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 , "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?

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" wrote in message
news
In article , "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 Mon, 19 Jul 2004 09:01:36 -0400, "G. Lewin" 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

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" wrote in message
...
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.

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" wrote in message
...

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" wrote in message
...
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...".

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

OK, pretty much as advertised. Lower velocity (sqroot) lower the pull, I
guess.

"G. Lewin" wrote in message
...

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

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" 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 , "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?

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" 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 , "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.

Nope, Newton.

We're moving solids, hopefully. That's Newton. Thus the concept passage
cited.

"DarylRos" wrote in message
...
It's not Bernoulli mainly that factors into this, it's Boyle.

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?

Nope, they behave as masses acted upon by outside forces.

The fluid is a lube to reduce friction.

"Dave Hinz" wrote in message
...
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?

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.

Chunks, man, think chunks.


"Dave Hinz" wrote in message
...
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.

"Dave Hinz" wrote in message
...
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)

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/biog...lliDaniel.html says Swiss,
but
http://www-groups.dcs.st-and.ac.uk/~...li_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" wrote in message
...
"Dave Hinz" wrote in message
...
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.

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
...
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/biog...lliDaniel.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/~...li_Daniel.html
goes with Netherlands

todd

"Todd Fatheree" 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=r esource_getitembrowse&interfacenameCHAR=browse.cfm &ISSUEID_CHAR=A4AD4ADB-2B35-221B-699D1485A73879AA&ARTICLEID_CHAR=A4B65445-2B35-221B-655AE8D9744434BC&sc=I100322

Scott Lurndal wrote:

"Todd Fatheree" 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=r esource_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)

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" wrote in message
...

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

"G. Lewin" wrote in message
...
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

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" wrote in message
...
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

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" wrote in message
...
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