Hank Murrow on tue 11 dec 07
On Dec 11, 2007, at 8:50 AM, Matt MacIntire wrote:
> A question for those knowledgeable about kiln design...
>
> Within normal limits, would a lower but wider chimney have (roughly)
> similar draw to a taller but narrower chimney?
Dear Matt:
What a chimney does is keep the hot gases hot until they leave the
chimney, thus inducing draft at the front end of the kiln. The volume
of effluent is another matter. A leaky uninsulated chimney wouyld
need to be taller than one which was well insulated, and any chimney
needs to be big enough to evacuate the kiln gases in proportion to
the input of fuel and air. Most studio-sized kilns seem to make do
very well with 10 or 12 foot chimneys, and I imagine that a small
kiln of 20 cuft might get by with a 10" diameter chimney, while a 40
cuft kiln might need a 12" diameter chimney. Remember that the
corners of a rectangular chimney do not 'count' in figuring the
needed area, because they contribute little to draft.
what are you building?
Cheers, Hank
www.murrow.biz/hank
Matt MacIntire on tue 11 dec 07
A question for those knowledgeable about kiln design...
Within normal limits, would a lower but wider chimney have (roughly)
similar draw to a taller but narrower chimney?
None of the books I've seen discuss this directly, but I have seen it
mentioned online. I'm not talking about a two foot tall chimney that is
four feet across. But if I increase the section, could I reduce the
height in some corresponding way? It would seem that the draft would be
weaker, but the volume of air drawn might remain similar.
Any opinions or experience would be greatly appreciated. Specific
examples of your chimney height and section compared to chamber size
would be fabulous. Thanks in advance.
Ivor and Olive Lewis on wed 12 dec 07
Dear Matt MacIntire,=20
This has been discussed and there should be something somewhere with my =
name on it in archives.
The draw of a Stack is related to height and temperature. The higher the =
temperature the lower the density in comparison the cold outer =
atmosphere. The effect of gravity on the dense air makes it sink. This =
causes hot low density gas up float up the stack.
There are equations relating Gravitational Source, Stack Height, Density =
of hot and cold gas (Air) Temperature Differential and Flue Velocity. =
Google under " Chimney Sizing ". In general the greater the height the =
faster the draught. Also, there is a correction for Elevation above Sea =
level. Remember to provide a good Damper.
Knowing the intended firing temperature and the volume of the kiln it is =
possible to calculate a workable value for the density of your kiln =
gases. G =3D 9.81 m/s^2. Flue velocities need to be in the region of 2 =
metres per second for a small kiln. From there is a mater of =
substituting in the equations and a few moments with a hand calculator =
to find out how high your stack should be. I like to have a flue =
aperture that is twice the area of my fire ports.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Lee Love on wed 12 dec 07
On 12/12/07, Hank Murrow wrote:
> the input of fuel and air. Most studio-sized kilns seem to make do
> very well with 10 or 12 foot chimneys,
I didn't see the original message, but the length is also
dependant on the type of fuel you are using. Also, with a woodkiln,
there is an optimal length for minimal smoke.
--
Lee in Minneapolis, Minnesota USA
http://groups.google.com/group/ClayCraft
"Men are born ignorant, not stupid. They are made stupid by
education." -- Bertrand Russell
David Woof on wed 12 dec 07
Dear Matt, one of my favorite subjects: yes read the archives and all t=
he expert advice you can get, then take the best formulas and build larger =
with more passive dampers (pull out bricks) with bigger chamber exit flue o=
penings than anyone recomendeds or you ever can imagine using or needing. Y=
ou will then have a kiln that will handle changing atmospheric conditions, =
moon phases, prevailing wind, changeing wind, hill, valley, mountain and se=
a level elevations, nearby physical and man created obstructions...all that=
effect the chimney draw. All the physical realities and conditions that =
the best minds, experience, and formulas can only anticipate but not always=
address are covered in "build it tight and build it bigger", then plug up =
what you don't need until that rare time when you are 2/3 of the way to the=
greatest fireing you've ever experienced and the barometric pressure chang=
es pushing your fire back down tha chimney and your spirits with it for a r=
ide it out an extra 8-24 hrs... or tap your extra controls and learn someth=
ing new.
