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## burner behaviour + why chimneys work

### Vince Pitelka on wed 21 mar 07

Michael Wendt wrote:
> Vacuum is the absence of air and thus exerts no
> force. It takes pressure or force to move things.

But as John said, "Nature abhors a vacuum," and therefore atmospheric
pressure will force air into the kiln to fill the vacuum. And of course
only an absolute vacuum is the complete absence of air and thus exerts no
force. We are talking about very slight vacuum within the firing chamber,
so there is still plenty of gases present to exert force.
- Vince

Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/

### Michael Wendt on wed 21 mar 07

John wrote:
"As this effluent exits the confines of the stack, it
leaves space in the
chimney for an equal volume of materials to replace it.
This creates a
pressure differential between the bottom of the chimney
and the
surrounding air. "Nature abhors a vacuum", so if the
base of the chimney
is open to the general surrounding atmosphere, the
"laws of nature" will
attempt to equalize this pressure differential back to
zero."

This is very close to the explanation with one minor
but
key clarification:
The reason the buoyancy effect exists is the
difference
in atmospheric pressure that ALWAYS exists between
the bottom and the top of the chimney.
The earth's atmosphere can be thought of as a
continuous stack of 1 cubic foot volumes reaching
all the way to the vacuum of space.
The cube of air at the bottom of the chimney "sees"
10 (or 20) more cubes stacked on top of it than the
cube
at the top of the chimney and this pressure difference
is
what actually pushes the "hot air balloons" upward.
Vacuum is the absence of air and thus exerts no
force. It takes pressure or force to move things.
Regards,

Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
USA
208-746-3724
http://www.wendtpottery.com
wendtpot@lewiston.com

### John Baymore on thu 22 mar 07

Michael,

This is true for sure. HOWEVER, this pressure differential value is of
such a small magnitude in a stack that is on a studio potter's kiln as to
be absolutely insignificant when compared to the magnitude of the stack
effect caused by the gas temperature differential.

This pressure difference is based on the atmospheric pressure at the
intlet to the chimney at the base when compared to the atmospheric
pressure at the outlet (top) of the chimney. The pressure change for a
height difference of something between ground level and from about 10 feet
or maybe up to 20 feeet above ground level is miniscule.

Calculate it out.........

Atmospheric pressure at any altitude (height above sea level) is computed
with the following formula:

p = p0 exp(-Mgy/RT)

Where: p = corrected pressure (mb)
p0 = Sea level pressure (about 1013mb)
y = Altitude (in metres above sea level)
R = Gas constant (J/mole/K)
M = Molar mass, air
T = Temperature (Kelvin)
g = Gravitational constant

The general difference works out for most lower level altitudes to about 1
Millibar per 27 feet of altitude differential. That's only about 0.03" of
Mercury. That's a "nothing" pressure differential.

This pressure differential is usually enough to barely.....just
BARELY..... get some flow started when the kiln is first turned on. Once
warm gases reach the chimney, this effect is diminished in magnitude
compared to the stack effect cause by the density of the hot gases
compared to the surrounding air. As the effluent temperature increases,
this pressure differential induced flow become lass and less significant.

And yes, the atmospheric pressure "pushes" to drive the system as I noted.
The chimney does not "suck" the gases out of the kiln.

best,

..................john

John Baymore
River Bend Pottery
Wilton, NH

JBaymore@compuserve.com
http://www.JohnBaymore.com

On Wed, 21 Mar 2007 17:05:41 -0800, Michael Wendt
wrote:

>This is very close to the explanation with one minor
>but
>key clarification:
> The reason the buoyancy effect exists is the
>difference
>in atmospheric pressure that ALWAYS exists between
>the bottom and the top of the chimney.
>The earth's atmosphere can be thought of as a
>continuous stack of 1 cubic foot volumes reaching
>all the way to the vacuum of space.
>The cube of air at the bottom of the chimney "sees"
>10 (or 20) more cubes stacked on top of it than the
>cube
>at the top of the chimney and this pressure difference
>is
>what actually pushes the "hot air balloons" upward.
>Vacuum is the absence of air and thus exerts no
>force. It takes pressure or force to move things.
>Regards,
>
>Michael Wendt

### Vince Pitelka on fri 23 mar 07

John Baymore wrote:
> And yes, the atmospheric pressure "pushes" to drive the system as I noted.
> The chimney does not "suck" the gases out of the kiln.

Doesn't this get into a level of semantics that is a little silly? I mean,
if hot gases rising create velocity and lift in the chimney, don't they also
create suction at the flue? Atmospheric pressure would not push secondary
air into the kiln unless the suction created by the stack was removing the
gases through the flue. As I have stated in an earlier post, if you open
the damper wide on a gas kiln with a tall chimney, the burner flames will
suddenly narrow down where they enter the burner port, because secondary air
is rushing in to fill the slight vacuum created by the hot gases rising in
the chimney. I have fired many kilns that had a slightly sloppy fit on the
damper, and you can hear the sound of air rushing in through the slot, like
an open passive damper port.

