:[email protected]...
> "Dave Hinz" <[email protected]> wrote in message
> news:[email protected]...
> > On Thu, 22 Jul 2004 10:21:13 -0400, George <george@least> wrote:
> > > Nope, Newton.
> > >
> > > We're moving solids, hopefully. That's Newton. Thus the concept
> passage
> > > cited.
> >
> > Solids suspended in air perform as a fluid, do they not?
>
> I guess we need to find out which laws apply to a non-colloidal
suspension.
> By the way...the Bernoulli equation is for frictionless, incompressible
> flow. It works well enough for fluids, but it's out for gases. A cursory
> look over my fluid mechanics info says that we might have better luck with
> the Euler equation.
>
"Dave Hinz" <[email protected]> wrote in message
news:[email protected]...
> On Thu, 22 Jul 2004 10:21:13 -0400, George <george@least> wrote:
> > Nope, Newton.
> >
> > We're moving solids, hopefully. That's Newton. Thus the concept
passage
> > cited.
>
> Solids suspended in air perform as a fluid, do they not?
I guess we need to find out which laws apply to a non-colloidal suspension.
By the way...the Bernoulli equation is for frictionless, incompressible
flow. It works well enough for fluids, but it's out for gases. A cursory
look over my fluid mechanics info says that we might have better luck with
the Euler equation.
Also, someone here pointed out that Bernoulli was Swiss (after someone else
said he was Italian). He lived much of his life in Switzerland, but he was,
in fact, Dutch.
todd
Todd Fatheree wrote:
> Also, someone here pointed out that Bernoulli was Swiss (after someone else
> said he was Italian). He lived much of his life in Switzerland, but he was,
> in fact, Dutch.
>
> todd
I guess it depends on the definition of nationality. I have no idea what
citizenship rules were like then, or if it's relevant. His dad, Johann,
was Swiss and was working in the Netherlands at the time of Daniel's
birth. Were I to move to, say, Sweden and have a child, I would still
consider my child an American. Would it be Swedish? Technically, I suppose.
Let's just say that he was a member of the Axis of Fine Chocolate
Producing Countries (not sure what the third would be)...
G
In metric:
V = 3000 ft./min. X 1 m/3 ft. X 1 min. / 60 sec. = 16.7 m/s
D = (1/3) ft X 1 m / 3 ft. = 0.11 m
nu = 1.46 E-5 m^2/s
Re = VD/nu = 16.7 X 0.11 / (1.46 E-5) = 125,000
A little higher than yours, but I rounded. So, you're correct.
G
Bill Leonhardt wrote:
> "G. Lewin" <[email protected]> wrote in message
> news:[email protected]...
> SNIP
>
>>Now, as for the important answer of which is more important for moving
>>chips: turbulence effects vs. friction effects? I can't say. But if you
>>work through the calculations, you find that the "recommended" flow
>>speed usually works out to the transition region between laminar and
>>turbulent flow. Coincidence? I suspect (and this is pure conjecture)
>>that some amount of turbulence is necessary to keep dust from sticking
>>to the sides of the duct. Obviously, though, the bulk motion of the air
>>is what moves the dust from A to B.
>>
>
>
>
> This didn't seem quite right to me so I took a look at the numbers. IIRC,
> recommended duct velocities are 3000 to 4000 fpm.
>
> Reynolds number = Re = (density)(velocity)(diameter)/(viscosity)
>
> At 70 deg F: density = 0.075 lbm/cu ft viscosity = 0.044 lbm/ hr ft
>
> A lower limit could be 3000 ft/min in a 4 inch duct.
>
> Re = (0.075 lbm/ cu ft) (3000 ft/min) (60 min/hr) 4 in) / (0.044 lbm/hr ft)
> (12 in/ ft)
>
> Re = 102,273
>
> Since transition from laminar to turbulent flow (in internal duct flow) is
> in the range 2,000 to 10,000, this is clearly turbulent. Higher values for
> the flow rate and/or duct diameter will yield higher Re numbers.
