Eva Gallagher on wed 12 sep 07
Hello,=20
I need to add about 2-3 feet to my chimney for my wood burning kiln to =
be well clear of the roof. Our newspaper delivery man got all interested =
in what I was building and gave a 3 foot stainless steel insulated =
chimney piece. It has an inside diameter of 10" but my flue up to the 12 =
1/2 foot level on the inside is 9 1/2 x 13 1/2" - so the stainless is =
about 60% of the area. Would that restrict the draft too much? I am =
building a long throat train according to Steve Harrison's plans and he =
has been extremely helpful all along the process but unfortunately has =
no experience with this sort of situation. Any advice would be greatly =
appreciated.
Eva Gallagher
Deep River, Ontario
Bruce Girrell on wed 12 sep 07
Eva Gallagher wrote
[The stainless steel insulated chimney piece] has an inside diameter of 10"
but my flue up to the 12 1/2 foot level on the inside is 9 1/2 x 13 1/2" -
so the stainless is about 60% of the area. Would that restrict the draft too
much?
See if you can find a copy of The Kiln Book, by Frederick Olsen. He talks
about reducing the diameter of the flue at the top by design. A 13 1/2 by 9
1/2 flue sounds excessively large to me anyway. You don't say what the
volume of your kiln is or how many/what size your burner ports are, but I
would think you would need a pretty darned big kiln to need that much flue
area. Sorry I don't have a copy of the book handy at the moment to give you
more info. Maybe the others can help.
Bruce Girrell
Mary & Wes Handrow on wed 12 sep 07
Everyone should also know that a 9" X 9" flue behaves like a 9" diameter
circular flue and a 9" X 13" behaves like an oval shaped flue because of
friction (turbulence) and you lose the corners of effective draft. As the
gases cool they reduce in volume so a smaller opening at the top tends to
increase the velocity of the gases exiting the top of the stack maintaining
or in some cases increasing the effective draft of the stack.
Eva Gallagher wrote
[The stainless steel insulated chimney piece] has an inside diameter of 10"
but my flue up to the 12 1/2 foot level on the inside is 9 1/2 x 13 1/2" -
so the stainless is about 60% of the area. Would that restrict the draft too
much?
See if you can find a copy of The Kiln Book, by Frederick Olsen. He talks
about reducing the diameter of the flue at the top by design. A 13 1/2 by 9
1/2 flue sounds excessively large to me anyway. You don't say what the
volume of your kiln is or how many/what size your burner ports are, but I
would think you would need a pretty darned big kiln to need that much flue
area. Sorry I don't have a copy of the book handy at the moment to give you
more info. Maybe the others can help.
Bruce Girrell
Elizabeth Priddy on wed 12 sep 07
My flue is small and short, the kiln is a cone 7 wood kiln.
It would not apply to the kiln in question here as it is a
vastly different scale
I am sorry I don't have a better answer. My flue is designed
to be removed easily due to hurrican and nor'easter wind
conditions. It works fine for me, but I doubt that it would be
a good model for very large kilns.
E
Elizabeth Priddy
Beaufort, NC - USA
Natural Instincts Conference Information:
http://downtothepottershouse.com/NaturalInstincts.html
http://www.elizabethpriddy.com
http://www.flickr.com/photos/7973282@N03/
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Ivor and Olive Lewis on thu 13 sep 07
Dear Eva Gallagher,
My understanding of the Physics is that the velocity of the escaping =
flue gas is determined by the "Equation of Continuity". Velocity of =
moving fluids, your flue gas, will increase when the area of flow is =
constricted. This allows the same volume of gas to pass in unit time =
without causing any pressure increase in your kiln chamber.
Hope your addition is successful.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
JL Webster on thu 13 sep 07
Forgive my ignorance to kiln workings but I have to ask about this. I keep
reading this bit about downsizing the top of a flue. Two things that I want
to make sure I am understanding. First is directly to do with Ivor's
comments. Are you saying that the same volume of gases pass through the kiln
when the top of the flue is restricted? The second is why higher velocity
exit from the flue stack is advantageous. What is the point of faster
movement right at the top of the stack? For some I'm sure this is obvious
but if the firebox and ware chamber is seeing no difference in the amount of
air flowing through I fail to see how this alters the firing.
Jared
PS. I can only assume that sufficient restriction would slow the gases, but
that we are talking about reducing the size by small percentages and not
some huge amount.
On 9/13/07, Ivor and Olive Lewis wrote:
>
> Dear Eva Gallagher,
>
> My understanding of the Physics is that the velocity of the escaping flue
> gas is determined by the "Equation of Continuity". Velocity of moving
> fluids, your flue gas, will increase when the area of flow is constricted.
> This allows the same volume of gas to pass in unit time without causing any
> pressure increase in your kiln chamber.
>
> Hope your addition is successful.
>
> 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 fri 14 sep 07
Dear Jared Webster,
You ask me the following questions : -
"Are you saying that the same volume of gases pass through the kiln when =
the top of the flue is restricted?" Per unit of time I would anticipate =
the same volume of gas to pass though the exit from the stack. But if =
there were sparks entrained then the ones from the smaller aperture =
would appear to travel faster than those from the greater aperture area.
"The second is why higher velocity exit from the flue stack is =
advantageous." I see no advantage, nor did I suggest there would be one. =
I responded to show that extending the height of a stack with a narrower =
tube would not significantly affect the rate of air flow into the kiln =
chamber. There is a marginal change but it is not significant. Effective =
use of the damper is more important.