It really does seem silly not to simply call this suction. If you were to
sift a handful of dust next to a passive damper port at the bottom of a hot
chimney, it would be aggressively sucked into that port, by all the
definitions of the word "suck" that we understand and accept.

In New England, there are lots of places where those huge old tapered brick
Industrial Revolution smoke stacks are standing all by themselves, with the
old flue opening at the bottom. At the end of a hot day, you can stand at
the bottom of one of those chimneys, and there is an incredible suction at
the flue opening, created by the hot air rising inside due to the sun
heating the brick all day long.

Someone is going to have to be a lot clearer about why it is inappropriate
to say that there is a slight vacuum in the firing chamber due to the
suction created by the chimney. As I see it, it's just a matter of common
sense to refer to these phenomena by those terms.
- Vince

Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/

### Lee Love on fri 23 mar 07

On 3/23/07, John Baymore wrote:

>
> And yes, the atmospheric pressure "pushes" to drive the system as I noted.
> The chimney does not "suck" the gases out of the kiln.

There is another aspect that should be taken into consideration.
Usually, the speed of the wind movement is faster the farther way from
the ground and any obstacles you get. This air movement does "suck"
air out of the stack, the same way a mouth sprayer sprays liquid.
If you've fired on a windy day, you can see it.

--
Lee in Mashiko, Japan
Minneapolis, Minnesota USA
http://potters.blogspot.com/

"To affect the quality of the day, that is the highest of arts." -
Henry David Thoreau

"Let the beauty we love be what we do." - Rumi

### Gerald O'Sullivan on fri 23 mar 07

It's the Venturi effect. When air moves at right angles across a gap
like the top of a chimney, there is a pressure drop which causes the air
in the chimney to move into the gap, effectively "sucking" the air from
the chimney.

On Fri, 2007-03-23 at 11:35 +0900, Lee Love wrote:
> On 3/23/07, John Baymore wrote:
>
> >
> > And yes, the atmospheric pressure "pushes" to drive the system as I noted.
> > The chimney does not "suck" the gases out of the kiln.
>
> There is another aspect that should be taken into consideration.
> Usually, the speed of the wind movement is faster the farther way from
> the ground and any obstacles you get. This air movement does "suck"
> air out of the stack, the same way a mouth sprayer sprays liquid.
> If you've fired on a windy day, you can see it.
>
>
> --
> Lee in Mashiko, Japan
> Minneapolis, Minnesota USA
> http://potters.blogspot.com/
>
> "To affect the quality of the day, that is the highest of arts." -
> Henry David Thoreau
>
> "Let the beauty we love be what we do." - Rumi
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>

### John Baymore on fri 23 mar 07

Yeah Lee, thanks. That is another aspect of this whole complex ball of
wax. It's called the Bernoulli Effect and it actually is the same
principle that makes airplanes fly.

Bernoulli Effect can sometimes be problematic for a kiln since it is
VARIABLE based on airflow velocity across the top opening of the chimney.
With a light breeze, it certainly may help the draft get going in the
early stages. A stiff gusty wind can be problematic though. Anyone who
has fired a wood kiln on a gusty, high wind day is intimately familiar
with this effect on an experiental level .

On some industrial units a draft regulator is used to help minimize this
effect...... they keep the negative pressure differential induced by a
chimney sort of constant. You can find an example of these on a home oil
burner chimney; its the T shaped thingy that has the flap that opens and
closes as the pressure varies.... and sometimes makes an annoying clicking
noise as it bounces.

best,

................john

John Baymore
River Bend Pottery
Wilton, NH

JBaymore@compuseve.com
http://www.JohnBaymore.com

### Michael Wendt on fri 23 mar 07

John,
Experiments done in the space station
have shown that in the absence of
gravity a flame burns and the hot
gases from that flame expand in a
totally different way from on earth
where gravity rules. This happens
because pressure in the space
station is uniform so there is no
convection. Change is density
drives convection, even minute
changes much smaller than the
seemingly small pressure differentials
seen (as calculated by your example)
between the top and the bottom
of a tall chimney.
That "totally miniscule" pressure differential
(on a still day) is the only reason gases
rise on earth. I'm not making this up. Check
any physics text.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
USA
208-746-3724
http://www.wendtpottery.com
wendtpot@lewiston.com
John wrote:
Michael,

This is true for sure. HOWEVER, this pressure
differential value is of
such a small magnitude in a stack that is on a studio
potter's kiln as to
be absolutely insignificant when compared to the
magnitude of the stack
effect caused by the gas temperature differential.

