Richard Aerni on sat 19 feb 11
Ivor,
It's a good idea in theory...but, most potters do not fire their kilns =3D
(and I'm talking gas kilns here, not electric) enough to make it pay. =3D
We looked into this concept in the early 1980s, but the cost of =3D
construction for the equipment was just too much money for the expected =3D
return. I imagine it would be a very functional and productive idea for =
=3D
a constantly firing kiln, such as one finds in industry, rather than for =
=3D
the rather smaller and less frequently fired kilns found in the average =3D
minimum-wage pottery shop. =3D20
Best,
Richard Aerni
Rochester, NY
Vince Pitelka on sun 20 feb 11
Richard Aerni wrote:
"It's a good idea in theory...but, most potters do not fire their kilns (an=
d
I'm talking gas kilns here, not electric) enough to make it pay. We looked
into this concept in the early 1980s, but the cost of construction for the
equipment was just too much money for the expected return. I imagine it
would be a very functional and productive idea for a constantly firing kiln=
,
such as one finds in industry, rather than for the rather smaller and less
frequently fired kilns found in the average minimum-wage pottery shop."
Richard is correct. This was researched in the latter part of the last
century, and was abandoned because it just wasn't financially feasible. I
remember a story about the Oregon School of Art and Crafts installing heat
exchangers in the flues to heat combustion air, but they improvised ceramic
tubes for the purpose, and soon had so much leakage that the system had to
be abandoned. For periodic kilns fired as rarely as most of ours get fired=
,
it doesn't pay.
The studio glass situation is a whole different situation. Our glass
program just installed an incredibly high-tech melt furnace that they built
from scratch, and it has the currently popular recuperation system and a
sealed-face burner. When the system is turned up full-blast you can hardly
even hear it. The heat-exchanger is just welded-stainless steel, and is
located a short distance away from the furnace.
I was surprised that stainless would work for this application, but on
Saturday a week ago I was visiting friends at their farm in the Smokies nea=
r
Johnson City, TN, and we took a drive over to Penland and wandered through
all the studios. There glass studio must be one of the finest in the US
after Pilchuck, and they have three or four melt furnaces and lots of glory
holes, and all of the melt furnaces had the same heat recuperation system
with a welded stainless heat exchanger and a sealed-face burner.
The glass-blowers are so far ahead of the potters on the technology of thei=
r
melt furnaces for the simple reason that the damn things run 24 hours a day
and cost a fortune to run, and because controlling all aspects of combustio=
n
and atmosphere on a melt tank is a simple matter in comparison to a
downdraft reduction kiln. For our purposes, we get by just fine with much
simpler burner systems and manual controls. Installing a fully-automatic
system of the type mentioned above on a glass-melt furnace is an expensive
proposition, but it pays for itself pretty quickly.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
John Britt on sun 20 feb 11
That is the Hugh Jenkins system. It was written up in Ceramics Monthly:
http://ceramicartsdaily.org/firing-techniques/kiln-plans-and-diagrams/hug=
=3D
h-
jenkins-volcano-kiln-recuperating-waste-heat-for-efficient-firing/
A very interesting cast burner which uses forced air.
Probably not economical for intermittent kilns but great for continuous k=
=3D
ilns.
www.johnbrittpottery.com
Taylor Hendrix on mon 21 feb 11
Vince,
Was this kiln featured in an article which had a picture of a dixie
cup, sitting atop the kiln's flue exhaust when the kiln was firing at
top speed, merely scorched? I remember the article had a picture of an
array of square tubes that were extruded into curves coming out then
reentering the back of the kiln. So a good idea but poor execution?
Taylor, in Rockport TX
wirerabbit1 on Skype (-0600 UTC)
http://wirerabbit.blogspot.com
http://wirerabbitpots.blogspot.com
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On Sun, Feb 20, 2011 at 8:30 AM, Vince Pitelka wrote:
...
> Richard is correct. =3DC2=3DA0This was researched in the latter part of t=
he l=3D
ast
> century, and was abandoned because it just wasn't financially feasible. =
=3D
=3DC2=3DA0I
> remember a story about the Oregon School of Art and Crafts installing hea=
=3D
t
> exchangers in the flues to heat combustion air, but they improvised ceram=
=3D
ic
> tubes for the purpose, and soon had so much leakage that the system had t=
=3D
o
> be abandoned. =3DC2=3DA0For periodic kilns fired as rarely as most of our=
s ge=3D
t fired,
> it doesn't pay.
...