>
> I would expect you would want to stay away from laminar flow, and certainly
> stay away from transition for good performance.
>
> Bill Leonhardt
>
>
Whoa! Slow down, everyone. Let's back up.
The most important governing equations here are the incompressible
Navier-Stokes equations. The Bernoulli equation, as noted, is for
frictionless, incompressible fluids (n.b., both liquids and gases are
classified as fluids). The Euler equations are for frictionless,
compressible gases, but air under these conditions is nearly
incompressible, so we can make that simplification (if you want to get
an anser down to the <1% error range, use the full compressible N-S).
As pointed out elsewhere, Boyle's Law is just a simplification of
compressible gas laws, and isn't appropriate here.
Now, the solids in the airstream don't substantially affect the flow.
That means that we can "decouple" the system and calculate how "pure"
air would flow and then throw the wood dust/small chips in and simply
track them through the ducts, using our solution for pure air. (again, a
prefect model would account for the fact that the wood chips can _cause_
turbulence, but this is a secondary effect).
Now, as for the important answer of which is more important for moving
chips: turbulence effects vs. friction effects? I can't say. But if you
work through the calculations, you find that the "recommended" flow
speed usually works out to the transition region between laminar and
turbulent flow. Coincidence? I suspect (and this is pure conjecture)
that some amount of turbulence is necessary to keep dust from sticking
to the sides of the duct. Obviously, though, the bulk motion of the air
is what moves the dust from A to B.
Greg
Todd Fatheree wrote:
> "Dave Hinz" <[email protected]> wrote in message
> news:[email protected]...
>
>>On Thu, 22 Jul 2004 10:21:13 -0400, George <george@least> wrote:
>>
>>>Nope, Newton.
>>>
>>>We're moving solids, hopefully. That's Newton. Thus the concept
>
> passage
>
>>>cited.
>>
>>Solids suspended in air perform as a fluid, do they not?
>
>
> I guess we need to find out which laws apply to a non-colloidal suspension.
> By the way...the Bernoulli equation is for frictionless, incompressible
> flow. It works well enough for fluids, but it's out for gases. A cursory
> look over my fluid mechanics info says that we might have better luck with
> the Euler equation.
>
> Also, someone here pointed out that Bernoulli was Swiss (after someone else
> said he was Italian). He lived much of his life in Switzerland, but he was,
> in fact, Dutch.
>
> todd
>
>
DarylRos wrote:
> It's not Bernoulli mainly that factors into this, it's Boyle.
I beg to disagree but at the velocities common in dust collection systems
the flow of air is assumed to be incompressible and Boyle doesn't enter
into the calculation. It's not until you have velocities approaching Mach
1 that you start having to consider compressibility.
> Pressure dynamics is the same whether it's for gas, liquid or even traffic
> patterns.
Maybe so, but Boyle's Law applies to static pressures, not dynamic.
> Reduce the size of the pipe, duct or road and you increase the pressure
> and reduce the velocity.
You've got it backwards. Reduce the size of the pipe or duct and you
decrease the pressure and increase the velocity.
> So the idea that a reduction at one point (be it the smaller pickup at a
> saw or a roadblock in the middle of the road), the pressure increases, the
> dust, car, water, whatever slows, but then, as the pressure decreases with
> the increase in the roadwork, the speed increases.
That may be _your_ idea but gases don't behave that way in ducts.
> The traffic analogy was not mine, but worked out by some highway
> engineers. They were surprised to learn that traffic flow basically obeys
> Boyle's law.
I'd like to see a reference to that.
>
> Which is why you want large main ductwork, this is your freeway. The
> smaller gates are your on ramps.
>
> The speed cannot be the same throughout. Just as traffic picks up after a
> slowdown. Sometimes when you hit traffic and then it speeds up, you wonder
> why. Well, there was a stoppage a while ago, and the system is simply
> recovering. It does not stay slow the entire way.
