C.C. Bookout on sun 22 feb 04
We built a 36 cu foot gas fired kiln inside an existing building. In
order to meet fire codes the chimney had to go a few feet above the roof
line which makes it rather tall at about 16 feet or a little more. I
recently bought an oxyprobe because I was not getting good results with
traditional reduction glazes such as copper red and celedon. Following
the instructions that come with the probe the initial few firing useing
their suggested setting to quide the firing I am not showing the
reduction effects in the glaze. I was wondering if anyone knew if this
is because of the tall chimney or any general information about how
chimneys affect reduction.
C.C. witing from Eastern Long Island where we are having a mild sunny
day and waiting eagerly for Spring.
Hank Murrow on sun 22 feb 04
On Feb 22, 2004, at 9:45 AM, C.C. Bookout wrote:
> We built a 36 cu foot gas fired kiln inside an existing building. In
> order to meet fire codes the chimney had to go a few feet above the
> roof
> line which makes it rather tall at about 16 feet or a little more. I
> recently bought an oxyprobe because I was not getting good results with
> traditional reduction glazes such as copper red and celedon. Following
> the instructions that come with the probe the initial few firing useing
> their suggested setting to quide the firing I am not showing the
> reduction effects in the glaze. I was wondering if anyone knew if this
> is because of the tall chimney or any general information about how
> chimneys affect reduction.
Dear C.C;
The chimney may be completely controlled by your damper, so surely you
can compensate for stronger draft by closing the damper more than you
used to before.
Also, do not fire 'by the numbers', rather...fire by your eye and smell
until you get the results you want............then fire by those
numbers. I consistently under-reduced at first after getting the
OxyProbe .
Cheers, and let us know,
Hank in Eugene
Dave Finkelnburg on sun 22 feb 04
Dear C.C.,
Your tall chimney will not keep you from getting good firings. Chimney
height is like kiln design at this point...it's what you have and you just
have to figure out how to work with it.
In general, the taller the chimney, the more draft you can get through
the kiln. This assumes the chimney cross section is large enough, of
course! Since you don't have a problem with the kiln stalling, the chimney
must be large enough.
The Oxyprobe reading and the chimney height are really separate items.
They work independently.
If the probe is located properly for the kiln, it should sense the
atmosphere accurately.
Since your chimney is tall, it can draw in more air than you may be used
to. You must have to use more damper than with a shorter chimney. If the
chimney cross section is too large, then you have to operate with the damper
nearly closed, and any tiny movement of the damper will have a big influence
on the kiln atmosphere.
However, you should see this with the probe. Copper reds are a special
case, but celadons are very straight-forward. If you aren't getting the
iron reduced before the glaze melts, you won't get the greenish-blue in the
celadon. If that's what you see, you need heavier, or earlier reduction, or
both. Just remember that at .55 on the oxyprobe you are very close to
making soot, which can cause carbon trapping. That would give you
liver-colored copper reds and other things you may not want. So don't
overdo the early reduction.
Good firing!
Dave Finkelnburg
----- Original Message -----
From: "C.C. Bookout"
Sent: Sunday, February 22, 2004 10:45 AM
> We built a 36 cu foot gas fired kiln inside an existing building. In
> order to meet fire codes the chimney had to go a few feet above the roof
> line which makes it rather tall at about 16 feet or a little more. I
> recently bought an oxyprobe because I was not getting good results with
> traditional reduction glazes such as copper red and celedon. Following
> the instructions that come with the probe the initial few firing useing
> their suggested setting to quide the firing I am not showing the
> reduction effects in the glaze. I was wondering if anyone knew if this
> is because of the tall chimney or any general information about how
> chimneys affect reduction.
Richard Aerni on sun 22 feb 04
cc,
16 feet of height does not strike me as way too much for a gas kiln of your
size. If you want to decrease the draft, simply damper the kiln more
severely. That will also help you achieve better results with your
reduction problem. Not enough heighth for a chimney, not enough cross
section, these are serious problems. Plenty of height and too much cross
section are easy to deal with.
As for the oxyprobe, don't go by "the book's" settings to govern your kiln
firings. Go by the way you have fired it and have gotten good results in
the past. Then, record those settings using the oxyprobe, and then
duplicate them in future. The suggested settings are just
that...everyone's kiln will be different, and give different readings on
the probe based upon where the probe is located, etc. Don't rely on an
instrument more than you rely on your own common sense.
Best,
Richard Aerni
On Sun, 22 Feb 2004 12:45:30 -0500, C.C. Bookout
wrote:
>We built a 36 cu foot gas fired kiln inside an existing building. In
>order to meet fire codes the chimney had to go a few feet above the roof
>line which makes it rather tall at about 16 feet or a little more. I
>recently bought an oxyprobe because I was not getting good results with
>traditional reduction glazes such as copper red and celedon. Following
>the instructions that come with the probe the initial few firing useing
>their suggested setting to quide the firing I am not showing the
>reduction effects in the glaze. I was wondering if anyone knew if this
>is because of the tall chimney or any general information about how
>chimneys affect reduction.
>
>C.C. witing from Eastern Long Island where we are having a mild sunny
>day and waiting eagerly for Spring.
John Britt on sun 22 feb 04
CC,
You might consider putting in passive dampers to adjust for the chimney
height. I usually put in about three bricks which I can then remove to
reduce the excessive draw from the chimney if I want strong reduction.
Hope it helps,
John Britt
Ron Roy on mon 23 feb 04
Hi CC,
The taller the chimney the stronger the draft and the more crucial the
damper settings of course.
Cooling will be faster as well because of the stronger draft - a good
fitting damper will help.
I found - when I had a tall chimney - that having barometric dampers
installed 6 feet above the damper helped a lot - they should be the
adjustable kind - close em when you need more draft and open em when you
want less.
RR
>We built a 36 cu foot gas fired kiln inside an existing building. In
>order to meet fire codes the chimney had to go a few feet above the roof
>line which makes it rather tall at about 16 feet or a little more. I
>recently bought an oxyprobe because I was not getting good results with
>traditional reduction glazes such as copper red and celedon. Following
>the instructions that come with the probe the initial few firing useing
>their suggested setting to quide the firing I am not showing the
>reduction effects in the glaze. I was wondering if anyone knew if this
>is because of the tall chimney or any general information about how
>chimneys affect reduction.
