Gavin Stairs on fri 13 mar 98
Ok, time for a little elementary insulation talk.
As others have said, heat transfer occurs by the following mechanisms:
contact/conduction, fluid flow/convection, and radiation. At ordinary
temperatures, the first two tend to dominate. However, as the temperature
rises to incandescent (glowing) heat, radiation takes over. This is
because radiative transfer goes as the difference of the fourth power of
the absolute temperatures of the radiating and accepting surfaces.
Translated, that means that the radiative transfer goes up rapidly with
temperature.
So, let's get the mind wrapped around these concepts.
Conduction: Think of the difference in feel when you touch a piece of foam
and a piece of metal, both at room temperature. The foam feels warm, and
the metal cool, right? That's because of the low heat conduction of the
foam, and the high heat conduction of the metal. The metal conducts heat
away from your fingers rapidly, while the foam does not. Similarly, a hard
brick conducts heat faster than an insulating fire brick (IFB), which is
sort of stone foam. (It's lighter, right? That means it contains a lot of
air: a foam.) And fibre conducts less than both of them.
Convection: Blow on your hand from about ten inches away. It feels cool.
That's because you are causing air to circulate rapidly past your warm
hand, which then heats the air, and is cooled as a result. That's what
wind chill is about. You and your warm skin heat the air about you. If
the air circulates, you have to keep heating more air, and you feel colder.
If the air does not circulate, but stays still, then you feel warm, or on
a hot summer day, hot. Air gaps can be efficient insulators, provided they
are not too big. I'm going to skip a lot of math here. In a house wall,
the optimum thickness of an air gap is about an inch or two. Bigger than
that, and the gap will begin to convect, that is, circulate because of the
density difference between warm and cool air. You can prevent this
convection by removing the air. Hey presto, a vacuum bottle. What us
scientific types call a Dewar flask, what you call a Thermos(R). Or by
breaking the air mass up into small cells: foam.
An IFB insulates better than a hard brick because of the air it contains.
Foams and fibre mats are very good insulators because of the air. Remove
conductive mass and replace it with air, and you reduce conductive loss, as
well as lowing the heated mass, which is another topic. Compress the fibre
mat and it will conduct more heat because of more and shorter conduction
paths. Compress to full density, and the fibre will conduct like a hard
brick (more or less). However, you can fit more fibre in a given space by
compressing it. This still does not improve the conduction heat loss, but
it does do two things: It fills up the (possibly not completely dead) air
spaces, cutting down on convection and drafts, and it shortens the
radiative path.
Remember radiation? At kiln temperatures, radiation is very important.
The glowing brick/fibre transmits lots of heat from hot particles to less
hot ones. In a confined space, this happens regardless of the distance at
about the same rate. So radiative heat transfer bridges over air gaps.
When very hot, the difference in insulation value of IFB and hard brick
diminishes. So also the insulating value of fibre. But the insulation
value of compressed fibre diminishes somewhat less than that of
uncompressed fibre. In this case you actually want more material in order
to make the air gaps smaller. So, at the hot face of a kiln, air gaps are
not so useful. Hard facing a kiln, whether IFB or fibre, will not diminish
the insulation value very much. It may also plug up air holes.
There is another factor in radiative transfer. That is the surface
emissivity. This just means that some surfaces emit radiation more
efficiently than others. So you can lower the radiative heat transfer by
lowering the emissivity of the radiating AND/OR the accepting surface(s).
For example, at room temperatures, a polished metal surface has a very low
emissivity. So does a white surface. Presto: space blankets and polar
bears. Also the insides of Thermos(R) bottles. A black surface has a very
high emissivity: that's the best color for a radiator. Also the best
color for heating by the sun, for example. So summer wear is seldom black,
because it's too hot. The question is, what's white and what's black at
kiln temperatures? Look inside and see. Actually, it's very difficult to
see, but there are some differences in material emissivities that can make
a difference in insulation value. So you can research these, and maybe
make a kiln coating that reduces the heat loss by radiation. Look for what
glows least brightly inside a kiln.
Now, what about electric kiln elements? Well, these are heaters: you want
them to be black, which is the opposite to what you want the brick face to
be. Think of the brick face as a reflector, and the element as the
radiator. White and black. Why does the element need to be black?
Because the more white it gets, the hotter it gets, without changing the
amount of heat that gets into the kiln. That means reduced element life.
But... there's a really big but here: Most element failures are by
oxidation of the element surface. Element alloys are designed to form a
coherent oxide film which protects the rest of the element from further
oxidation. Like stainless steel and aluminum. So, you want to avoid
damaging this oxide film. That's why elements get brittle (one of the
reasons, anyway) and that's why you need to keep them clean (fluxes tend to
disrupt the coating, just like they do in glazes). If you could coat the
elements, you would choose a material which forms a cohesive film at kiln
temperatures (an effective oxygen block), AND which has a high emissivity.
This is not impossible. Remember that elements are just metal (mostly
aluminum and refractory metals), and their oxides are just what we are used
to putting in our bodies and glazes. So your kiln elements are already
ceramic coated, whether you do anything to them or not, essentially with
Al2O3 plus other oxides. So treat them kindly, and preserve them from
glaze drips and dust. And from reducing atmospheres, which paradoxically
promote oxidation by destroying the oxide coating on the elements.
So, the ITC type coating can do some good, by playing with the emissivity,
and by being good, coherent, durable coatings. As Tony Clennell has said,
less is more here: most of what the coating does can be done by a very
thin coat. This doesn't amount to a testimonial on my part since I have no
experience with these coating. I just want to say that there is some
technological room for them to work, so I don't discount the anecdotes
entirely. A good comaprative test would be nice. Even better would be a
controlled experiment by someone at arms length. I would like to do this,
but I can't afford it. Can you?
Gavin
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Gavin Stairs
Toronto, Ontario, Canada
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