Paul Gerhold on sun 18 aug 02
Daer Gavin
What is a "Hot Zone" are we talking thermodynamics here, Also for a kiln to
reach thermal equilibrium the heat energy in will have to be equal to the
heat energy out which means essentially that the kiln will have been fired to
its absolute maximum temp. which I doubt many potters actually strive for.
When I added extra insulation to my kiln bottom it was to help even the
temperature from top to bottom and it did help quite a bit. Heat loss is a
function of temperature difference between the surface of the kiln and the
surrounding air (dQ=hAdT) so any insulation will help but in the case of
cheap stainless it will be at a cost. Maybe one of the kiln people who
monitor the site can give authoratative answers to different experiences.
Paul
Gavin Stairs on sun 18 aug 02
At 07:00 AM 18/08/2002 -0400, Paul Gerhold wrote:
>What is a "Hot Zone" are we talking thermodynamics here, Also for a kiln to
>reach thermal equilibrium the heat energy in will have to be equal to the
>heat energy out which means essentially that the kiln will have been fired to
>its absolute maximum temp. which I doubt many potters actually strive for.
Hi Paul,
By hot zone I simply meant the region in the wall insulation which reaches
a significant temperature, like something above 500C, which will begin to
do nasty things to metal skins and bands. Just outside the hot zone is the
wet zone, where condensate forms. As we have heard on the list recently,
this can also do really bad things to metal. The condensate is mostly
water, but is acidic from the carbon and sulphur in the clay.
If the insulation is uniformly effective, then at equilibrium, the
temperature from the inside to the outside will fall at a uniform rate: so
many C/ mm, or F/in. When the inside temperature is 1000C and the outside
is 200C, the gradient over a 63.5mm (3") wall will be
800C/63.5mm=12.6C/mm. So if you put an inch of insulation on outside
that, and the outer wall only reaches 150C at equilibrium, you will get
about 850C/89mm=9.6C/mm, and the temperature at the old wall will be
150C+25mm*9.6C/mm=394C. This is a simplified picture, since insulation
does not behave the same at all temperatures. Above the incandescent
temperature, most insulations begin to lose effectiveness at a dramatic
rate, so the gradient is much flatter toward the interior of the
kiln. Also, fibre is a much better insulation than brick at lower
temperatures, maybe 2 to 4 times better. So you may get temperatures as
high as 700C at the old wall. These are the limit cases: the wall
temperature will not exceed these temperatures, provided the interior of
the kiln remains at the 1000C used for the calculation. However, it will
take some time for the kiln wall to reach these temperatures, as it takes
heat to raise the temperature of the insulating brick and fiber.
If you are firing only to bisque temperatures, the kiln wall will probably
never get near these temperatures, except very near the inner kiln
wall. In this case, the condensation temperature (<100C) may be reached
well within the kiln insulation. The outer brick and fibre will become
waterlogged, and the kiln skin will get wet with acidic water. This is one
of the degradation modes for batch kilns with metal outer skins. More
insulation outside the skin may help by forcing the hot zone out and moving
the condensation zone out with it, but it is more likely to simply trap the
moisture in more effectively.
If you are firing stoneware, the actual temperatures may reach something
near these temperatures. In this case, the metal wall may be subject to
heat stresses for which it was not designed. There are two effects of
interest. The first is transitory, and may not involve permanent damage to
the metal. That is, the metal will expand, and the kiln brick it was
designed to hold tight will become loose. Especially in the lid and floor,
this can mean eventual damage to the brick. The second effect is actual
heat damage to the metal. This may include discoloration, distortion and
stretching, and in extreme cases may lead to increased corrosion, failure
of welds and failure of the metal.
While the wall is in dis-equilibrium, the effect of additional insulation
on the outside is minimal, because the heat loss from the interior is not
reaching the outer zones of the kiln: it is being absorbed by the
insulation farther in. The interior of the kiln, and your electricity
bill, will not see any effect until the insulation is almost completely
heated to a good fraction of its equilibrium temperature. So in firing
bisque, it is unlikely to have any significant effect on power
consumption. It is only when the full gradient has formed at its
equilibrium value that any economies will be seen.
This is only a precis of the effects. If you have seen significant evening
effects from fiber top and bottom, it is because you are firing to high
temperatures on longer firing profiles which allow the insulation to get
hot throughout. In such a case, one inch of fiber is about tripling the
insulation value of your kiln top or bottom, and the effect can be
dramatic. In this case, it is better not to insulate the metal banding at
the edge of the top or bottom, as doing this may cause long term damage.
Please understand that I am not saying you should never use fibre on the
outside of the kiln. I am saying that you need to be aware of what effect
the fiber will have, and use it only when your kiln use warrants it. I
think that fiber applied around the body of a metal clad batch kiln is
almost never a good idea, for the reasons cited above. Fiber top and
bottom may well be useful, but care must be taken not to damage the metal
and brick. An outside support for the bottom slab, such as a metal or
brick plate underneath, will probably do a lot to relieve the extra stress
put on the floor by adding insulation. So, an air gap, followed by a
structural layer of sheet metal or brick, followed by an inch of fiber and
then the kiln slab. The top can most likely be treated simply with a fiber
slab laid loosely on top, but this may lead to premature failure of the top
slab. Also, beware of fiber dust!
Gavin
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