If highways behaved like air ducts then you'd see people going 180 MPH
though construction zones.
--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)
Scott Lurndal wrote:
> "Todd Fatheree" <[email protected]> writes:
>>And to make it even more confusing, it is sometimes better to induce a
>>turbulent boundary layer to get better flow than something that is
>>strictly laminar.
>
> Like Professor Fish's work with humpback[*] flippers. The nobs on the
> leading edge perform much better than the typical smooth leading edges on
> modern
> aircraft wings. Expect to see knobby wings on future aircraft :-)
Don't. It's called a "turbulator" and it works fine in low reynolds number
flows. Put them on high speed aircraft and they create all manner of
chaos. Been tried, repeatedly, in various forms. A whale is not an
airplane.
> [*]
>
<http://www.sciamdigital.com/browse.cfm?sequencenameCHAR=item2&methodnameCHAR=resource_getitembrowse&interfacenameCHAR=browse.cfm&ISSUEID_CHAR=A4AD4ADB-2B35-221B-699D1485A73879AA&ARTICLEID_CHAR=A4B65445-2B35-221B-655AE8D9744434BC&sc=I100322>
--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)
"G. Lewin" <[email protected]> wrote in message
news:[email protected]...
SNIP
> Now, as for the important answer of which is more important for moving
> chips: turbulence effects vs. friction effects? I can't say. But if you
> work through the calculations, you find that the "recommended" flow
> speed usually works out to the transition region between laminar and
> turbulent flow. Coincidence? I suspect (and this is pure conjecture)
> that some amount of turbulence is necessary to keep dust from sticking
> to the sides of the duct. Obviously, though, the bulk motion of the air
> is what moves the dust from A to B.
>
This didn't seem quite right to me so I took a look at the numbers. IIRC,
recommended duct velocities are 3000 to 4000 fpm.
Reynolds number = Re = (density)(velocity)(diameter)/(viscosity)
At 70 deg F: density = 0.075 lbm/cu ft viscosity = 0.044 lbm/ hr ft
A lower limit could be 3000 ft/min in a 4 inch duct.
Re = (0.075 lbm/ cu ft) (3000 ft/min) (60 min/hr) 4 in) / (0.044 lbm/hr ft)
(12 in/ ft)
Re = 102,273
Since transition from laminar to turbulent flow (in internal duct flow) is
in the range 2,000 to 10,000, this is clearly turbulent. Higher values for
the flow rate and/or duct diameter will yield higher Re numbers.
I would expect you would want to stay away from laminar flow, and certainly
stay away from transition for good performance.
Bill Leonhardt
And to make it even more confusing, it is sometimes better to induce a
turbulent boundary layer to get better flow than something that is strictly
laminar.
"George" <george@least> wrote in message
news:[email protected]...
> Guess we need to think about how much energy we want to waste in turbulent
> flow to get things in suspension versus what we'd like to have to get them
> flowing in a more laminar pattern toward the impeller.
>
> That is what velocity is, is it not? Motion in a direction?
Four and a half years of engineering school say "yes". The question is,
what equations govern this type of flow? It sure isn't Bernoulli and I'm
not sure Boyle's strictly applies. Boyle's Law is more applicable to a
pressure cooker or a engine cylinder. I'm not sure it can be extended to a
flow such as what we're discussing. But then, my specialization was solid
mechanics, not fluids.
> http://scienceworld.wolfram.com/biography/BernoulliDaniel.html says
Swiss,
If you define "Swiss" by living in Switzerland, then this one is correct.
Most people, howeve, define "Swiss" to mean, "born in Switzerland". It's
clear he was born in the Netherlands.
>
http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Bernoulli_Daniel.html
> goes with Netherlands
todd
"Todd Fatheree" <[email protected]> writes:
>And to make it even more confusing, it is sometimes better to induce a
>turbulent boundary layer to get better flow than something that is strictly
>laminar.
