Ted Neal on fri 31 aug 07
Hi Paul
I heard on NPR the other day "All science is Physics...the rest is stamp collecting"
So lets think about this looking at a couple "natural" laws.
Chimneys work because warm air, like any heated gas, is excited, the molecules orbits
increase in size and everything expands a little...and here is the key, becomes a little less dense,
aka lighter. Warm air rises.
The second a trickle of warm air hits your chimney it begins to travel skyward. That being said,
the second principle is displacement or in this case what we as potters call draw or draft. Think
about it. As air is heated, expands and rushes up your chimney air is pulled into the kiln from any
opening to displace the volume that is heading out, if it didn't. it would try to create a vacuum.
"Nature abhors a vacuum" so "it" is really just trying to keep a balance.
As soon as you look at it this way it becomes very easy to see what happens with dampers whether
they are passive or regular. When you first light a kiln the movement of air is directly influenced by
the heat generated from your fuel but as you progress in the firing, the kiln itself participates by
reradiating the heat that it has absorbed. Same thing is happening in the chimney. The hotter the
chimney brick get the more it aids in the "draw" (which is why some woodfire potters light a little
fire in the chimney)
So to address your question, think about it this way. If your chimney were a river, what would
narrowing the river in the middle of its length do? (Kind of difficult to answer completely) but you
know it wouldn't really slow the river, in fact the volume of water behind is still trying to get
through a smaller opening so it actually increases in velocity (and stifles the flow of what cant get
through.) Same thing in the kiln.
A passive damper doesn't really effect how much air is heading out the chimney, just where that
air originates. If it is being pulled in from the base of the chimney , then it is not being pulled
in from the rest of the kiln and therefore it stifles the air flow throught the kiln body.
In the end I think it is better to do the research and find out exactly what size your flue needs to
be, and make it a little bigger than that....don't rely on the damper, turbulence in the stream so to
speak, to get the size right. Sorry to go on and on...
Best
Ted
Ted Neal
Ball State University
On Wed, 29 Aug 2007 17:00:51 -0500, Paul Vernier wrote:
>I have been reading the threads on chimney sizing and have a related
>question. I recently completed building a new kiln, a MFT. I initially
>followed Nils guidelines on sizing the flue. But, I was warned that the
>smaller flue size has contributed to stalling. So, I made the flue a little
>larger thinking I could close it off some later as I experiment and learn to
>fire my kiln.
>
>OK, the question. Doesn't closing the damper effectively reduce the flue
>size? Or is it more complicated and dependent on things like the chimney
>base design and the distance between the flue and damper. I am sure there is
>something to do with turbulence as well.
>
>I really enjoy and appreciate the contribution people make to this forum.
>
>Paul, in HOT Santa Cruz
>
>______________________________________________________________________________
>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.
Luke Nealey on fri 31 aug 07
Hey Ted,
This is a quote of Earnest Rutherford's(1870s-1930S), the discoverer of the
atomic nucleus, Nobel prize winner etc. basically saying that in light
of the tools that were being developed at that time just watching and
describing stuff was not where science was anymore. Scientists could figure
out what was really happening, not just wave their hands. I think this must
be one of Ivor's mantras.
I had some issues with your description of expanding gases, I won't venture
into fluid flow of gases, its beyond me.
When anything is heated the mean kinetic energy of its atoms/molecules
increases. It is this motion that makes things expand. If we were talking
about solids or liquids we would have to talk alot about intermolecular
forces etc., however since we are talking about gases, and are not at some
extreme of temperature or pressure we can think about the gas molecules as
infintesimally small spheres cruising about in a very empty space. As their
kinetic energy increases they start moving faster (KE = 1/2 mv^2) (their
mass won't change), as they zoom around faster there exert more force on the
stuff they run into by hitting it with more force(they are moving faster)
and hitting it more often(rate of collision with whatever increases), ie
their pressure goes up (pressure is force/area). If T increases in a closed
container the pressure increases, in the kiln some of this happens but since
the gas can expand against the ambient pressure of the outside,via holes,
chimney etc it expands and pushes against air in the great beyond. However,
that is only part (and I think a small part of the story), but the only part
I can speak to. Other fluid dynamic folks will have to talk about bouyancy,
venturi effect, turbulance, friction etc. that is also going on.