>
>C.C. witing from Eastern Long Island where we are having a mild sunny
>day and waiting eagerly for Spring.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
John Britt on mon 23 feb 04
Ron,
I think we are talking about the same thing, but I always refer to it as a
passive damper. I have never heard of it called a Barometric damper? That
is interesting.
John Britt
Lee love on tue 24 feb 04
John Britt wrote:
>Ron,
>
>I think we are talking about the same thing, but I always refer to it as a
>passive damper. I have never heard of it called a Barometric damper? That
>is interesting.
>
>
John, the barometric dampers I am familar with are used with
heaters and boilers (and sometimes on wood drying kilns.) Tthey work on
a hinge and use an ajustible counterweigh to keep them closed until the
draft is strong enough to open them. They are a passive damper that
uses a counterweight to keep it closed.
I've never seen them on a pottery kiln, but it wouldn't be
difficult to include one during the building of the chimney.
Lee in Mashiko, Japan http://mashiko.us "It seems to me what you lose
in mystery you gain in awe" -- Francis Crick
william schran on tue 24 feb 04
C.C. wrote:>We built a 36 cu foot gas fired kiln inside an existing
building. In
order to meet fire codes the chimney had to go a few feet above the roof
line which makes it rather tall at about 16 feet or a little more. I
recently bought an oxyprobe because I was not getting good results<
Way back in the early 70's we built a 40 cu. ft. downdraft at school
with a chimney that was about 15-20' high, everything built out of
hard firebrick. Had loads of problems getting up to temperature.
Finally decide chimney was too big - inside dimensions. Put couple of
broken shelves on top of stack narrowing the opening. Was able to get
to temperature and seemed to achieve more even temperature and better
reduction. Might give it a try to see if it would help with your
reduction problem.
Bill
Steve Mills on tue 24 feb 04
Can I add to this:
In a similar urban situation I Swiss cheese'd the bottom section of the
chimney by removing bricks to reduce the velocity of the gases. This was
with a Natural Gas kiln which relied on *draw* to fire properly. I
subsequently used the process of removing or replacing bricks as passive
(and much more controllable) dampers to obtain reduction. My current
small wood kiln uses a wider & shorter chimney, and burns effectively
without the need for a lot of draught. Such control as is needed is
achieved by controlling the primary air only.
Steve
Bath
UK
In message , Ron Roy writes
>Hi CC,
>
>The taller the chimney the stronger the draft and the more crucial the
>damper settings of course.
>
>Cooling will be faster as well because of the stronger draft - a good
>fitting damper will help.
>
>I found - when I had a tall chimney - that having barometric dampers
>installed 6 feet above the damper helped a lot - they should be the
>adjustable kind - close em when you need more draft and open em when you
>want less.
>
>RR
>
>>We built a 36 cu foot gas fired kiln inside an existing building. In
>>order to meet fire codes the chimney had to go a few feet above the roof
>>line which makes it rather tall at about 16 feet or a little more. I
>>recently bought an oxyprobe because I was not getting good results with
>>traditional reduction glazes such as copper red and celedon. Following
>>the instructions that come with the probe the initial few firing useing
>>their suggested setting to quide the firing I am not showing the
>>reduction effects in the glaze. I was wondering if anyone knew if this
>>is because of the tall chimney or any general information about how
>>chimneys affect reduction.
>>
>>C.C. witing from Eastern Long Island where we are having a mild sunny
>>day and waiting eagerly for Spring.
>
>
>Ron Roy
>RR#4
>15084 Little Lake Road
>Brighton, Ontario
>Canada
>K0K 1H0
>Phone: 613-475-9544
>Fax: 613-475-3513
--
Steve Mills
Bath
UK
Ivor and Olive Lewis on wed 25 feb 04
I know why hot gas exhausts up the stack, but what I don't understand
is why the draft gets stronger the higher the chimney is built. Can
someone please explain.
Thanks and best regards,
Ivor Lewis. Redhill, South Australia
Bruce Girrell on wed 25 feb 04
> I know why hot gas exhausts up the stack, but what I don't understand
> is why the draft gets stronger the higher the chimney is built. Can
> someone please explain.
Hot gas is less dense than cool gas. Granted, the chemical composition of
the exhaust gas is different from the air surrounding the kiln, but I think
you would still agree that the density of the gas in the flue is lower than
the density of the air outside the kiln. Because of its lower density, the
gas in the chimney is buoyant relative to the ambient air. A taller chimney
means a larger volume of low density gas, hence more buoyancy and more
draft. A larger hot air balloon can carry a heavier load. In this case the
"load" is the denser incoming air.
Just because I know that you care about such details, I should mention that
the whole affair is driven by gravity and should, in reality, be described
in terms of the cooler air displacing the less dense hot gas, But this
distinction seems silly to most and confusing to many others so I used the
buoyancy approach. It all works out the same in the end.
Bruce "buoy oh buoy" Girrell
Earl Krueger on thu 26 feb 04
All,
So, I'm trying to _understand_ about kiln chimneys and the
types and effects of dampers.
The kind of damper I am familiar with, from stoves not kilns,
restricts the flow through the kiln by partially blocking
the path the gasses must take. Not knowing a better or more
proper terminology I shall call this an "obstruction" damper.
From what I have gleaned by reading this thread there is
another method used. This method I shall call the "dilution"
damper as the effect of reducing the gas flow through the
kiln is accomplished by diluting the hot gases in the chimney
with cool ambient air thereby reducing their buoyancy. This
reduced buoyancy then produces less "draw" on the kiln proper.
I have read in other posts that reduction levels in a kiln are
very sensitive to damper adjustments. I assume that
obstruction dampers were being referred to. From my
limited knowledge of physics I can see why this would be
true.
Question #1. When would you use an obstruction damper
versus a dilution damper? Would there be an advantage
of having both available? When would you adjust one
as opposed to the other?
Question #2. I have read of kilns stalling due to low atmospheric
pressure, as when a storm is approaching. Would a taller
chimney, with a dilution damper allow better control in situations
like this ( i.e. ports open when barometric pressure is high and
closed when pressure is low)?
Thanks.
Earl K...
Bothell, WA, USA
Hank Murrow on fri 27 feb 04
Dear Earl;
Despite my friend Dave's caution, I LOVE your idea of calling the
passive damper a 'dilution' damper, because that is what it does as you
correctly point out in your post. Any time we can find a word that
reveals the process.....I say go with it.