Like Professor Fish's work with humpback[*] flippers. The nobs on the leading
edge perform much better than the typical smooth leading edges on modern
aircraft wings. Expect to see knobby wings on future aircraft :-)
[*]
<http://www.sciamdigital.com/browse.cfm?sequencenameCHAR=item2&methodnameCHAR=resource_getitembrowse&interfacenameCHAR=browse.cfm&ISSUEID_CHAR=A4AD4ADB-2B35-221B-699D1485A73879AA&ARTICLEID_CHAR=A4B65445-2B35-221B-655AE8D9744434BC&sc=I100322>
Damn. I'm sure I calculated a much lower Re once, but I can't find my
notes to see where I made the mistake (I assume it was me, but I'll
check yours). Probably got screwed up on the whole lbm/lbf thing...
G
Bill Leonhardt wrote:
> "G. Lewin" <[email protected]> wrote in message
> news:[email protected]...
> SNIP
>
>>Now, as for the important answer of which is more important for moving
>>chips: turbulence effects vs. friction effects? I can't say. But if you
>>work through the calculations, you find that the "recommended" flow
>>speed usually works out to the transition region between laminar and
>>turbulent flow. Coincidence? I suspect (and this is pure conjecture)
>>that some amount of turbulence is necessary to keep dust from sticking
>>to the sides of the duct. Obviously, though, the bulk motion of the air
>>is what moves the dust from A to B.
>>
>
>
>
> This didn't seem quite right to me so I took a look at the numbers. IIRC,
> recommended duct velocities are 3000 to 4000 fpm.
>
> Reynolds number = Re = (density)(velocity)(diameter)/(viscosity)
>
> At 70 deg F: density = 0.075 lbm/cu ft viscosity = 0.044 lbm/ hr ft
>
> A lower limit could be 3000 ft/min in a 4 inch duct.
>
> Re = (0.075 lbm/ cu ft) (3000 ft/min) (60 min/hr) 4 in) / (0.044 lbm/hr ft)
> (12 in/ ft)
>
> Re = 102,273
>
> Since transition from laminar to turbulent flow (in internal duct flow) is
> in the range 2,000 to 10,000, this is clearly turbulent. Higher values for
> the flow rate and/or duct diameter will yield higher Re numbers.
>
> I would expect you would want to stay away from laminar flow, and certainly
> stay away from transition for good performance.
>
> Bill Leonhardt
>
>
Edwin,
One of the recent wood mags (not sure which one) says PVC is not good
for DC. Just can't remember why not. I know it's easy to just glue up
and you're ready, but I would like to know why PVC won't work.
Mark L.
Edwin Pawlowski wrote:
> "Phisherman" <[email protected]> wrote in message
> news:[email protected]...
>
>> I was looking at T-7 Santoprene tubing which is
>>supposed to be good at abrasion and contains a steel wire which I can
>>ground. It is about $4.50 a foot. Is this a good choice/value?
>
>
> Rigid PVC for the long runs is a hell of a lot cheaper. Use the flex for
> connections and drops to the machines. .
>
>
"Fly-by-Night CC" <[email protected]> wrote in message
>
> 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.
>
> Wadya think?
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
suction.
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.
Ed
In article <[email protected]>,
[email protected] wrote:
> On Tue, 13 Jul 2004 18:04:09 GMT, Bob Brogan <[email protected]> wrote:
>
>
> >
> >So while PVC doesn't "conduct", it can generate "Static electricity"
> >under the right circumstances. That is what you're collecting and
> >grounding.
>
>
> nope.
Yup.
--
Thanks,
Ham
In article <[email protected]>,
Wes <n7ws@_yahoo.com> wrote:
> On Mon, 12 Jul 2004 15:21:40 GMT, "patrick conroy"
> <[email protected]> wrote:
>
> |
> |"Mark L." <[email protected]> wrote in message
> |news:DQlIc.3236$m%[email protected]...
> |
> |> and you're ready, but I would like to know why PVC won't work.