Expansion of the gas is not caused by increasing molecular orbital or
molecular size. This is quantum mechanics! Electrons have set energy
orbitals and will go up and down them (excited electrons falling from higher
energy states will give off the energy we see as light and other
electromagnetic radiation) but electrons moving to higher energy levels has
no effect on gas volume. At these fairly mild conditions we can think of
molecules of gas as being point masses, having no volume. Twice as big of
nothing is still nothing. This is a big assumption of the ideal gas laws
and works well except under very high pressure(100+x atmospheric) or very
low temperature (close to absolute zero).
Regards,
Luke Nealey
Rankin Co. MS
On 8/31/07, Ted Neal wrote:
>
> Chimneys work because warm air, like any heated gas, is excited, the
> molecules orbits
> increase in size and everything expands a little...and here is the key,
> becomes a little less dense,
> aka lighter. Warm air rises.
>
> 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
> melpots2@visi.com
>
Ted Neal on fri 31 aug 07
Hi luke
I just thought it was funny....
Thanks for the info. True. Trying to keep it simple I guess. Most
folks understand that warm air rises. I know that the variance in pressure
between heated gases inside and the air outside of a chimney is really one of the
driving principles at work here, but 'warm air rising' seems to get the point across.
My use of expanding 'orbits' was a poor choice of words to describe the expansion and
pressure increase due to increased molecular movement not orbits. Easily confused with the
electron orbits etc Good point!
Thanks for clarifying. What do you think about the original question? Is closing the
damper the same as having a smaller flue...outside of the turbulence created by the damper?
Curious???
Best
Ted
On Fri, 31 Aug 2007 16:12:08 -0500, Luke Nealey wrote:
>Hey Ted,
>
>This is a quote of Earnest Rutherford's(1870s-1930S), the discoverer of the
>atomic nucleus, Nobel prize winner etc. basically saying that in light
>of the tools that were being developed at that time just watching and
>describing stuff was not where science was anymore. Scientists could figure
>out what was really happening, not just wave their hands. I think this must
>be one of Ivor's mantras.
>
>I had some issues with your description of expanding gases, I won't venture
>into fluid flow of gases, its beyond me.
>
>When anything is heated the mean kinetic energy of its atoms/molecules
>increases. It is this motion that makes things expand. If we were talking
>about solids or liquids we would have to talk alot about intermolecular
>forces etc., however since we are talking about gases, and are not at some
>extreme of temperature or pressure we can think about the gas molecules as
>infintesimally small spheres cruising about in a very empty space. As their
>kinetic energy increases they start moving faster (KE = 1/2 mv^2) (their
>mass won't change), as they zoom around faster there exert more force on the
>stuff they run into by hitting it with more force(they are moving faster)
>and hitting it more often(rate of collision with whatever increases), ie
>their pressure goes up (pressure is force/area). If T increases in a closed
>container the pressure increases, in the kiln some of this happens but since
>the gas can expand against the ambient pressure of the outside,via holes,
>chimney etc it expands and pushes against air in the great beyond. However,
>that is only part (and I think a small part of the story), but the only part
>I can speak to. Other fluid dynamic folks will have to talk about bouyancy,
>venturi effect, turbulance, friction etc. that is also going on.
>
>Expansion of the gas is not caused by increasing molecular orbital or
>molecular size. This is quantum mechanics! Electrons have set energy
>orbitals and will go up and down them (excited electrons falling from higher
>energy states will give off the energy we see as light and other
>electromagnetic radiation) but electrons moving to higher energy levels has
>no effect on gas volume. At these fairly mild conditions we can think of
>molecules of gas as being point masses, having no volume. Twice as big of
>nothing is still nothing. This is a big assumption of the ideal gas laws
>and works well except under very high pressure(100+x atmospheric) or very
>low temperature (close to absolute zero).