This pressure difference is based on the atmospheric
pressure at the
inlet to the chimney at the base when compared to the
atmospheric
pressure at the outlet (top) of the chimney. The
pressure change for a
height difference of something between ground level and
or maybe up to 20 feet above ground level is miniscule.

Calculate it out.........

Atmospheric pressure at any altitude (height above sea
level) is computed
with the following formula:

p = p0 exp(-Mgy/RT)

Where: p = corrected pressure (mb)
p0 = Sea level pressure (about 1013mb)
y = Altitude (in metres above sea level)
R = Gas constant (J/mole/K)
M = Molar mass, air
T = Temperature (Kelvin)
g = Gravitational constant

The general difference works out for most lower level
Millibar per 27 feet of altitude differential. That's
Mercury. That's a "nothing" pressure differential.

This pressure differential is usually enough to
barely.....just
BARELY..... get some flow started when the kiln is
first turned on. Once
warm gases reach the chimney, this effect is diminished
in magnitude
compared to the stack effect cause by the density of
the hot gases
compared to the surrounding air. As the effluent
temperature increases,
this pressure differential induced flow become lass and
less significant.

And yes, the atmospheric pressure "pushes" to drive the
system as I noted.
The chimney does not "suck" the gases out of the kiln.

best,

..................john

John Baymore

### John Windus on fri 23 mar 07

John,

I always love it when someone intones the Laws of Thermodynamics to keep the
natives in line. Hey lightening up, dude. Those Laws are only for the High
Priest Engineers, not the lay people. Ok, you can say them, but you have to
say them in Latin.

I agree with what you say about kiln physics, but want to add a couple of
things. First, Bernoulli's equation does relate pressure, velocity and
elevation in a "fluid" flow, but it is based on the assumptions that the
fluid flow is steady, irrotational, nonviscous and incompressible. Throw
these out and you are back to the Navier-Stokes equations and it makes my
head hurt just to look at those squiggly symbols. The flow in a kiln might
be nonviscous, but it certainly is not steady, irrotational or
incompressible. So does this mean the conclusions drawn are wrong? No, it
just means that you have to throw a few more variables into the mix, such as

And do a gas kiln and a wood kiln act the same? Yes and No. The convective
flow is the same, the heat transfer is similar, but in a gas kiln you are
injecting a pressurized, expanding stream of gas under going an exothermic
chemical reaction. In a wood kiln you have only the exothermic chemical
reaction.

In heat transfer, it is usual to categorize the transfer into convection,
conduction and radiation. Convection is the cool or heat you feel from a
breeze, conduction is the cold floor you feel on your bare feet and
radiation is the warmth you feel from the sun. The mode of heat transfer
within a fuel kiln changes as the temperatures rise. At first, the dominate
transfer is from convection, but as it gets up there, the other two start to
become increasingly significant. As you pointed out, that is why stalling a
kiln can even the temperature out. It is also an argument for watching the
rates at which you climb or fall in temperature. Too fast and you might
have cold spots in the kiln. Interestingly, electric kilns start out using
radiation and then convection and conduction come into play.

And yes, there is a lot of nasty science involved, but I hate the thought
that science is something different and set apart. To me science,
engineering, art and craft are all the same thing. Humans striving to
understand and create; some just use different tools.

Regards,

Jody Windus
2nd Chance Art
Smithville, TN 37166
615-215-6175

-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of John Baymore
Sent: Friday, March 23, 2007 5:41 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Burner Behaviour + why chimneys work

Yeah Lee, thanks. That is another aspect of this whole complex ball of
wax. It's called the Bernoulli Effect and it actually is the same
principle that makes airplanes fly.

Bernoulli Effect can sometimes be problematic for a kiln since it is
VARIABLE based on airflow velocity across the top opening of the chimney.
With a light breeze, it certainly may help the draft get going in the
early stages. A stiff gusty wind can be problematic though. Anyone who
has fired a wood kiln on a gusty, high wind day is intimately familiar
with this effect on an experiental level .

On some industrial units a draft regulator is used to help minimize this
effect...... they keep the negative pressure differential induced by a
chimney sort of constant. You can find an example of these on a home oil
burner chimney; its the T shaped thingy that has the flap that opens and
closes as the pressure varies.... and sometimes makes an annoying clicking
noise as it bounces.

best,

................john

John Baymore
River Bend Pottery
Wilton, NH

JBaymore@compuseve.com
http://www.JohnBaymore.com

____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org

You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/

Moderator of the list is Mel Jacobson who may be reached at

### Ivor and Olive Lewis on sat 24 mar 07

Dear John Baymore,=20

Given the equation you provide and allowing for my modest ability with =
elementary mathematics, could you provide some help with a worked =
example.

p =3D p0 exp(-Mgy/RT)

Where:

p =3D corrected pressure (mb)
p0 =3D Sea level pressure (about 1013mb)
y =3D Altitude (in metres above sea level)
R =3D Gas constant (J/mole/K)
M =3D Molar mass, air
T =3D Temperature (Kelvin)
g =3D Gravitational constant

I can cope with sea level pressure and altitude. I have a value of =
8.31441 for R. I am unsure about the molar mass of air, where does this =
value come from? Deg K is no worry, just add 273 to Deg C.And the tables =
book tells me the Gravitational Constant G is 6.672 .