As to 'obstruction' dampers, I have gravitated exclusively to that
type. The reason I like them above 'dilution' types is that they
encourage turbulence and mixing of the atmosphere in the chamber, thus
creating very even effects in the ware. When I fire with wood, I like
those long soft steamers of flame which give the variables of colour.
When I fire in my Doorless Fiberkiln I love the even atmospheric
effects that the 'obstruction' damper provides. Even reduction
throughout the kiln need not be equated with boring, if anyone wants to
check on this, they only have to go to www.wlotus.com and click on
'now showing' and then 'HankMurrow' to see how exciting the surfaces
can be.
I can't answer your other question, as I have noticed no difference on
stormy days with my kiln.
On Feb 26, 2004, at 12:36 AM, Earl Krueger wrote:
> So, I'm trying to _understand_ about kiln chimneys and the
> types and effects of dampers.
>
> The kind of damper I am familiar with, from stoves not kilns,
> restricts the flow through the kiln by partially blocking
> the path the gasses must take. Not knowing a better or more
> proper terminology I shall call this an "obstruction" damper.
>
> From what I have gleaned by reading this thread there is
> another method used. This method I shall call the "dilution"
> damper as the effect of reducing the gas flow through the
> kiln is accomplished by diluting the hot gases in the chimney
> with cool ambient air thereby reducing their buoyancy. This
> reduced buoyancy then produces less "draw" on the kiln proper.
> Question #1. When would you use an obstruction damper
> versus a dilution damper? Would there be an advantage
> of having both available? When would you adjust one
> as opposed to the other?
Ivor and Olive Lewis on fri 27 feb 04
Yes Bruce Girrell,
But why the higher velocity?
<> I know
<< but I think you would agree that the density of the gas in the
flue is lower than the density of the air outside the kiln.>> I do, I
Do !
<more buoyancy and ....>> Yes, I agree. But <<..... more draft. >>
Why?
<> There is a
mechanical connection. The heavy load does not push the balloon
upwards. The sum of the mass of the balloon, the mass of hot air and
the mass of the load is equal to the mass of cold air displaced. Cut
that connection and the balloon rises rapidly. Dense air is displacing
less dense air.
So I ask again "Why do you get an increase in velocity as the chimney
is extended upwards?" Archimedes says nothing to us about this
Best regards,
Ivor Lewis. Redhill, South Australia
Steve Slatin on sat 28 feb 04
Bruce -- That the heavier air displaces (supports) the lighter air can
be demonstrated by a simple test. Get a small helium balloon. Put it
in you car, where it "floats" up against the roof. Take some fairly
tight turns while driving -- the lighter balloon is displaced by the
heavier normal air, and the balloon moves in the counter-intuitive
direction -- you turn left, you feel your body pushing to the right,
and the balloon slips to the left inside your car. This test creates
the requisite "aha moment" in folks who don't initially get the concept.
Regards -- Steve S
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Bruce =
Girrell
Sent: Saturday, February 28, 2004 6:19 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: height of chimney affecting gas fired reduction results
Correct. I was trying to create an analogy. I've said this before but =
most
people think I'm off my rocker when I do: it's not about hot air rising,
it's about cold air sinking. Gravity is the driving force. The cold, =
dense
air displaces the less dense hot air. Because of this displacement, the
ballon is capable of carrying a load, but the real workhorse here is the
unseen dense air.
Note that there is a larger volume of dense air displacing the larger =
volume
of hot air in either case - balloon or chimney. The chimney has a fixed
cross-sectional area. Given that there is no pressurizing mechanism
available in a natural draft system, the only way to move the greater =
volume
through the same cross-section is to increase the velocity.
Bruce Girrell on sat 28 feb 04
> But why the higher velocity?
Ah, I hadn't understood that it was mainly the velocity that was bugging
you.
> <> There is a
> mechanical connection. The heavy load does not push the balloon
> upwards.
Correct. I was trying to create an analogy. I've said this before but most
people think I'm off my rocker when I do: it's not about hot air rising,
it's about cold air sinking. Gravity is the driving force. The cold, dense
air displaces the less dense hot air. Because of this displacement, the
ballon is capable of carrying a load, but the real workhorse here is the
unseen dense air.
Note that there is a larger volume of dense air displacing the larger volume
of hot air in either case - balloon or chimney. The chimney has a fixed
cross-sectional area. Given that there is no pressurizing mechanism
available in a natural draft system, the only way to move the greater volume
through the same cross-section is to increase the velocity.
Does that work for you?
Bruce
Ron Roy on sat 28 feb 04
Hi Earl,
The "obstruction" damper is straight forward - it affects the amount of air
being drawn into the kiln through the primary and secondary burner ports
and any openings in the kiln.
The "dilution" dampers certainly do have an effect by cooling the chimney
gases but that is - I would think - not the main effect.
Effect #1 would be - cuts down on the amount of air being drawn through the
kiln (through the burners and ports) - that would be the main effect.
#2 If the barometric dampers are free swinging (and to some extent when
fixed) - they will also dampen the effect of wind and gusts that pass over
the top of the chimney - a very important factor if you have atmospheric
type burners. They tend to lessen the effect if wind comes up during a
firing. If you are using atmospheric burners the increased draft may upset
your reduction for example.
#3 - if the kiln is inside a room the barometic damper - because it is
taking air from inside the room - will lower the pressure in that room -
and reduce the pull through the kiln.
Leaving a barometric damper open all the way will naturally slow down
cooling inside the kiln by cutting down the draw through the flue. That
will also take heat out of the room - in other words the chimney becomes an
exit for the excess heat in the kiln room.
Close them up and the draft increases - open em up and the draft decreases
- a very flexible system.
Hope that is clear - RR
>So, I'm trying to _understand_ about kiln chimneys and the
>types and effects of dampers.
>
>The kind of damper I am familiar with, from stoves not kilns,
>restricts the flow through the kiln by partially blocking
>the path the gasses must take. Not knowing a better or more
>proper terminology I shall call this an "obstruction" damper.
>
> From what I have gleaned by reading this thread there is
>another method used. This method I shall call the "dilution"
>damper as the effect of reducing the gas flow through the
>kiln is accomplished by diluting the hot gases in the chimney
>with cool ambient air thereby reducing their buoyancy. This
>reduced buoyancy then produces less "draw" on the kiln proper.
>
>
>I have read in other posts that reduction levels in a kiln are
>very sensitive to damper adjustments. I assume that
>obstruction dampers were being referred to. From my
>limited knowledge of physics I can see why this would be
>true.