> |
> |
> |Remember that guy who strapped a JATO to his Chevy Impala out in AZ? He was
> |a woodworker too, had ungrounded PVC dust collection in shop. It blew up one
> |day.
> |
>
> Folks, PVC is an insulator. You CAN NOT GROUND an insulator. If you
> could, you would not be reading this message because none of our
> computers (or anything else electronic) would be working.
>
> Repeat after me:
>
> "I cannot ground PVC"
> "I cannot ground PVC"
> "I cannot ground PVC"
> "I cannot ground PVC"
> "I cannot ground PVC"
> "I cannot ground PVC"
> "I cannot ground PVC"
> "I cannot ground PVC"
> .....
Yes, PVC is more a dielectric. That said, one puts wire through the
ducts so that it "collects" the charge that is generated by the movement
of the air/wood through the duct. You ground the wire to dissipate the
"static charge".
So while PVC doesn't "conduct", it can generate "Static electricity"
under the right circumstances. That is what you're collecting and
grounding.
--
Thanks,
Ham
In article <[email protected]>,
[email protected] says...
> igor <[email protected]> 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.
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.
Greg
George wrote:
> Italian fellow name of Bernoulli, I believe, has some good words to say on
> the subject.
>
> 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.
>
> "Fly-by-Night CC" <[email protected]> wrote in message
> news:[email protected]...
>
>>In article <[email protected]>, "George" <george@least>
>>wrote:
>>
>>
>>>Not an engineer, but imagine the optimum transport pipe is probably ~5".
>>>Force/unit area calcs show 6" less than half the four.
>>
>>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.
>>
>>Wadya think?
>
>
>
OK, you seem to have two contradictory statements: The first paragraph
(correctly) states that the "carry" is related to the flow rate. But
then in the last paragraph (and the quote about Bernoulli) suggests that
the pressure _itself_ is responsible for carrying the particles. The
correlation that high speed == low pressure and vice versa (Bernoulli's
principle) is not really relavent, and for a ducted system, only
marginally applicable.
Yes, the pressure and flow rate do change (and you can use Bernoulli's
principle on a limited basis at the junction) when you change duct size.
But pressure is just a means to an end (in that pressure differences are
what move the air, of course). It is air speed that is responsible for
carrying the particles (turbulence and particle friction, in
particular). So when you say "Lower vacuum (large pipe), pieces drop" it
should really be "Lower speed...".
Greg
George wrote:
> Well, fluid dynamics was not my prime concern. The concern was with the
> "carry", which of course is related to the flow rate. You are concerned
> with the fluid, I with the solid, which, at least to me, is the reason for
> having a collector, not to move air around.
>
> "Bernoulli's principle can be explained in terms of the law of conservation
> of energy (see conservation laws, in physics). As a fluid moves from a wider
> pipe into a narrower pipe or a constriction, a corresponding volume must
> move a greater distance forward in the narrower pipe and thus have a greater
> speed. At the same time, the work done by corresponding volumes in the wider
> and narrower pipes will be expressed by the product of the pressure and the
> volume. Since the speed is greater in the narrower pipe, the kinetic energy
> of that volume is greater. Then, by the law of conservation of energy, this
> increase in kinetic energy must be balanced by a decrease in the
> pressure-volume product, or, since the volumes are equal, by a decrease in
> pressure."
>
>
> Will you go this? Lower vacuum (large pipe), pieces drop - higher vacuum
> (narrower pipe) , pieces move.
>
>
>
> "G. Lewin" <[email protected]> wrote in message
> news:[email protected]...
>
>>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.
>
>
>
On the show " Ultimate Workshop" on the DIY network they said that the =
flex hose slows down the air current thus robbing power from your DC.
Puff
"Phisherman" <[email protected]> wrote in message =
news:[email protected]...
> I have used a single 50 flex 4" diameter flex dust collection tubing
> (with an internal steel grounded wire) for all my machines for the
> past 15 years but now it is wearing out with many small holes. I have
> a 1.5 HP DC with a trash can cyclone. Are there any recommendations
> for replacement? I was looking at T-7 Santoprene tubing which is
> supposed to be good at abrasion and contains a steel wire which I can
> ground. It is about $4.50 a foot. Is this a good choice/value?