" What is the point of faster movement right at the top of the stack? " =
It might be significant in dissipating noxious or toxic pollutants by =
encouraging a greater degree of mixing with the surrounding atmosphere =
and projecting the blend to a greater height above ground level.
Sometimes a gas, oil or wood fired kiln will stall and no amount of =
juggling with fuel input or damper setting will push the temperature =
higher. The solution is to increase the height of the stack. This =
generates greater gravitational force which drives the virtual pump that =
pushes air into the kiln. This is assisted by the decrease in density of =
the heated air and combustion products. So the higher the stack the =
greater the gravitational differential and the greater the potential =
velocity of the draught in the fireboxes. The damper is used to control =
the volume of air flowing into the chamber.
I hope this satisfies your curiosity.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Michael Wendt on fri 14 sep 07
Jared,
Ivor referred to the ideal gas law which is expressed:
PV= nrT
P = pressure
V= volume
n= the number of molecules of the gas present
r = the ideal gas constant
T= temperature expressed in degrees Kelvin
(the degrees are the same as Celsius but start
at absolute zero = - 273.16 degrees Celsius)
Consider the typical chimney. As the gas rises,
it give up heat to the chimney and its volume
decreases in direct proportion to the temperature
in degrees Kelvin.
If the chimney is the same cross section from top to
bottom, the gases near the top flow slower.
In very large cross section chimneys, this can lead
to convection cells and turbulence. The goal is to have
the
gas stream be as close to the same velocity from bottom
to top.
In very tall chimneys, this fact can also help save
cost and
tends to make for a more stable shape, wider at the
bottom than at the top.
For more detailed math, see below.
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
Jared wrote:
Forgive my ignorance to kiln workings but I have to ask
about this. I keep
reading this bit about downsizing the top of a flue.
Two things that I want
to make sure I am understanding. First is directly to
do with Ivor's
comments. Are you saying that the same volume of gases
pass through the kiln
when the top of the flue is restricted? The second is
why higher velocity
exit from the flue stack is advantageous. What is the
point of faster
movement right at the top of the stack? For some I'm
sure this is obvious
but if the firebox and ware chamber is seeing no
difference in the amount of
air flowing through I fail to see how this alters the
firing.
Jared
PS. I can only assume that sufficient restriction would
slow the gases, but
that we are talking about reducing the size by small
percentages and not
some huge amount.
Examples:
Let T1 at the bottom be 1587 K
Assume T2 at the top is 1387 K
the ratio of volumes is V2/V1=T2/T1
so V2 is 1387/1587xV1=0.874 V1.
From this you can see the cross sectional
area of the chimney at the top can be smaller by
about 13% and still not pose any added restriction.
My numbers are arbitrary and not to be construed to
be actual measured values, merely math examples.
mw
om on fri 14 sep 07
On Sep 13, 2007, at 5:12 PM, JL Webster wrote:
> The second is why higher velocity
> exit from the flue stack is advantageous. What is the point of faster
> movement right at the top of the stack?
Guessing mind you, but I would think a faster velocity at the chimney
mouth would make downdrafts less likely because the escaping gasses
would be more able to "push back" against the outside air trying to
go down the chimney.
Odin
anagama blog:
http://www.anagama-west.com/firing_log
Eva Gallagher on fri 14 sep 07
Hello Ivor - I guess that means the narrower metal tube won't act like a
plug - which is what I was worried about.
Thanks for the info.
Eva
----- Original Message -----
From: "Ivor and Olive Lewis"
To:
Sent: Friday, September 14, 2007 3:05 AM
Subject: Re: decreasing diameter of chimney flue at top
Dear Jared Webster,
You ask me the following questions : -
"Are you saying that the same volume of gases pass through the kiln when the
top of the flue is restricted?" Per unit of time I would anticipate the
same volume of gas to pass though the exit from the stack. But if there were
sparks entrained then the ones from the smaller aperture would appear to
travel faster than those from the greater aperture area.
"The second is why higher velocity exit from the flue stack is
advantageous." I see no advantage, nor did I suggest there would be one. I
responded to show that extending the height of a stack with a narrower tube
would not significantly affect the rate of air flow into the kiln chamber.
There is a marginal change but it is not significant. Effective use of the
damper is more important.
" What is the point of faster movement right at the top of the stack? " It
might be significant in dissipating noxious or toxic pollutants by
encouraging a greater degree of mixing with the surrounding atmosphere and
projecting the blend to a greater height above ground level.
Sometimes a gas, oil or wood fired kiln will stall and no amount of juggling
with fuel input or damper setting will push the temperature higher. The
solution is to increase the height of the stack. This generates greater
gravitational force which drives the virtual pump that pushes air into the
kiln. This is assisted by the decrease in density of the heated air and
combustion products. So the higher the stack the greater the gravitational
differential and the greater the potential velocity of the draught in the
fireboxes. The damper is used to control the volume of air flowing into the
chamber.
I hope this satisfies your curiosity.
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 sat 15 sep 07
Dear Eva Gallagher,=20
Thirty years ago the first large kiln that I built was not drawing =
well, even though I was using fan driven oil burners. A friend provided =
me with a 12 foot tube, rolled mild steel 8 ins dia. The flue was 9x9. A =
flange on the bottom of the tube set on plastic clay gave me a good seal =
. Never had any more trouble.
Always a pleasure to put forward ideas.
Best regards,
Ivor
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