>
>Question #1. When would you use an obstruction damper
>versus a dilution damper? Would there be an advantage
>of having both available? When would you adjust one
>as opposed to the other?
>
>Question #2. I have read of kilns stalling due to low atmospheric
>pressure, as when a storm is approaching. Would a taller
>chimney, with a dilution damper allow better control in situations
>like this ( i.e. ports open when barometric pressure is high and
>closed when pressure is low)?
>
>Thanks.
>
>Earl K...
>Bothell, WA, USA
>
>______________________________________________________________________________
>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.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Bruce Girrell on sat 28 feb 04
I wrote:
> Note that there is a larger volume of dense air displacing the larger
volume
> of hot air in either case - balloon or chimney.
Let me rephrase that.
Note that when the size of the balloon increases or when the height of the
chimney increases there is a larger volume of dense air displacing the
increased volume of hot gas in the larger balloon or taller chimney.
I hope that makes more sense.
Bruce "second time's a charm?" Girrell
Hank Murrow on sat 28 feb 04
Dear Bruce;
I have appreciated your posts on this. When I was teaching, I used to
demonstrate 'draft' with pingpong balls in the shop sink. One like
normal to represent hot flue gases, one almost filled with water to
demonstrate cooled gases, and one half-filled with water to demonstrate
moderately buoyant gases. Put them all at the bottom of a filled sink
and see how they rise to the top.
Cheers, Hank
On Feb 28, 2004, at 1:24 PM, Bruce Girrell wrote:
> Note that when the size of the balloon increases or when the height of
> the
> chimney increases there is a larger volume of dense air displacing the
> increased volume of hot gas in the larger balloon or taller chimney.
Ivor and Olive Lewis on sun 29 feb 04
Dear Bruce,
To increase the Volume of the balloon you have to increase the
pressure. The volume of the interior of the chimney does not change.
I understand fully that it is Gravity acting on density differential
that drives the process forcing hot gas up the stack.
Doubling the altitude of the stack does not alter either the density
of the cold air or the density of the hot gases. The motive force
(PE=mg) is constant.
Best regards,
Ivor Lewis. Redhill, South Australia
Ivor and Olive Lewis on sun 29 feb 04
Dear Lee Love,
What amazing experiences.
Do you find Eagles and other birds utilise the "thermal" effect of
your kilns to gain altitude ?
Best regards,
Ivor Lewis. Redhill, South Australia
Ivor and Olive Lewis on sun 29 feb 04
Dear Bruce,
If there is no fire in the hole then the density differential ceases
to exist.
So vertical motion upwards stops.
So, why in my cool house with the temp 10 degrees below shade temp
outdoors do I get a strong updraught when I open the ceiling trap ?
If the cross sectional area is constant the velocity remains constant.
If velocity increases then density has to decrease.
I think we are missing something here?
Best regards,
Bruce Girrell on sun 29 feb 04
> To increase the Volume of the balloon you have to increase the
> pressure.
Absolutely not. I am talking about a hot air balloon, not a pressurized
helium balloon.
>The volume of the interior of the chimney does not change.
Ivor, what are you talking about today? You asked why a taller chimney has
more draft. Are you saying that a taller chimney has no more interior volume
than a shorter one? Volume = height times area. The cross-sectional area of
the two chimneys is the same, the taller one certainly does have a larger
volume.
> I understand fully that it is Gravity acting on density differential
> that drives the process forcing hot gas up the stack.
> Doubling the altitude of the stack does not alter either the density
> of the cold air or the density of the hot gases. The motive force
> (PE=mg) is constant.
The density of each remains the same, but the mass is different because the
volume has changed.
Ivor, push the reboot button on your brain and try again.
Bruce "I've had those days" Girrell
Bruce Girrell on sun 29 feb 04
> If there is no fire in the hole then the density differential ceases
> to exist. So vertical motion upwards stops.
OK.
> So, why in my cool house with the temp 10 degrees below shade temp
> outdoors do I get a strong updraught when I open the ceiling trap ?
You have made a major step between a simple chimney and your house. My
belief though, without coming to your house and peforming the requisite
tests, would be that the temperature throughout your entire house is not
constant, with a pocket of warm air near the top, just itching to escape
through your ceiling trap.
Bruce "I'll be checking my mailbox for those Qantas tickets" Girrell
John Jensen on sun 29 feb 04
Another thought on the updraft when you open your attic: There is a
build up of heat in the attic which cannot escape through the attic
vents because there I no supply of air to replace it. When you open the
attic door you supply replacement air and the warm attic air escapes
through your attic vents.
John Jensen, Mudbug Pottery
mudbug@toadhouse.com , http://www.toadhouse.com
> So, why in my cool house with the temp 10 degrees below shade temp
> outdoors do I get a strong updraught when I open the ceiling trap ?
Lee love on sun 29 feb 04
Bruce Girrell wrote:
>Note that when the size of the balloon increases or when the height of the
>chimney increases there is a larger volume of dense air displacing the
>increased volume of hot gas in the larger balloon or taller chimney.
>
>I hope that makes more sense.
>
>
I mentioned here before, when a hot air balloon rally came
across Mashiko while we were firing the noborigama. The first one we
saw, we waved at and we could see the little people hanging in the
basket wave back at us. As they came over the kiln, the balloon
started falling out of the air. They hit the "micro-climate" the
heat the noborigama was generating. As soon as they noticed their
predicament, they opened their burners full, but kept on falling.
Eventually, they pulled out of the fall, and continued on at a much
lower altitude.
When we saw the next balloon, it was higher in the sky,
but it was coming more directly over the firing noborigama. We
shouted at them "Abunai!" "Danger!" They didn't seem to understand,
but just waved back at us, until they came over the kiln. The balloon
sack went limp and they fell much faster than the first balloon. We
all thought they were going to crash and be hurt. They opened up
their burners full. The balloon was so limp, I was afraid that the
burner flames would set it on fire! When they were very close to the
ground, they lost some velocity. As they drifted to the South West,
they came very close to hitting power lines. We ran after them and
saw that they landed safely in the Togei-Mura parking lot, a very small
space surrounded by electric and power lines. They were very lucky!
We couldn't stick around, to see if they got back off the ground, or if
a ground crew came to pack them up. We had to stoke the noborigama
again.