So what is exp and how do I use it. There is a button on my calculator =
EXP . My barometer reads 1015 hectopascals today, how do I correct it. =
We are about 100m metres above sea level.

By the way, logically, a down draft kiln should not draw because the =
entry point and exit points for air movement are at the same elevation. =
Just a thought.

Best regards,

Ivor Lewis.
Redhill,
South Australia.

### Michael Wendt on sat 24 mar 07

Vince,
I have no problem accepting suction
as the term used .
All I was trying to point out was the
underlying mechanism which is distinct
from the observed effect is being ignored.
People are changing what I was saying.
First, the old fashioned statement:
"Nature abhors a vacuum"
comes from a time before people
knew that most of space is a nearly
perfect vacuum. Nature consists almost
entirely of a vacuum would be more
accurate.
Here on earth a different set of conditions
exist.
Mel posted that a person he knew
believed that atmospheric pressure
had an effect on how well their
kiln fired. I agree and here's why.
The cause behind hot gases rising in air
(and hotter liquids rising in the same fluid)
is the fact that there is a pressure gradient
that increases as you get closer to the ground
(or in the case of water in a lake or pool,
get deeper beneath the surface)
This unequal pressure due to the force
of gravity causing the stacking effect
is the reason convection and floating occur.
In freefall in space, you can place a
ping pong ball in a jar of water and push
it into the middle of the liquid and it will stay
where you put it.
Archimedes (circa 287-211 BC) is credited
with developing the needed understanding
for why things float.
Everyone also knew that to float,
the thing had to be lighter than water.
He used the fact that even objects
heavier than water are still buoyed
by it to realize that the only way
this could happen is if the force
upward exerted by the water was
greater than the force exerted
downward due to increasing pressure
with increasing depth.
I think this is easy to
see in water but seems too small
in air to matter.
It does matter. It alone causes hot air to rise.
Air, like water, is subject to a pressure
into the vacuum created when heated
air rises up a chimney and exits the top.
If the air pressure at the top of the chimney
were identical to the pressure at the bottom,
hot air would not rise but merely expand
outward in all directions.
No "suction".
You employ the same principle when you
drink with a straw.
you suck on the top of the straw.
air pushes down on the top of the
drink and forces it up through the straw
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
USA
208-746-3724
http://www.wendtpottery.com
wendtpot@lewiston.com
Vince wrote:
John Baymore wrote:
> And yes, the atmospheric pressure "pushes" to drive
the system as I noted.
> The chimney does not "suck" the gases out of the
kiln.

Doesn't this get into a level of semantics that is a
little silly? I mean,
if hot gases rising create velocity and lift in the
chimney, don't they also
create suction at the flue? Atmospheric pressure would
not push secondary
air into the kiln unless the suction created by the
stack was removing the
gases through the flue. As I have stated in an earlier
post, if you open
the damper wide on a gas kiln with a tall chimney, the
burner flames will
suddenly narrow down where they enter the burner port,
because secondary air
is rushing in to fill the slight vacuum created by the
hot gases rising in
the chimney. I have fired many kilns that had a
slightly sloppy fit on the
damper, and you can hear the sound of air rushing in
through the slot, like
an open passive damper port.

### Lee Love on sat 24 mar 07

On 3/24/07, John Windus wrote:

> And do a gas kiln and a wood kiln act the same? Yes and No. The convective
> flow is the same, the heat transfer is similar, but in a gas kiln you are
>> injecting a pressurized, expanding stream of gas under going an exothermic
> chemical reaction. In a wood kiln you have only the exothermic chemical
> reaction.....

>...Interestingly, electric kilns start out using
>radiation and then convection and conduction come into play.

Actually, you also have pressurized gas where wood is concerned.
Wood releases gases and most of the energy is from the burning gases,
not from the coals. A wood fire's flame is much longer than a gas
flame. The flame is made of burning gases.

An important feature of wood fuel is the water in the fuel
that turns into steam and causes pressure. At temperature, you have
much larger bursts of gas pressure compared to a gas kiln. When you
stoke at temperature, you can "feel" the kiln expand. You get the
sensation that you are feeding something living, like a fire breathing
dragon.

--
Lee in Mashiko, Japan
Minneapolis, Minnesota USA
http://potters.blogspot.com/

"To affect the quality of the day, that is the highest of arts." -
Henry David Thoreau

"Let the beauty we love be what we do." - Rumi