=20
Perhaps you also wish to consider in your choices that a taller chimney wit=
h passive damper air controls along with judicious stoking is a natural for=
a virtually smokeless firing with consistant heat rise.
=20
Don't be a slave to formulas, scientists need them to explain things, we =
all need them as points of reference but they are confines as well. It's g=
ood to look and venture beyond. That's where the new discoveries wait for t=
he curious ones willing to ask "what else is there? I wonder what would hap=
pen if?" and then pay attention.
David __________________________________________________________________Dav=
id Woof StudioClarkdale, ArizonaPh. 928-821-3747 __________________________=
______________________________________peering over the edge, reverently tak=
ing an irreverent look at everything.
_________________________________________________________________
Share life as it happens with the new Windows Live.
http://www.windowslive.com/share.html?ocid=3DTXT_TAGHM_Wave2_sharelife_1220=
07=
Matt MacIntire on wed 12 dec 07
Hank,
Thanks for the explanation. Good to know about the corners.
Since you asked, here are all the details... I'm building a smallish
two chambered climbing kiln. With semicircular sprung arches. About
17" rise from one chamber to the next. Roughly 25 cubic feet of ware
space in each chamber, not counting the firebox and combustion space. I
will be using pairs of 14" X 28" shelves. Not planning to have a bag
wall unless absolutely necessary, but I will leave space for one.
I'm planning to fire primarily with wood, so I intend to make good size
fireboxes with deep ash pits. However I also plan to experiment with
some alternate fuels a bit.
I hope two chambers will be more efficient, but primarily I'm doing two
chambers so I can do different things in each chamber. I'll probably
salt lightly in the second chamber. I want the kiln small enough that I
can fire fairly often. 50 cubic feet (total) feels about right (for me)
in that respect. Big enough that I can have friends come, but not so
big that I couldn't easily fill it myself.
I've got several good books, the usual ones. I can find "rules" about
firebox size compared to floor area. I find rules about chimney height
in relation to down draft and cross draft flame paths. I find rules
about exit flue size in comparison to chimney section and for exit flue
size in relation to air intakes. I've even got some experience firing
wood kilns.
Even though the rules are fairly consistent from source to source, they
seem sketchy to me. Using formulas would feel delusional. I doubt any
custom kiln I build using scavenged materials will be standardized
enough that formulas would apply directly. (but thanks Ivor!) So I
sorta feel like I've just got to take a wild ass guess when I pick
dimensions for the chimney and so forth. I'm actually more worried that
I'll overcompensate and make the chimney too big. Openings I can close.
But I don't want to have to rebuild the chimney.
I was thinking of making the chimney 13 1/2 by 13 1/2 for the inside
opening. And I was guessing (based on the "rules") it would probably
need to be about 16 feet tall. I'm envisioning multiple exit flues
across the bottom of the chamber that would collect in a plenum (or
whatever it is called) at the base of the chimney. Then the chimney
would narrow to the final dimensions at about six feet or so just below
which point I'd put two active dampers. A second passive damper
slightly above the active dampers. I've fired wood kilns with similar
chimneys, but they had bigger chambers.
Based on all that, does a 13 X 13 chimney 16' tall sound excessive?
Obviously, I'll build in adjustable openings where ever I can. But I'd
like the dimensions to to be appropriate enough that firing is not a
huge struggle.
Lee,
Would you care to elaborate on your suggestion that there is an optimal
length for wood fired kiln chimneys? I've noted some people (14'
chimney to start) who said they had trouble till they added a couple
feet. Their kilns were bigger than my tiny thing though. So any
experience you'd share will be helpful.
Thank you gentlemen, for your advice. I appreciate how willing you
always are to help.
Matt MacIntire on wed 12 dec 07
Hi David,
Thanks for the sound advice, "build it tight and build it bigger." I'll
take that to heart, for sure. Hard to go to wrong with that maxim.
Ultimately, I'm doing this to enjoy firing, not to earn a degree in
fluid dynamics. Thanks for the reminder.
Hank Murrow on thu 13 dec 07
On Dec 12, 2007, at 11:38 AM, Matt MacIntire wrote:
> Hank,
> Thanks for the explanation. Good to know about the corners.