It made me aware of how much a large woodkiln effects the
local weather and how the little burners in a hot air balloon are no
match for the waste heat coming off of a large woodkiln. The balloon
rally is an annual event. No doubt, they told the balloonist to steer
clear of Mashiko when they see the billowing smoke of the noborigama
kilns.
Lee In Mashiko
Lee@Mashiko.org
http://Mashiko.us
"With Humans it's what's here (he points to his heart) that makes
the difference. If you don't have it in the heart, nothing you make will
make a difference." ~~Bernard Leach~~ (As told to Dean Schwarz)
Ivor and Olive Lewis on mon 1 mar 04
Dear Bruce,
It seems obvious that your Balloon analogy has confused my brain and
addled it into an even greater state of agitation.
I thought the envelope of a hot air balloon had a fixed volume.
Ah well, Back to basics.
Best regards,
Ivor
Ivor and Olive Lewis on mon 1 mar 04
Dear John,
The roof space (We do not have an attic) is fitted with a ridge
ventilator which is always open and the temperature of the air below
the ceiling is at least ten degrees cooler than air in the roof space
so it has to be denser.
The updraught is quite intense.
Best regards,
Ivor
Logan Oplinger on mon 1 mar 04
On Sun, 29 Feb 2004 16:42:48 -0500, John Jensen wrote:
>Another thought on the updraft when you open your attic: There is a
>build up of heat in the attic which cannot escape through the attic
>vents because there I no supply of air to replace it. When you open the
>attic door you supply replacement air and the warm attic air escapes
>through your attic vents.
>
>John Jensen, Mudbug Pottery
>mudbug@toadhouse.com , http://www.toadhouse.com
>
>> So, why in my cool house with the temp 10 degrees below shade temp
>> outdoors do I get a strong updraught when I open the ceiling trap ?
>
John, Bruce, Ivor,
My two cents worth. I've noticed in empty, cold kilns with a chimney, a
draft going up the chimney. My only conclusion is that there must be air
movement across the top of the chimney causing a slight venturi effect.
This is assuming negligable differential heating of a metal chimney stack
due to differences in atmospheric air temperature. In the early morning
before sunrise, or in the shade, solar heating can be discounted.
Could this venturi effect also be happening when the ceiling/roof vents are
opened?
Logan Oplinger
Another Pacific Island
Bruce Girrell on mon 1 mar 04
> I thought the envelope of a hot air balloon had a fixed volume.
When these threads get strung out over a long period of time some of the
original context gets lost. Remember me thinking that you had water in your
magnet/iron powder plasticity model?
In my first response I said "A taller chimney
means a larger volume of low density gas, hence more buoyancy and more
draft. A larger hot air balloon can carry a heavier load."
I changed the size of the hot air balloon to correspond to the increasing
size of the chimney, thus the change in the envelope of the balloon.
Cheers,
Bruce " 'bout hammered this one to death" Girrell
pdp1@EARTHLINK.NET on mon 1 mar 04
Hi Ivor,
An outside breeze, even if slight...could do it...
What sort of up draft have you if there is definately still
air outside...?
Phil
Las Vegas
----- Original Message -----
From: "Ivor and Olive Lewis"
> Dear John,
> The roof space (We do not have an attic) is fitted with a
ridge
> ventilator which is always open and the temperature of the
air below
> the ceiling is at least ten degrees cooler than air in the
roof space
> so it has to be denser.
> The updraught is quite intense.
> Best regards,
> Ivor
>
>
____________________________________________________________
__________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your
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> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached
at melpots@pclink.com.
Ivor and Olive Lewis on tue 2 mar 04
Dear Logan,
Not in the case of the Ceiling. It happens even on days when the
atmosphere is still.
But I think you have a valid point about there being a Venturi Effect.
Almost like those mouth operated spray devices.
Yet we are still left with that quandary about height and flue
velocity.
Best regards,
Ivor Lewis. Redhill, South Australia
Ivor and Olive Lewis on tue 2 mar 04
Phil,
just about good enough to put one into orbit.
Interior temp usually about 28, Shade temp 42, Tin temp, 60 plus.
Double ceiling insulation helps keep the place cool.
If the breeze gets up we get "Willi Willis" which travel across the
Condowie and can reach several hundred feet into the air. Almost like
miniature tornados.
Interestin' though
Best regards,
Ivor
----- Original Message -----
From:
To:
Sent: Tuesday, 2 March 2004 5:21
Subject: Re: height of chimney affecting gas fired reduction results
> Hi Ivor,
>
>
>
> An outside breeze, even if slight...could do it...
>
>
> What sort of up draft have you if there is definately still
> air outside...?
>
>
>
> Phil
> Las Vegas
>
> ----- Original Message -----
> From: "Ivor and Olive Lewis"
>
>
> > Dear John,
> > The roof space (We do not have an attic) is fitted with a
> ridge
> > ventilator which is always open and the temperature of the
> air below
> > the ceiling is at least ten degrees cooler than air in the
> roof space
> > so it has to be denser.
> > The updraught is quite intense.
> > Best regards,
> > Ivor
> >
> >
> ____________________________________________________________
> __________________
> > 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.
>
>
______________________________________________________________________
________
> 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.
Ivor and Olive Lewis on tue 2 mar 04
And we still do not know if the speed of the draught changes if we
change the height of the chimney !!! :-))
Best regards
Ivor ..."What was the question to which the answer is....?" Lewis.
In Redhill, South Australia, with another long hot week ahead
Joseph Herbert on tue 2 mar 04
One effect that plays into air movement is called Bernoulli's Principle. In
a venturi device, the most famous is the automotive carburetor, relies on
the observation that air moving at a higher velocity does so at lower
pressure. In the classic illustration, air moving through a closed tube
goes from an area of larger cross section, through an area of smaller cross
section, and back into an area of larger cross section. Since the same
amount of air moves through all the sections of the device, the velocity of
air in the part that has a smaller cross section must be greater. When the
air pressure is measured each of the areas, the pressure is lower in the
smaller cross section part where the velocity is greatest. In the
automotive carburetor or in the Cricket mouth spray device, the inlet for a
liquid supply is located in that area so the lower pressure draws the liquid
into the air stream. This principle is used in all sorts of devices from
pressure measuring instruments to the jet pumps in Boiling Water Nuclear
Reactors. Then there is the classic explanation of how airplanes fly: air
over the curved wing travels further ( and therefore faster) making the top
of the wing a low pressure area compared to the bottom of the wing.
