mel jacobson on mon 20 aug 07
i want to add a few thoughts about fuel kiln chimneys.
hank has advocated tight chimney design.
i totally agree. the tighter the better.
there seems to be a misunderstanding about
free stacking ifb kilns that they are loose.
it could not be farther from the truth.
a well made, double walled ifb kiln that is free
stacked is tight. it just does not need mortar to
hold it together. and, normal expansion will not
cause the kiln to crack.
often, when potters get to the stack, they think that
any old rotten used hard brick will do.
wrong. in fact, i think the best bricks in the
kiln should be in the firebox, flue area. tight
as a drum head. and using mortar to fill the
voids is best. and it is good economy to use a quality
commercial mortar. it is good stuff. in fact, put the rotten
bricks under the floor...hide them.
some of the worst kilns i have seen in america are
built with bad chimneys. over and over i see it.
anyone can see the hundreds of cracks and voids
in the stack as it fires. (some look like a fire works display.)
it is a series of passive dampers.
if you add up those cracks an inch at a time you have
maybe 80 square inches of open space. it is like
making a 60 sq.inch window in the back of the stack.
wide open.
i have been very excited about the spiral pipe stacks i have
designed. spiral pipe is now the sheet metal standard for duct
work across the industrial world. any size, any gauge, any length.
match that pipe with kaowool flue liners dipped in itc, and you
will have a stack that is tight and will last for years.
not to mention, perhaps the safest stack going. in fact, an oversized
flue liner would make a great safety element going through a ceiling.
just make sure there is good air gap.
i have lined my chimney area with quarter inch kaowool paper.
just made a cone, stapled it in place...i also hung a 12 inch x 24 inch
piece of koawool paper over my wooden header.
i actually soak the ends of the flue liners in a thick mix of itc100.
get them a bit soggy (like two minutes). drop them down the stack and
they squish
together...making a monolithic unit. no gaps. what you want to avoid
with metal stacks and flue liners are those tiny quarter inch voids...the
flame pushes through those and melts the metal pipe.
i solved that problem.
over sized flue openings...like 80 inches and beyond, with huge
chimney size...it a heat robber. a kiln like that will never
fire well. all the heat is going up the stack. no matter how much
gas pressure you add, it all goes up the stack.
and remember, if you are using compressed air/blowers/ all the rules
change. it is a totally different kiln. then the chimney is just a super
vent system....taking smoke and guck away. and learning to fire
a compressed air kiln has a nice learning curve. it has its own system
and style. each one is different. there is never `one size fits all` with
kilns...it all depends.
mel
from: mel/minnetonka.mn.usa
website: http://www.visi.com/~melpots/
Clayart page link: http://www.visi.com/~melpots/clayart.html
Corinna Laird on tue 21 aug 07
Mel and everyone,
I'm wondering if you could elaborate further on free-stacked ifb kilns. You
said,
"there seems to be a misunderstanding about
free stacking ifb kilns that they are loose.
it could not be farther from the truth.
a well made, double walled ifb kiln that is free
stacked is tight. it just does not need mortar to
hold it together. and, normal expansion will not
cause the kiln to crack."
I spent a semester trying to learn to fire a small free stacked ifb kiln
(about 4 cu. ft of ware space), which pretty much represents 90% of my
fuel-fired kiln experience. It was only a single-brick thickness, and
definitely pretty loose and riddled with cracks, which seemed to get worse
as the semester progressed. I could literally look straight through the
cracks in the chimney and see the lawn on the other side.
I can see how a double walled kiln could/would be tighter, but could you
describe a little of how and why? Would the tighter build allow the bricks
to last longer?
thanks,
Corinna
Luke Nealey on wed 22 aug 07
Corinna,
There are many good posts in the archives re chimneys, brinks etc. I have
found mel's and david hendley's the clearest on these matters but the
threads have a lot of good and different solutions to these issues. One I
remember is titled "chimney thoughts". I have just finished a fast fire
wood kiln and know that it gets where I want it to go because I followed the
advice here. There are also many pictures of fuel burning kilns on potters
web sites.
One of the things that helped me understand the importance of the tight
chimney is that the main issues in my kiln are mass transfer not heat
transfer issues. The chimney has to be tight so it can suck enough air into
the kiln to provide sufficient oxygen to burn the fuel. The chimney only
has to be insulative enough to keep the gases hot enough to keep going up
and out the stack. Regular fire brick or a cermanic fiber lined metal stack
seems to work for many. The first 5 feet of my stack is mortared double
thick fire brick, the next 3 feet is single fire brick with about an inch of
mortar on the outside to seal it, the last 8ft is a lined metal stack. It
works because it is sealed well.
Luke Nealey
Rankin Co. MS
Marcia Selsor on wed 22 aug 07
Dear Corinna,
Double walled loose stacked bricks can last for years. That is what I
built and used in the program at MSUB in Montana. They were braced
with angle iron with threaded rods w/ car valve springs and large
washers that allowed movement of the arch during the firings. They
could be tightened when needed.
I fired them twice a week starting early in the semesters, sometimes
more than that. Over one 20-year period, those kilns were rebuilt
twice. That is a lot of wear /ware! One was about 36 cu ft. of
functional ware space and the other, a car kiln, was about 48 cu. ft
of functional ware space. The arches had two layers of IFB plus
ceramic blanket. Both had solid doors to avoid bricking up (ganglia
cyst on my wrist and bilateral carpal tunnel was the reasoning behind
this.)
The car kiln door was two layers of brick and insblock insulation.
The smaller Hinged door for the sprung arch kiln was fiber sprayed
with rigidizer and insblock insulation. Insblock is rigid board but
only good for a hot face of 1900 degrees. By my calculations it was
good enough after the inside insulation.
If I could see color through a crack during a firing, I would not
like it. But at high temps, sometimes I might see a little color here
and there. Certainly could see through the kiln out the other side.
These kilns fired evenly, and like clockwork.
A kiln that you could look through would seem to me to be a gross
waste of fuel!
Marcia Selsor
http://marciaselsor.com
On Aug 21, 2007, at 11:11 PM, Corinna Laird wrote:
> Mel and everyone,
>
> I'm wondering if you could elaborate further on free-stacked ifb
> kilns. You
> said,
>
> "there seems to be a misunderstanding about
> free stacking ifb kilns that they are loose.
> it could not be farther from the truth.
> a well made, double walled ifb kiln that is free
> stacked is tight. it just does not need mortar to
> hold it together. and, normal expansion will not
> cause the kiln to crack."
>
>
> I spent a semester trying to learn to fire a small free stacked ifb
> kiln
> (about 4 cu. ft of ware space), which pretty much represents 90% of my
> fuel-fired kiln experience. It was only a single-brick thickness, and
> definitely pretty loose and riddled with cracks, which seemed to
> get worse
> as the semester progressed. I could literally look straight through
> the
> cracks in the chimney and see the lawn on the other side.
>
> I can see how a double walled kiln could/would be tighter, but
> could you
> describe a little of how and why? Would the tighter build allow
> the bricks
> to last longer?
>
> thanks,
> Corinna
>
Marcia Selsor
http://marciaselsor.com
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