Vince Pitelka on mon 21 feb 11
Taylor Hendrix wrote:
:Was this kiln featured in an article which had a picture of a Dixie =3D
cup, sitting atop the kiln's flue exhaust when the kiln was firing at =3D
top speed, merely scorched? I remember the article had a picture of an =3D
array of square tubes that were extruded into curves coming out then =3D
reentering the back of the kiln. So a good idea but poor execution?"
Taylor -=3D20
I don't think it was poor execution, just a bad idea because of the =3D
expansion and contraction. How do you guarantee a seal over time if the =
=3D
component parts are constantly expanding and contracting? I think that =3D
the stainless steel heat exchanger makes a whole lot more sense, =3D
especially if a person is able to find some scrap stainless pipe that =3D
will work.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu=3D20
http://iweb.tntech.edu/wpitelka
jonathan byler on mon 21 feb 11
most stainless steel alloys don't handle being at red heat for long.
you get carbide precipitation, which is essentially formation of
chrome carbide in the steel. this is really hard stuff and tends to
form cracks that start along the carbide particles. it will
eventually fail, from what I understand, often faster than regular non
stainless alloys. I'm sure someone has invented an alloy that
doesn't do this, but I'm also sure it is crazy expensive. Stainless
steel exposed to a reduction atmosphere is probably also going to have
problems from embrittlement as carbon diffuses into the surface at
high temperature and aids in the forming of chrome carbides.
stainless steel has weird failures due to stress cracks that regular
high strength alloys are not prone to. this is why they don't build
bridges out of stainless (other than the insane cost). and why any
trains or cars that are made with stainless do no use it for
structural parts, like the frame.
your heat exchanger needs to handle the almost 2000F flue gasses, and
also handle the temperature differential. stainless steel really
doesn't expand and contract very conveniently, since it transfers heat
poorly and tends to warp badly when there are temperature
differentials (which there inevitably are). try welding on the stuff
and you will see what I mean. the stresses from the heat on complex
parts are going to probably crack and fail even sooner than if it was
just a simple rod/pipe used in a fire box.
I'm not sure how the dixie cup kiln worked out long term, but he made
all the parts for the heat exchanger himself using kyanite and some
other clays and an extruder to make the parts. if he used fibrefrax
at the joints where those pieces went in, there would be some room for
flex. I read his stuff, and have been looking at making kilns more
efficient for quite a while now, and haven't found anything better
than what he came up with. the only better design I can imagine is
some kind of heat exchanger on a wood kiln, where you don't have to
worry about redesigning the burners to take the hot air. you just
route your super heated air under the coal bed and take it from there.
the biggest problem on a gas kiln like that is the burners, since you
can't go mixing fuel gas with air in your venturi burners above the
temperature at which you get combustion. and if all he has is a
dixie cup getting scorched, his exhaust temperature has got to be
under 450-500F. which means if the internal temperature is at 2300F
the gas going into the burner is going to be at least 1700F probably
more. he shows on his website how the burners were designed, and it
starts with venturi burners and switches to the hot air and the fuel
being mixed inside the firebox as the kiln heats up.
I think y'all are putting too much weight on leaks in the heat
exchanger. it would be a big deal in your house where you don't want
to breath CO exhaust fumes, but for a kiln, it is not going to hurt
anything if a little of the exhaust gas leaks into the intake. there
will be such an overwhelmingly large volume of fresh air pushed down
the stack that you will still have plenty of oxygen for combustion
purposes. again, stainless steel, despite what some people thing is
to fragile if it is going to be as efficient as that guy made his. it
just gets too hot and fails. it has been a while since I read about
it, but I recall that he tried it, it failed, and he went to the
ceramic exchanger instead.
-jon
On Feb 21, 2011, at 4:28 PM, Vince Pitelka wrote:
> Taylor Hendrix wrote:
> :Was this kiln featured in an article which had a picture of a Dixie
> cup, sitting atop the kiln's flue exhaust when the kiln was firing
> at top speed, merely scorched? I remember the article had a picture
> of an array of square tubes that were extruded into curves coming
> out then reentering the back of the kiln. So a good idea but poor
> execution?"
>
> Taylor -
> I don't think it was poor execution, just a bad idea because of the
> expansion and contraction. How do you guarantee a seal over time if
> the component parts are constantly expanding and contracting? I
> think that the stainless steel heat exchanger makes a whole lot more
> sense, especially if a person is able to find some scrap stainless
> pipe that will work.
> - Vince
>
> Vince Pitelka
> Appalachian Center for Craft
> Tennessee Tech University
> vpitelka@dtccom.net; wpitelka@tntech.edu
> http://iweb.tntech.edu/wpitelka
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