In chimneys, Count Rumford revolutionized interior heating by recognizing
that there can be both updrafts and downdrafts in a single chimney at the
same time. Rumford invented the smoke shelf in the back of a properly
constructed fire place chimney that had the role of deflecting cold down
drafts coming down the back of the chimney into the rising heated stream of
gasses from the fire. The other role of the smoke shelf, along with a
specially constructed smooth, curved throat piece in the front of the inside
of the fire box, created a venturi situation that accelerates smoke from the
fire box into the chimney.
The situation in non-fireplace chimneys can be different. One might expect
that the case of a pottery kiln, the volume of gas entering the kiln, the
expansion from combustion, and the expansion from heating excess air
produces a positive pressure in the kiln at some times (obvious during
reduction). This should mean that there are sufficient hot gasses to fill a
properly sized chimney. The density difference does drive the draft in a
chimney but it is not the only thing. Once the draft is established, the
inertia of the moving air assists the draft. This can be a significant
factor. For an extreme example, a deep well (23,000') was being tested by
allowing natural gas from the well to flow into a pipe line. The bottom
hole pressure of the well was in excess of 10,000 psi so the well head
equipment was rated for 15,000 psi. The well was producing gas at a rate of
a million cubic feet per day during the test. The person conducting the
test closed the valve in the well head too quickly (not giving the rapidly
moving gas to slow) and the inertia of the moving 20,000 foot column of gas
acted on the well head and broke the valve. The good news was that the gas
was flowing into the pipeline but the bad news that there was no way to stop
it. Once the well head equipment failed, all the other piping had a maximum
pressure rating of 3000 psi. , far less than the static pressure the well
would produce.
The effects that work in a chimney are pretty localizes. Often one sees the
smoke from a home chimney rise from the chimney for a short distance (50
feet) before traveling horizontally to disburse. This is evidence of a
temperature inversion where the overlying air is warmer than the surface
layer. As the flue gases and smoke cool to the ambient temperature of the
surrounding air, they may encounter an upper layer of warmer, lower density
air and there stop rising and spread out along the interface between the two
air masses. Both electrical generating power plants and ore smelters tend
to have tall chimneys. In both cases, the rising gasses may not continue to
rise and the strategy is to get the stuff up high so it will be far away and
mixed with lots of air before it gets back to the ground. This works with
relative degrees of success from some to none. The neighborhood around a
Dallas battery making plant and lead smelter is relatively contaminated with
lead from the stack that didn't get all that far away from the stack and did
not "disperse" all that much. Under some conditions, deposits of fly ash
from power plants on their employees cars caused significant problems for
the paint on the cars. Basically, the flue gasses were leaving the stack
and coming directly to the ground.
For a chimney to be successful, the effect of the density difference between
the flue gasses exiting the chimney and the air above the chimney only has
to be great enough to prevent a buildup of hot combustion products at the
top of the chimney. This is a pretty low bar to have success. In modern
electrical generating power plants, the heat exchangers are effective enough
that there is not really enough heat to drive a good draft in the stack.
These plants have both induced draft and forced draft blowers to move the
air and combustion products through the furnace.
Joseph Herbert
Ivor and Olive Lewis on wed 3 mar 04
Dear Joseph Herbert,
Thank you for your contribution. The Bernoulli principle and the
Venturi Effect we are aware of, but the Rumford inventions are new
information to me. I wonder if that could be adapted to the stack of a
kiln?
The remainder was interesting but does not answer the question about
the speed of smoke up the stack.
Nils Lou makes some interesting observations which are well worth
reading.
Best regards,
Ivor Lewis. Redhill, South Australia
Michael Wendt on wed 3 mar 04
If I understand the question, then here is the answer:
Atmospheric pressure is higher at sea level than any
point above it. What is not immediately obvious to
most of us is atmospheric pressure is greater at the
bottom of a chimney than at the top and the taller
the chimney, the greater the difference in atmospheric
pressure.
As to why this would be true, a simple thought
experiment helps us see:
Imagine the atmosphere is a stack of books some
immense number high. Since gasses compress with
pressure, the books at the bottom of the stack are
very heavy and dense. Each successive book higher
in the stack is slightly less dense. Consider the weight
on the face of the bottom book. On the floor it is the
weight of all the book stack, but where the book
touches the one above, it is the weight of all the books
minus the bottom book. The chimney is just the same.
If a gas is heated, it expands, weighs less than the
surrounding air and as a result, has a tendency to
rise because the pressure on the bottom of the gas
is greater than the pressure on the top. The more
force you apply to a system
(the taller you make the chimney),
the greater the resultant acceleration.
The chimney absorbs heat from the gas and cools it
so that can play a role too but in general, once the
chimney reaches a temperature where this cooling
effect drops out, the draw from a tall chimney has
more pull than a short one.
Simplistic, I realize, but observations support it.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
Ivor and Olive Lewis on thu 4 mar 04
Dear Michael,
An interesting analogy and a thought provoking imaginary experiment.
But surely, if the taller chimney cools effluent gas that will
increase in density and help to push rising effluent downwards. So,
would not the upward velocity decrease? Is that what you wish me to
believe?
By the way, in considering your information <<. What is not
immediately obvious to most of us is atmospheric pressure is greater
at the
bottom of a chimney than at the top and the taller the chimney, the
greater the difference in atmospheric pressure.>> This is true. But
displacement of kiln effluent gas is because of differences in
Density, not difference of Pressure.
So we are still left wondering about the change in speed or velocity
as the height increases.
Best regards,
Ivor Lewis. Redhill, South Australia
pdp1@EARTHLINK.NET on thu 4 mar 04
Hi Ivor, all...
In my layman's imagination...I suppose a sort of boundary
layer of intermediatly heated Gasses, as sort of 'roll'
along the interior sides of the disparately cooler
Chimney...
Would you suppose something similar as well...?
Phil
Las Vegas
----- Original Message -----
From: "Ivor and Olive Lewis"
> Dear Michael,
> An interesting analogy and a thought provoking imaginary
experiment.
> But surely, if the taller chimney cools effluent gas that
will
> increase in density and help to push rising effluent
downwards. So,
> would not the upward velocity decrease? Is that what you
wish me to
> believe?
> By the way, in considering your information <<. What is
not
> immediately obvious to most of us is atmospheric pressure
is greater
> at the
> bottom of a chimney than at the top and the taller the
chimney, the
> greater the difference in atmospheric pressure.>> This is
true. But
> displacement of kiln effluent gas is because of
differences in
> Density, not difference of Pressure.