On 12 Jul 2004 21:50:11 -0700, [email protected] (Joe Emenaker) wrote:
snip post about how pvc isn't a static explosion hazard, but metal
duct is better anyway.
>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.
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....
On Mon, 19 Jul 2004 09:01:36 -0400, "G. Lewin" <[email protected]> wrote:
>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.
>
Next you'll try to tell us that Columbus was from Brooklyn. Mama mia!!!
PS: The comments you made about a temp. reduction in pipe diam. was
helpful. I don't understand much of the physics, but it has proven out in
practice - e.g., putting a 4" pipe on machine's 2" duct fitting is better
than putting a 2" pipe on it to the DC. In fact, this *suggests* a reason
why my Dewlat TS has a small fitting - increased air speed perhaps improves
dust capture over what it would otherwise be. Just a thought. -- Igor
"Mark L." <[email protected]> wrote:
> Edwin,
> One of the recent wood mags (not sure which one) says PVC is not good
> for DC. Just can't remember why not. I know it's easy to just glue up
> and you're ready, but I would like to know why PVC won't work.
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
(http://dsc.discovery.com/fansites/mythbusters/mythbusters.html).
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
decision process.
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.
"Edwin Pawlowski" <[email protected]> wrote:
>
> Because people will lead you to believe that if you use non-grounded plastic
> there will be an explosion bigger than the H-bomb.
Actually, it just occured to me that you *can* purchase
electrically-conductive paint... probably in a spray, too. So you
could make your PVC system grounded without needing wires in it.
... for those so concerned about grounding, that is.
- Joe
"Puff Griffis" <[email protected]> wrote:
> On the show " Ultimate Workshop" on the DIY network they said that the
> flex hose slows down the air current thus robbing power from your DC.
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
90-degree).
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.
- Joe
I... (Joe Emenaker) wrote:
> 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.
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:
http://mywebpages.comcast.net/rodec/woodworking/articles/DC_myths.html
Enjoy!
- Joe
igor <[email protected]> wrote:
>
> .... 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?
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.
- Joe
Joe Emenaker wrote:
> igor <[email protected]> wrote:
>>
>> .... 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?
>
> 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%!!!
But you'd also get more clogs. 3" is getting down to the shop-vac hose
territory.
> 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.
>
> - Joe
--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)
igor <[email protected]> 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.
igor said:
>>"Puff Griffis" <[email protected]> wrote:
>> o 4" is good. 5" is gooder. 6" is gooderer.
>
>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 correct. When velocity drops too low, you cannot maintain the
material in suspension - it just builds up in the pipes instead of
flowing to the DC/pre-collector.
Greg G.
On Tue, 13 Jul 2004 14:39:24 -0400, "George" <george@least> wrote:
>How many places per foot do you have to anchor the ground wire to the PVC to
>dissipate the charge?
>
>Hint. This is a trick question.
an infinite number.......
>"Bob Brogan" <[email protected]> wrote in message
>news:[email protected]...
>> In article <[email protected]>,
>>
>> Yes, PVC is more a dielectric. That said, one puts wire through the
>> ducts so that it "collects" the charge that is generated by the movement
>> of the air/wood through the duct. You ground the wire to dissipate the
>> "static charge".
>>
>> So while PVC doesn't "conduct", it can generate "Static electricity"
>> under the right circumstances. That is what you're collecting and
>> grounding.
>> --
>
Greg, while I agree with your statements, per se, I'd like to toss in one more
item. Specifically, the intake bypass in a 2-bag DC. We have a single fan
(impeller), and if the air line to that was fully (or even mostly) blocked for
some reason, the upper bag would collapse. To avoid this, there appears to be
a partial intake bypass. The air movement would then split between the main
duct and the bypass by the relative resistance of the two paths.
