Shelley S on mon 12 jul 99
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From: John Baymore[SMTP:JBaymore@compuserve.com]
Sent: Sunday, July 11, 1999 11:21 AM
To: CLAYART@LSV.UKY.EDU
Subject: Kiln chimney
----------------------------Original message----------------------------
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(clip)
Due to concerns about earthquakes, I've decided to terminate my firebrick
chimney at six feet, (nine feet above the ground) and finish the final six
feet with a single wall stainless chimney of the same dimensions as the
interior of the fire brick. My question is will I encounter any problems
with this? Will it effect the draw? I like the idea of being able to take
the metal part down between firings. I'm also afraid a firebrick chimney
wouldn't last long in our two hundred plus inches of rain. Any input will
be greatly appreciated.
(snip)
Bill,
Do you GET regular earthquakes there? Or since you are located in an
active volcanic zone, are you just taking precautions sort of "in case"?
You might have a governmental office there that has an engineer on staff
that would be glad to discuss the best option for earthquake-proofness with
you. Show him your choices..... and ask him/her which one seems best. Or
what else could he/she suggest. Might be a good resource.
As to the firing aspect of your idea:
The main issue you might have is that Draft is determined to a large degree
on the temperature of the gasses in the stack and the exit temperature (at
the top opening of the chimney) is a big determinant. A single wall
stainless chimney will pass heat through itself pretty readily and
convect/radiate it to the surrounding air, wood, shingles, etc........
thereby reducing the temperature in the stack on the average. This will
decrease the draft for the given other factors of the kiln/chimney design.
50 percent of your chimney will be a good heat "loser". That is a good
portion of it. At almost any given point in the firing curve, the Draft
figure will be less than if you had a more insulated chimney.
Building any fuel burning kiln is about air handling capacity first and
foremost. No air....no combustion. If the wood kiln is overdesigned quite
a bit as to air handling capacity (will entrain way more primary air than
it needs to combust adeguate BTU's per hour to get desireable climb rate at
peak fire conditions) then you probably have some margin to play with.....
and the stainless will likely suffice. If the kiln is marginal at all in
air handling, then the stainless tube might just be the factor that pushes
it over to the "not enough air" side of the equation and makes it
unforgiving to fire and a "one trick pony" type kiln ( Has one way it will
work and that is it...stacking and stoking is critical). If the kiln is
low on air handling to start with, the uninsulated stainless may make it so
that you can't beg, borrow, or steal cone 9-10.
Mel's suggestion on using fiber liners in the pipe is a great one. It does
make a good inexpensive solution appropriate for many situations. It goes
up fast too. Donovan is also correct that in critical situations a
homemade insulated chimney might not be the best choice as to fire safety
issues. Depends a lot on the specific installation. Sometimes an
insulated brick chimney (real thick one) is the best choice.
As Mel correctly mentioned.... a wood kiln usually has a hot chimney
temperature. This is due to long flame length and inadequate mixing. The
flame front is often burning off in the chimney or at the top. In some
cases I have seen on big kilns.........20 feet out of the chimney .
Your chimney is going to get HOT if you fire like most wood firers I have
met do (often because their kiln will only fire that way). I have seen the
radiant heat through a single wall metal stack on someone's kiln catch 2 by
8's on fire at a distance of 3-4 feet away....and not in an enclosed dead
air space. That stainless could be glowing at red heat ....not readily
noticeable in daylight but clearly apparent in the dark. Some insulation
would be desireable if you are ANYWHERE near a roof structure.
If you are set on the pipe thing........... I'd take Mel's suggestion and
place fiber flue liners inside the pipe. (Get the MSDS from the supplier
on the fiber and READ it! Nasty stuff. Exercise extreme caution in using
any fiber product.) They are light and pretty cheap and even if you have
to have them shipped, it might not cost all that much. This will give some
insulation at little cost and little weight factor. They will sustain the
original brick chimney draft figure, most likely. (This heat loss figure
can be calculated if you need to.) Because they are light, the angular
momentum generated in a bit of a shaking from an earthquake might be low
enough that with sufficient guying the chimney would stay up if the rest of
the kiln and shed stayed up. Not too much different from the pipe by
itself in total mass and lever arm.
I'd just leave the guyed chimney sections in place no matter what....unless
you get shook good every day. In that case.....I'd move .
Listen to Donovan's really good advice........ be really careful about fire
danger with kiln stacks. Most of the fires I have heard of or have gone to
see, originated at the stack thru roof point or right near there.
Like Donovan, in critical situations I have been known to use UL Listed Van
Packer units.....expensive, but if the kiln is in an interior building
location, cheap insurance. In some less critical situations where the flue
gasses are being diluted with room air vai a pickup hood, I have used UL
Listed Metalbestos units of stainless and fiber (no asbestos). If I were
building in Hawaii...... cost to ship Van Packer there along with the high
original cost might make them prohibitive. They are heavy. The weight
would be a bad factor in the angular momentum bit too. Never shipped a
special order item to Hawaii . Have shipped pots to Japan
though........ takai desu....it's expensive! Bet it is somewhat similar
.
Not sure why you think a firebrick chimney would not last long in the rain
there? Hardbrick can take lots of rain, no problem. Softbrick is another
story! You are building it on a solid foundation aren't you? If not you
are correct that the rain could erode the base enough to let it tip. The
heavy rain might also be a factor with the fiber flue liners. I'd
recommend, like Mel, that you coat them with ITC 100HT and then before
installing them, pre-fire them in your gas kiln to "set" the ITC. This
would make them pretty waterproof, I think. Without that they will not
like to repeatedly get very wet. If you use them and don't do the ITC
thing, you'll have to cap the chimney for sure, they probably will
deteriorate far faster than soft firebrick would in the torrential rains.
Like Donovan, I build kilns with an eye to the aesthetic too. Whatever you
do, remember that you have to LOOK at this kiln. Probably and hopefully
for a long time . Don't only make it well designed and constructed, but
make it a pleasure to be around too. Sort of like good functional pots
. His advice is well given on that one.
Best,
........................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore@compuserve.com
John.Baymore@GSD-CO.COM
"Earth, Water, and Fire climbing kiln firing workshop Aug. 20-29,1999
-one space available"
John,
Can you talk a bit more about how one can determine where the flame front
is burning off? I think you mentioned the best place is right before the
exit flue. Does this give you better mixing, more even temperature, etc.?
And, would these assumptions be the same for venturi vs. forced air
burners?
thanks!
Shelley
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