> So we are still left wondering about the change in speed
or velocity
> as the height increases.
> 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
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> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached
at melpots@pclink.com.
Susan Giddings on thu 4 mar 04
Hello everyone,
I've been following this thread with a lot of interest, but I am completely
lost. Some of the messages got technical to a point well past my
understanding. That's OK, I really don't need to understand all the details
of all the science. But, I just don't have any idea what the answer to the
question is.
I did not start this thread. I don't know who did and I am not trying to
undermine or intrude in their understaqnding. I just don't get it - and I
want to. What I'd like to know is: does the height of the chimney have an
effect on reduction atmosphere in a gas fired kiln? Yes or no?
If yes, then, what is the effect? Does it tend to increase reduction, or
reduce it to a point where it's hard to get reduction in the kiln? While
we're at it, what about the width of the chimney? (I guess it's overall
chimney capacity I'm asking about here.)
Why I am asking: we have a very small Olympic updraft in the co-op to which
I belong (like a torchbearer). I think it is just under 8 cubic feet
capacity. The chimney is long and goes up through the roof and about 10-12"
diameter. I'd guess 10' in length but could be more. In addition, we have a
"hood" directly over the kiln that goes directly up to the ceiling where the
chimney starts. This hood has a flap which is dropped down when the kiln is
fired so that the area directly over the kiln and under the opening for the
chimney is completely enclosed. (This was done in attempt to confine CO
escaping into our neighbor's office.)
There is another issue with the burners and orifices, and I don't think that
can be brushed aside. I don't know if the enclosure between the kiln and
chimney has an effect or not. Notwithstanding these other factors, I really
think that there is an effect we are seeing in reduction atmosphere and
effects in the ware that is directly the result of the chimney "capacity".
To me, reduction effects are just not good or just not even present. I would
like to know what we could do to change this - if anything. That's where I
think the burners and orifices come into play. Would we want to open or
close the orifices? (In this style kiln, you can not change anything with
the burners once the kiln is lit. Whatever they are at, they must stay that
way for the duration of the firing.) There are 8 burners around the
circumference of the kiln that are all mounted on a ring but, aside from
that, are independent from each other.
So even though I didn't start this thread, I am very interested in these
answers. I am sorry gentlemen, I am just not understanding, not able to put
it all together.... I'm looking for an answer that will lead me to a path of
how to fix what we've got.
So just a little bit more, please?
------------------
Susan Giddings
"There are painters who transform the sun into a yellow spot, but there are
others who, thanks to their art and intelligence, transform a yellow spot
into the sun." — Pablo Picasso
_________________________________________________________________
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Snail Scott on thu 4 mar 04
At 10:06 AM 3/4/04 -0500, you wrote:
>...does the height of the chimney have an
>effect on reduction atmosphere in a gas fired kiln? Yes or no?
>If yes, then, what is the effect?
>...we have a very small Olympic updraft...
>...we have a "hood" directly over the kiln that goes directly
>up to the ceiling where the chimney starts...
>...reduction effects are just not...present...
Yes, it does affect the atmosphere, because a taller
chimney typically means a greater draw (or the potential
for greater draw) and with the same gas/air settings,
and the greater draw will increase air flow through the
kiln and reduce reduction (so to speak). ;)
I have seen changes in chimneys affect kilns this way.
Especially at high altitude and low atmospheric pressure,
a tall chimney can make the difference between a 'stock'
kiln which will reduce heavily and fail to reach
temperature, and one that can be fired normally with the
option to achieve neutral atmosphere if desired. I think
it's reasonable to extrapolate and assume that it could
also make the difference (in a kiln with adequate air)
between achieving reduction and not.
Even with an updraft kiln, an exhaust stack (which is
not, strictly speaking, a 'chimney') WILL affect the
firing properties of the kiln, causing a draft and
effectively increasing air flow, even if the stack is
not tight to the kiln.
If you are having trouble achieving reduction, and
you have an unavoidably tall exhaust stack, consider
restricting the space in the burner ports to take
in less secondary air.If this is insufficient, try
reducing the air intake for the burners themselves.
-Snail
Michael Wendt on thu 4 mar 04
Dear Ivor,
In a chimney of uniform cross section the velocity of the gas stream can be
deduced by inserting a thermocouple probe at different heights, measuring
the temperature and applying the Ideal Gas Law:
PV = nrT
where:
P = the pressure
V= the Volume
n = the number of molecules present
r = the gas constant
T = the temperature in degrees Kelvin (absolute scale)
I recently did some flash calciner design work for a company and had to run
these very calculations so bear with me please:
P in our case is so close to constant that it drops out
V will be the variable sought
n does not change regardless of temperature so it drops out
r is a constant so it drops out
T is a variable and one we can measure so it stays and determines V
Further, since V is proportional to T, a pair of equations allow the
approximation of the volume at T2 if The volume at T1 is known.
Example:
My burner fan is rated at 50 cubic feet per minute. The gas flow was about 3
cubic feet per minute. The starting temperature was 20 degrees C or 293
degrees Kelvin. The goal temperature was 1100 degrees C or 1373 degrees K.
So...
V2/V1 = T2/T1
solve for V2
get V2 = T2/T1 * V1 here V2 = 1373/293 x 53 = 248 cubic feet.
if this much gas is produced each minute and flows through a 1 square foot
chimney, then at that point, it flows at a rate of 248 feet per minute =
4.13 feet per second.
The exit point temperature was measured and found to be 1230 degrees K so
the velocity at the exit point had to be V2' = T2'/ T1* V1 = 1231/293 x 53
=
222 cubic feet per minute = 3.7 feet per second. This is slower than the
hooter part of the calciner.
At any point in the calciner (or chimney), the volume of the gas is directly
proportional to the absolute temperature so as the calciner cooled the gas
stream, the volume of the gas shrank and the velocity dropped. To keep the
velocity constant in a chimney, the cross sectional area of the chimney must
take into account the temperature of the gas at various points. It does not
matter if the cross section is constant but a tapered chimney requires less
material to build it and so saves money.
Ivor wrote in part:
>Dear Michael,
An interesting analogy and a thought provoking imaginary experiment.
But surely, if the taller chimney cools effluent gas that will
increase in density and help to push rising effluent downwards. So,
would not the upward velocity decrease? Is that what you wish me to
>believe?
Point 2
The observation that an object must be lower in density than the medium it
is in in order to float or rise is correct but it is NOT an explanation of
WHY it floats.