>
>Regards,
>Luke Nealey
>Rankin Co. MS
>
>
>
>On 8/31/07, Ted Neal wrote:
>>
>> Chimneys work because warm air, like any heated gas, is excited, the
>> molecules orbits
>> increase in size and everything expands a little...and here is the key,
>> becomes a little less dense,
>> aka lighter. Warm air rises.
>>
>> 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
>> melpots2@visi.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 melpots2@visi.com
Ivor and Olive Lewis on sun 2 sep 07
Dear Ted Neal=20
It seems I have been wrong in believing that it was Gravity pulling =
dense, cold air downwards which displaced the less dense hot air, =
forcing it up the stack.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ted Neal on mon 3 sep 07
Hello Ivor
Of course you are correct...and cheeky.
Isn't everything in Austrailia back to front anyway... joking
I guess as I think about it 'displacement' probably isn't the most accurate term either.
What do you think?
The pressure differential is the driving force here... correct? As I remember my studies of
high and low pressure systems, (of which the kiln could be considered a microcosm I suppose)
'wind' develops as an air mass moves from high pressure toward low pressure...seeking a balance
within the overall atmospheric pressure gradient. So does it really 'push' out the less dense hot
air? or is it a balance that factors the two different air masses seeking equilibrium coupled with
factors of buoyancy that drive the motion of gasses into and out of the kiln.
I have read some old threads and realize I am in over my head (based on old discussions with
yourself, Micheal W and others. Teach me Ivor....
Also any thoughts about the original question below?
>OK, the question. Doesn't closing the damper effectively reduce the flue
>size? Or is it more complicated and dependent on things like the chimney
>base design and the distance between the flue and damper. I am sure there is
>something to do with turbulence as well.
I still think waterflow is a good analogy, although I recognize the difference as being one of
positive vs negative forces.
Best
Ted Neal
Luke Nealey on mon 3 sep 07
Ted and Ivor:
I was truly only addressing the expansion of gases. I can't speal to all
the other stuff going on.
But, there certainly is no cool gas running down the stack to force hot
light air out. So in my wood kiln it must be pressure that forces the
ambiant air into the fire boxes, since it has to move horizontally and then
upward, gravity neutral then against gravity? I understand that once it
leaves the stack heavier cooler air will rsuh down and underneath it and
displace it upward, but this shouldn't happen in the stack right?
Talking about pressure, unlike gravity or electromotive forces, nothing can
suck, it can only push.
Regards,
Luke
Rankin, Co. MS
On 9/2/07, Ivor and Olive Lewis wrote:
>
> Dear Ted Neal
>
> It seems I have been wrong in believing that it was Gravity pulling dense,
> cold air downwards which displaced the less dense hot air, forcing it up the
> stack.
>
> Best regards,
>
> Ivor Lewis.
> Redhill,
> South Australia.
>
>
> ______________________________________________________________________________
> 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
> melpots2@visi.com
>
Ivor and Olive Lewis on tue 4 sep 07
Dear Odin,
"Drinking straw with a hole near your mouth.
Canister vacuum cleaner with that little hole near the handle opened."
I was even entertained yesterday when I was drinking a soft drink =
through a straw to find that I was entraining air as I drank. These are =
examples of a hole or break in continuity of the conduit. But an =
example is not a Mechanical Principle.
When I drink through a straw it is the pressure of the atmosphere that =
forces the water into my mouth in response to the increase in volume of =
my chest cavity as my diaphragm contracts and intercostal muscles raise =
my ribs.
So, what is the mechanical principle of the Passive Damper ?
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ted Neal on tue 4 sep 07
Ivor and Odin
Why would the mechanical principle of the passive damper system be different than
that of the rest of the flue? Same forces are at work....just a new hole, which changes the
distribution (for lack of a better word) of the effect of those forces.
Opening a passive damper shuts down the engine (as I have heard it described) that drives
the kiln. If the pressure differential between the inside/outside of the chimney causes "x" volume
of more dense cold air to enter the kiln. Part of that volume is satisfied through the passive
opening and the net effect would be less coming in through other locations. Thinking of wood
kilns, or more specifically natural draft kilns, this diminished air volume is equated to decreased
primary air supply which slows the entire combustion process.