Now, I imagine a pressure limit valve could be used in the bypass, but I doubt
they do this.
Haven't seen this mentioned before in discussions. But it explains why a 2-hp
DC cannot match the static vacuum of even a medium shop vacuum, no matter how
much you restrict the opening. It would also impact some of your conclusions
(by degree, not type), in that moving from a 5- to 4-inch hose would be worse
than expected since more air would flow though the intake bypass.
Does this make sense, or am I missing something?
GerryG
On Mon, 19 Jul 2004 08:28:54 -0400, "G. Lewin" <[email protected]> wrote:
>No, the system is not limited by the narrowest pipe. It's not a "weakest
>link" analogy. The reason is that the speed of the air will vary
>inversely to the area of the duct. So the 4 inch restriction will just
>accelerate the air. There is some (small) loss with restrictions, mostly
> because it is easy to accelerate flow efficiently but hard to slow it
>down.
>
>Others have properly noted the fact that while large ducts can pass a
>lot of air, the speed of the air drops so that dust can settle out in
>the pipe.
>
>Greg
>
>
>Fly-by-Night CC wrote:
>> In article <[email protected]>, "George" <george@least>
>> wrote:
>>
>>
>>>Not an engineer, but imagine the optimum transport pipe is probably ~5".
>>>Force/unit area calcs show 6" less than half the four.
>>
>>
>> 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.
>>
>> Wadya think?
>>
On 12 Jul 2004 21:52:32 -0700, [email protected] (Joe Emenaker) wrote:
>"Edwin Pawlowski" <[email protected]> wrote:
>>
>> Because people will lead you to believe that if you use non-grounded plastic
>> there will be an explosion bigger than the H-bomb.
>
>Actually, it just occured to me that you *can* purchase
>electrically-conductive paint... probably in a spray, too. So you
>could make your PVC system grounded without needing wires in it.
but you cannot, repeat cannot ground the cloud of dust. grounding the
duct is like putting your belt on under your pants. it's a stupid
waste of time.
>
>... for those so concerned about grounding, that is.
>
>- Joe
On Mon, 12 Jul 2004 15:21:40 GMT, "patrick conroy"
<[email protected]> wrote:
|
|"Mark L." <[email protected]> wrote in message
|news:DQlIc.3236$m%[email protected]...
|
|> and you're ready, but I would like to know why PVC won't work.
|
|
|Remember that guy who strapped a JATO to his Chevy Impala out in AZ? He was
|a woodworker too, had ungrounded PVC dust collection in shop. It blew up one
|day.
|
Folks, PVC is an insulator. You CAN NOT GROUND an insulator. If you
could, you would not be reading this message because none of our
computers (or anything else electronic) would be working.
Repeat after me:
"I cannot ground PVC"
"I cannot ground PVC"
"I cannot ground PVC"
"I cannot ground PVC"
"I cannot ground PVC"
"I cannot ground PVC"
"I cannot ground PVC"
"I cannot ground PVC"
.....
"Wes" <n7ws@_yahoo.com> wrote in message
news:[email protected]...
> Repeat after me:
> "I cannot ground PVC"
Can I just sit in detention instead?
On Thu, 15 Jul 2004 22:58:20 GMT, igor <[email protected]> wrote:
>On 15 Jul 2004 15:19:48 -0700, [email protected] (Joe Emenaker) wrote:
>
>>"Puff Griffis" <[email protected]> wrote:
>>> On the show " Ultimate Workshop" on the DIY network they said that the
>>> flex hose slows down the air current thus robbing power from your DC.
>>
>>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.
>
>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're not missing something. the bigger the duct the bigger the
collector you need to have sucking on it. if the duct is
oversize/collector is undersize sawdust will settle out in the duct.
In article <[email protected]>, "George" <george@least>
wrote:
> Not an engineer, but imagine the optimum transport pipe is probably ~5".
> Force/unit area calcs show 6" less than half the four.
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.
Wadya think?