The only mechanism that can account for floating is the pressure gradient
that exists in all fluids and gasses in a gravitational field. In a true
sense, you are seeing the effect of gravity since a low density object
suspended in water remains static in freefall or zero G.
Ivor wrote: in response to my post:
>greater the difference in atmospheric pressure.>> This is true. But
displacement of kiln effluent gas is because of differences in
>Density, not difference of Pressure.
If you want further clarification of the reason objects float, I will do the
balloon example in the next post to illustrate it.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
Ivor and Olive Lewis on fri 5 mar 04
Dear Susan,
Your "Olympic set up" interests me.
Are you able to send me a sketch showing the relationships between,
kiln hood and stack. .(jpeg or gif file please)
I need to know if there is an air space, a measurable gap, between the
exit point from the kiln to the hood and another gap between the hood
and the stack which goes through the roof.
Another point to be clarified. From your description this is an
"Updraught Kiln". Is the exit from the kiln in the centre of the roof?
'Tis I keeps harping on about the velocity of effluent gases.....
Best regards,
Ivor Lewis. Redhill, South Australia
Ivor and Olive Lewis on fri 5 mar 04
Dear Phil,
Perhaps in a chimney all things are possible !
We lived in a 15 ct cottage from which the original fire place had
been removed and replace by a small CI "All Night, Constant hot water"
stove.
Olive was well overdue with our second when she lifted the lid and
shot some fatty scraps into the fire box. There was a "Whoosh", a very
loud report and a lot of soot. No, it did not induce the awaited
"Labour" and we though nothing more about it, except to clean up the
soot, until I went to get something from our bedroom, which was above
the kitchen.
A seemingly solid wall had disintegrated with stonework all over the
bed and a large flag stone lifted and tilted sideways into a large
cavity. This had been the "Smoke Hole" for curing meat. A hundred
years or more of accumulated coal dust had been stirred up by the
"Whoosh". Flames carried into the cavity had exploded.
Read Encyclopedia Brit. for a run down on "Chimney".
Yes, it would be possible to have air rising on one side and a down
draught on the other. The throttle plate would increase velocity and
cause slip-streaming.
Best regards,
Ivor Lewis. Redhill, South Australia
Ivor and Olive Lewis on fri 5 mar 04
Dear Michael,
You have changed the conditions of the experiment. I cry "Foul"
We were discussing a natural draught. Now you are using a blower (Fan)
to induce motion.
Which way do you want to go, that is, do you want a mechanically
induced draught or do you wish to stay with an open atmospheric
system. If we follow your current argument many people will be scared
away so it becomes a private matter, and a topic I am not particularly
interested in even though I do have a big centrifugal blower to hook
up.
By the way, with your new system, if you double the cross sectional
area of your stack you halve the velocity of the gas flow. If you
double the length of your stack but keep to a 1 sq ft cross section
the velocity remains constant.
Back to the Wind Tunnel l!!!
Best regards,
Ivor Lewis. Redhill, South Australia
----- Original Message -----
From: "Michael Wendt"
To:
Sent: Friday, 5 March 2004 4:50
Subject: Re: height of chimney affecting gas fired reduction results
> Dear Ivor,
> In a chimney of uniform cross section the velocity of the gas stream
can be
> deduced by inserting a thermocouple probe at different heights,
measuring
> the temperature and applying the Ideal Gas Law:
>
> PV = nrT
> where:
> P = the pressure
> V= the Volume
> n = the number of molecules present
> r = the gas constant
> T = the temperature in degrees Kelvin (absolute scale)
>
> I recently did some flash calciner design work for a company and had
to run
> these very calculations so bear with me please:
> P in our case is so close to constant that it drops out
> V will be the variable sought
> n does not change regardless of temperature so it drops out
> r is a constant so it drops out
> T is a variable and one we can measure so it stays and determines V
> Further, since V is proportional to T, a pair of equations allow the
> approximation of the volume at T2 if The volume at T1 is known.
> Example:
> My burner fan is rated at 50 cubic feet per minute. The gas flow was
about 3
> cubic feet per minute. The starting temperature was 20 degrees C or
293
> degrees Kelvin. The goal temperature was 1100 degrees C or 1373
degrees K.
> So...
> V2/V1 = T2/T1
> solve for V2
> get V2 = T2/T1 * V1 here V2 = 1373/293 x 53 = 248 cubic feet.
> if this much gas is produced each minute and flows through a 1
square foot
> chimney, then at that point, it flows at a rate of 248 feet per
minute =
> 4.13 feet per second.
> The exit point temperature was measured and found to be 1230 degrees
K so
> the velocity at the exit point had to be V2' = T2'/ T1* V1 =
1231/293 x 53
> =
> 222 cubic feet per minute = 3.7 feet per second. This is slower
than the
> hooter part of the calciner.
> At any point in the calciner (or chimney), the volume of the gas is
directly
> proportional to the absolute temperature so as the calciner cooled
the gas
> stream, the volume of the gas shrank and the velocity dropped. To
keep the
> velocity constant in a chimney, the cross sectional area of the
chimney must
> take into account the temperature of the gas at various points. It
does not
> matter if the cross section is constant but a tapered chimney
requires less
> material to build it and so saves money.
> Ivor wrote in part:
> >Dear Michael,
> An interesting analogy and a thought provoking imaginary experiment.
> But surely, if the taller chimney cools effluent gas that will
> increase in density and help to push rising effluent downwards. So,
> would not the upward velocity decrease? Is that what you wish me to
> >believe?
>
> Point 2
> The observation that an object must be lower in density than the
medium it
> is in in order to float or rise is correct but it is NOT an
explanation of
> WHY it floats.
> The only mechanism that can account for floating is the pressure
gradient
> that exists in all fluids and gasses in a gravitational field. In a
true
> sense, you are seeing the effect of gravity since a low density
object
> suspended in water remains static in freefall or zero G.
> Ivor wrote: in response to my post:
> >greater the difference in atmospheric pressure.>> This is true. But
> displacement of kiln effluent gas is because of differences in
> >Density, not difference of Pressure.
> If you want further clarification of the reason objects float, I
will do the
> balloon example in the next post to illustrate it.
> Regards,
> Michael Wendt
> Wendt Pottery
> 2729 Clearwater Ave
> Lewiston, ID 83501
> wendtpot@lewiston.com
> www.wendtpottery.com
>
>
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