Thoughts?
Best
Ted
Ted Neal
Ball State University
Michael Wendt on tue 4 sep 07
Ivor,
Passive dampers work by the principle of
sources. In Physics, we learned that flow
through a pipe is continuous as long as there
are no sinks or sources between the inlet and
the outlet.
A passive damper represents another source
for flow through the chimney and so provides
a means for high pressure air to enter the chimney
(other than the kiln) and force the less dense air out.
As someone also mentioned, adding cool air to
the chimney reduces stack temperatures and in
this way reduces the amount of "draw" the
chimney exerts. I stick with the terminology even
though in physical terms, gravitational forces
create the pressure gradient that accounts for
the buoyancy effect in fluid media.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave.
Lewiston, Id 83501
U.S.A.
208-746-3724
wendtpot@lewiston.com
http://www.wendtpottery.com
http://UniquePorcelainDesigns.com
om on tue 4 sep 07
On Sep 3, 2007, at 9:30 PM, Ivor and Olive Lewis wrote:
> I was even entertained yesterday when I was drinking a soft drink
> through a straw to find that I was entraining air as I drank. These
> are examples of a hole or break in continuity of the conduit. But
> an example is not a Mechanical Principle.
>
> When I drink through a straw it is the pressure of the atmosphere
> that forces the water into my mouth in response to the increase in
> volume of my chest cavity as my diaphragm contracts and intercostal
> muscles raise my ribs.
I actually just use my mouth --- tongue like a piston -- to draw up
my drink. That prevents the uncomfortable sensation of breathing
liquids. :-)
> So, what is the mechanical principle of the Passive Damper ?
Well - I'm neither a physicist nor and engineer and as I mentioned
before, not a chimney expert. I can only guess how it works from my
observations of things flowing through tubes, so my knowledge is
purely empirical, although only loosely empirical because my
observations are "by eye", not instrument. I'm also unclear exactly
what your looking for when you say "mechanical principle". But I'll
take a shot regardless.
As I mentioned previously, given a constant rate of outflow at the
chimney mouth, you will have a certain amount of inflow at the air
inlet points (outside air will replace that expelled from the
chimney). Assuming the inlets are not so tight they prevent entrance
of the maximum amount of air possible, a volume of air equal to the
effluent volume leaving the kiln (minus the volume fuel residues
occupy in the effluent) should enter. In other words:
Ev-Fv=Iv
(effluent volume - burned fuel contribution to that volume = inlet
air volume)
Plainly, the exiting gasses will be very much expanded due to heat --
fewer molecules per given volume. But if we focus on volumes rather
than molecules, we shouldn't have to worry that inlet air is denser
and exhausted effluent is expanded. We should be able to compare
simple volume measurements.
So, if you have 25 sq cm of inlet, and 10 sq cm of passive damper,
you would expect 25/25*Iv to enter the kiln with the passive damper
closed, and 25/35*Iv to enter the kiln with the passive damper open.
I'm assuming that air shows no preference for one inlet over the
other (which may not be accurate, e.g., the passive damper inlet
pathway may be less obstructed and thus favored), just that the inlet
air volume is related to what is exhausted, and that the inlet air
will be divided proportionally across the openings.
It still comes down to the same idea behind a straw or hole in the
vacuum cleaner hose -- a given amount of suction spread over a wider
area reduces the amount of suction at the point of entry. Or as I
said originally: "You get less suction at the business end of the
device because of additional inlets."
Can I prove any of this? No -- I don't have enough hard knowledge
about gasses or the equipment to do actual measurements.
Odin
anagama blog
http://www.anagama-west.com/firing_log
Ivor and Olive Lewis on wed 5 sep 07
Dear Michael Wendt,
Thanks for your explanation.
Seems to me as though an aperture at the base of the stack acts as a =
shunt. This prevents motion by having identical pressure at the =
firemouths and the base of the stack. The effect it to stall the =
heating.
Regards,
Ivor
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