--
Owen Lowe and his Fly-by-Night Copper Company
"Mark L." <[email protected]> wrote in message
news:DQlIc.3236$m%[email protected]...
> and you're ready, but I would like to know why PVC won't work.
Remember that guy who strapped a JATO to his Chevy Impala out in AZ? He was
a woodworker too, had ungrounded PVC dust collection in shop. It blew up one
day.
On Tue, 13 Jul 2004 18:04:09 GMT, Bob Brogan <[email protected]> wrote:
>
>So while PVC doesn't "conduct", it can generate "Static electricity"
>under the right circumstances. That is what you're collecting and
>grounding.
nope.
In article <[email protected]>,
"Edwin Pawlowski" <[email protected]> wrote:
> "Mark L." <[email protected]> wrote in message
> news:DQlIc.3236$m%[email protected]...
> > Edwin,
> > One of the recent wood mags (not sure which one) says PVC is not good
> > for DC. Just can't remember why not. I know it's easy to just glue up
> > and you're ready, but I would like to know why PVC won't work.
> > Mark L.
>
> Because people will lead you to believe that if you use non-grounded plastic
> there will be an explosion bigger than the H-bomb.
>
> Where I work, we move 20,000 pounds of material a day through 4" pvc.
> Probably half of our industry does. I use pvc in my shop with good
> results. Just emptied the DC last night, in fact.
> Ed
> [email protected]
> http://pages.cthome.net/edhome
I hope you grounded that posting before you hit the send button. I
don't want any of your static blowing up my hard drive.
On 15 Jul 2004 15:19:48 -0700, [email protected] (Joe Emenaker) wrote:
>"Puff Griffis" <[email protected]> wrote:
>> On the show " Ultimate Workshop" on the DIY network they said that the
>> flex hose slows down the air current thus robbing power from your DC.
>
>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.
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
On 15 Jul 2004 15:19:48 -0700, [email protected] (Joe Emenaker) wrote:
>"Puff Griffis" <[email protected]> wrote:
>> On the show " Ultimate Workshop" on the DIY network they said that the
>> flex hose slows down the air current thus robbing power from your DC.
>
>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.
Maybe. It really depends upon distance and the size of your dust
collector. 6" may be *too* good and reduce velocity to the point where
chips settle out. You really need to do the velocity/static pressure
computations to be sure. In my case, 6" was too large, 5" was the ideal
size. Unfortunately, that meant I could not use cheap PVC from the Borg.
> 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
>90-degree).
Yep
> 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.
>
Again, yep
>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.
>
>- Joe
Check out the various web sites like
http://cnets.net/~eclectic/woodworking/cyclone/
it has an Excel spreadsheet:
http://billpentz.com/woodworking/cyclone/StaticCalc.xls that is invaluable
in sizing your system. Pay attention as well to sizing the ductwork for
each machine.
I just went through this late last year and have been very satisfied with
the results. I wound up buying metal spiral pipe from a local fabricator
along with Y's, T's and elbows. I can now see plumes of dust being sucked
into the table saw through the insert and the shaper table chips are sucked
into the system with few residuals left behind.
"Phisherman" <[email protected]> wrote in message
news:[email protected]...
> I was looking at T-7 Santoprene tubing which is
> supposed to be good at abrasion and contains a steel wire which I can
> ground. It is about $4.50 a foot. Is this a good choice/value?
Rigid PVC for the long runs is a hell of a lot cheaper. Use the flex for
connections and drops to the machines. .
I thought it was some kind of a commie plot..... So 4" PVC it is :-)
Edwin Pawlowski wrote:
>
>
> Because people will lead you to believe that if you use non-grounded plastic
> there will be an explosion bigger than the H-bomb.
>
> Where I work, we move 20,000 pounds of material a day through 4" pvc.
> Probably half of our industry does. I use pvc in my shop with good
> results. Just emptied the DC last night, in fact.
> Ed
> [email protected]
> http://pages.cthome.net/edhome
>
>