rocky mann on tue 27 apr 99
Couldn't agree more with Jonathan Kaplan about electric kilns. They are
under built
and a pain in the butt to work on. How come when we build our gas kilns we
use 9=B2 of insulating brick but we buy electric kilns (rated for cone 10)
with only 3=B2 of brick?
-- Rocky Mann
Bar Harbor, ME
rockymann=40acadia.net
Gavin Stairs on wed 28 apr 99
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At 07:47 AM 4/27/99 EDT, you wrote:
=3E----------------------------Original message----------------------------
=3ECouldn't agree more with Jonathan Kaplan about electric kilns. They are
=3Eunder built
=3Eand a pain in the butt to work on. How come when we build our gas kilns =
we
=3Euse 9=B2 of insulating brick but we buy electric kilns (rated for cone =
10)
=3Ewith only 3=B2 of brick?
Part of the reason is this: All such kilns are underinsulated if you
consider the equilibrium heat flow. However, these kilns are NOT designed
to reach equilibrium. They are designed to reach maximum temperature long
before the insulation is heat saturated. So, the outside temperature never
reaches the 400C or so that it would at eqilibrium, and the kiln doesn't
glow in the dark. Adding the extra insulation would add substantially to
the cost, but would not add very much to the kiln efficiency. So they
leave it out. However, if you seriously overfire such a kiln, the exterior
can get VERY hot, as several subscribers to this list have found out (see
the archives).
So-called industrial kilns may add the extra insulation, partly in
consideration of the safety issue, and partly for the slight extra fuel
efficiency. The higher quality round kilns are a compromise, with some
extra insulation, usually as a thin fibre layer on the outside, and thicker
brick. Continuous kilns, and kilns intended to fire on a long profile need
the extra insulation, as they are heated long enough to saturate the
insulation, and reach heat equilibrium. This would destroy an
underinsulated kiln. Therefore, most modern industrial kilns use very
thick brick walls or thick fibre batts.
The problem of the element quality is another compromise. It is difficult
to design 110V or 220V elements for a small kiln while maintaining
recommended conductor size. The element manufacturers recommend wire
diameters above 1mm, which makes the elements much more robust. But the
element resistance in such thick elements is very low, and does not limit
the current to the value required for the small kiln power. To use large
diameter element wire, you need to use a transformer or semiconductor
voltage regulator to reduce the element voltage. This is very expensive,
so none of the less expensive kilns do this. Instead, they use thin wire,
which wears out more often. The same remarks apply to ceramic elements,
which are very durable (but mechanically fragile and expensive), but also
very low resistance.
I've been designing some laboratory kilns lately, and these kilns, although
designed to last, are also designed to fire fast, and are usually very
underinsulated. This makes for a nice, compact, bench-top package. I
would prefer a kiln capable of a longer cycle, and also one that uses
robust elements, so, for myself, I design ones with thicker insulation and
expensive volage converters. But these would be hard to sell to potters, I
think. I can design a minimum price test kiln, but I would prefer one
designed to last and to be fail-safe, etc. That costs double or treble
what the cheap one does.
Gavin
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D==
3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Gavin Stairs =3Cstairs=40stairs.on.ca=3E
Stairs Small Systems (S3)
921 College Street, =23 1-A
Toronto, Ontario, Canada M6H 1A1
(416)530-0419
millie carpenter on wed 28 apr 99
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rocky
I think that the answer is in your comment, when WE build.. we don't
design our own. we don't build our own. we have a choice of about 6
different kiln companies who make basically the same kilns. Until I
move some where that I can build something, where am I going to get
something in the electric 7-12 cubic foot range for a price that I can
afford, that I can get down the steps and into my basement other than
what we have? Or do the gas/wood people over engineer their kilns?
=3E ----------------------------Original message----------------------------
=3E How come when we build our gas kilns we
=3E use 9=B2 of insulating brick but we buy electric kilns (rated for cone =
10)
=3E with only 3=B2 of brick?
=3E
=3E -- Rocky Mann
=3E Bar Harbor, ME
=3E rockymann=40acadia.net
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