Matt;
If a square chimney is 9" x 9", the area of the square is 81sqin.
Actual area('rounding the square' to eliminate the corners) is 64
sqin. Similarly, for a 13.5" x 9", the area is 122 sqin, rounding to
96 sqin. A 13.5 x 13.5 would have 182 sqin, actual area is 143. 13.5
x 18 gives an area of 243 sqin, with usable area of 192. You can see
that the difference is .79 approx. in each case.
Now, you can see that there will be a considerable increase in
material to build a flue whose corners are not in play.
So a round chimney will likely be cheaper to build, and have less
surface area also. That can be important, because remember that the
idea is to preserve buoyancy of the flue gases so draft is maximized.
Less surface=less heat loss as the gases travel up the chimney. In
addition, you can see the need for a well-insulated chimney. Perhaps
Mel's culvert pipe lined with fiber is the most efficient type of
chimney for a unit of length. I recently designed and built a chimney
for the anagama at the U of Oregon. The 18" x 18" brickwork(usable
area=254 sqin) stopped about 3' up from the exit, where we used a
castable joint to transfer to a round 20" diameter stainless steel
pipe lined with 2" of ceramic fiber to wind up with a 16" round
inside..... actual area is 210 sqin, and the length of the chimney is
25'. There was plenty of draft for a 22' long anagama about 6' wide.
The inside was stepped, but there was no other elevation from ground
level..... as the site is flat. I will send you pics privately which
illustrate this project. What you might infer from my suggestions is
that a well-insulated round chimney can be smaller in diameter than a
poorly insulated rectangular design..... and maybe even less tall.
>
> Since you asked, here are all the details... I'm building a smallish
> two chambered climbing kiln. With semicircular sprung arches. About
> 17" rise from one chamber to the next. Roughly 25 cubic feet of ware
> space in each chamber, not counting the firebox and combustion
> space. I
> will be using pairs of 14" X 28" shelves. Not planning to have a bag
> wall unless absolutely necessary, but I will leave space for one.
>
> I'm planning to fire primarily with wood, so I intend to make good
> size
> fireboxes with deep ash pits. However I also plan to experiment with
> some alternate fuels a bit.
I know of a wood kiln here whose second chamber is fired with bio-
diesel once the front chamber has gotten to temp.
>
> I hope two chambers will be more efficient, but primarily I'm doing
> two
> chambers so I can do different things in each chamber. I'll probably
> salt lightly in the second chamber. I want the kiln small enough
> that I
> can fire fairly often. 50 cubic feet (total) feels about right
> (for me)
> in that respect. Big enough that I can have friends come, but not so
> big that I couldn't easily fill it myself.
>
> I've got several good books, the usual ones. I can find "rules" about
> firebox size compared to floor area. I find rules about chimney
> height
> in relation to down draft and cross draft flame paths. I find rules
> about exit flue size in comparison to chimney section and for exit
> flue
> size in relation to air intakes. I've even got some experience firing
> wood kilns.
>
> Even though the rules are fairly consistent from source to source,
> they
> seem sketchy to me. Using formulas would feel delusional. I
> doubt any
> custom kiln I build using scavenged materials will be standardized
> enough that formulas would apply directly. (but thanks Ivor!) So I
> sorta feel like I've just got to take a wild ass guess when I pick
> dimensions for the chimney and so forth. I'm actually more worried
> that
> I'll overcompensate and make the chimney too big. Openings I can
> close.
> But I don't want to have to rebuild the chimney.
>
> I was thinking of making the chimney 13 1/2 by 13 1/2 for the inside
> opening. And I was guessing (based on the "rules") it would probably
> need to be about 16 feet tall. I'm envisioning multiple exit flues
> across the bottom of the chamber that would collect in a plenum (or
> whatever it is called) at the base of the chimney. Then the chimney
> would narrow to the final dimensions at about six feet or so just
> below
> which point I'd put two active dampers. A second passive damper
> slightly above the active dampers. I've fired wood kilns with similar
> chimneys, but they had bigger chambers.
>
> Based on all that, does a 13 X 13 chimney 16' tall sound excessive?
No, it sounds feasible, and I like the 'plenum' idea.
Cheers, Hank
www.murrow.biz/hank
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