Gavin Stairs on sun 18 oct 98
At 10:18 AM 17/10/98 EDT, you wrote:
>----------------------------Original message----------------------------
>The Skutt rep told me that the ITC coating acts as insulation.
>Thus preventing the radiation of heat out into the main area of the kiln
>holding it in the groove and element.
This is both true and false. Here's how it goes:
The element heats up because of current passing through it. So long as the
resistance of the element remains constant (it actually rises slightly with
temperature) and the current does also (it actually falls slightly, owing
to the rise in resistance with temperature: we will ignore these small
effects), the power dissipated from the element remains constant. No
matter what you do to the element surface, this will remain true. There
are only two things that can happen with this heat: it can be stored
internally by a rise in temperature, or it can escape from the surface,
mostly by radiation.
Suppose you coat the surface with a substance that makes it harder for heat
to radiate from the element (e.g., ITC 100 or 296A). What happens then?
The element gets hotter until the heat radiated once again balances the
resistive heat. So, ITC coated elements are presumably hotter than
elements not coated. Is this a good or a bad thing?
In general, the hotter the element, the more reactive and susceptible to O2
diffusion it is. So excess temperature is undesirable. However, if the
excess temperature is accompanied by an increase in the impermeability of
the surface to O2 and to a lesser extent N2, then the net effect may be
good (i.e., longer element life).
So, if the ITC 100 or 296A is forming a thin barrier layer which resists
the passage of O2 and other vapors and gasses, then the net effect could be
beneficial, provided the temperature rise does not compromise the element
mechanical stability. Most pottery kilns use fully supported coil elements
which are not sensitive to increased creep, so this is not an issue in most
cases.
The second part of the issue is whether the coating on the lip of the slot
is a significant barrier to the radiation of heat from the element. The
result is the same: if there is an increased barrier, then the element
will get hotter.
I don't know what ITC is made of, but a guess is a combination of zirconia
and alumina with minor constituents to modify the flow, adhesion, etc. The
several formulations are probably optimized for low emissivity at kiln
temperatures (that's the part that makes it a kiln insulator even in thin
layers) as well as for thermal expansion matching to various substrates
(that's the part that resists flaking and spalling). The zirconia and
alumina make it very refractory and very resistant to kiln atmospheres.
They also make it quite expensive.
The transfer of heat from a surface is proportional to the 4th power of the
difference in temperature between the hot body (the element) and the cold
body (the rest of the kiln), and directly proportional to the product of
their emissivities. A normal element can be considered to be a pure
alumina coating over iron alloy. An ITC coated element is not very
different: it is an alumina-zirconia coating over iron alloy. This is
probably not a great difference. The rest of the kiln, in the case of an
uncoated brick kiln, is a combination of clays and glazes, including the
ware. The coated kiln is pretty much the same, from the element's point of
view, except for the kiln wall and groove in which it sits. The wall
behind it will reflect more (i.e., remain cooler relative to the element),
and so will the lip in from of it. So the heat transfer to the brick will
be less efficient, which will tend to raise the element temperature, while
the heat transfer to the ware will remain much the same. In net terms, the
element temperature will probably rise slightly, and the heat transfer to
the ware will also rise slightly. The effect will be to increase the kiln
efficiency.
The above is for a thin coating of ITC. For a thicker coating, the element
will look more like pure alumina-zirconia, and less like a thin
(transparent) coating over iron alloy, so the emissivity will fall, and the
element temperature will rise. There will also be a slight temperature
drop over the thick coating, so the element metal will become that much
hotter. The thicker the coat, the hotter the element, until all the good
effects of coating are "used up" by the adverse effects of the temperature
rise. So a thickly coated element may fail prematurely.
Hope this helps,
Gavin
Nils Lou on thu 22 oct 98
Gavin,
Your assumption that ITC is a coating of alumina/ zirconia is not
the case. If it were, your other assumptions would be true, ie., providing
a barrior and so on. The ITC does not, in fact, make the element hotter;
it is much more complicated. What it does do is resists the tendency for
the element to lose mass, gain resistance and age. There are ion transfers
and a building of new atomic clusters which develops an entirely new
ceramic material interface which has the property of resisting atmospheric
degradation effects. This is poorly said, as I am rushing, but since you
seem interested I wanted to apprise you of the nature of the material. It
is not a simple "coating". Best regards, Nils
Stuart Ridgway on wed 28 oct 98
There has been recent discussion of the usefulness of ITC coatings
on kiln heating elements. The coatings are expected to protect the element
against kiln atmosphere caused degradation, but it may hinder the heat flow
out of the wire raising its temperature thus accelerationg wire aging.
I have a new element for a AIM kilon ring that measures 35 ohms
resistance, and the wire was 0.038 in diameter and 66 feet long. At 230
volts the heat power to be radiated to the kiln is 2.54 watts/cm sq of wire
surface. For an emissivety of 1, which is perhaps 10% high for oxidized
Kanthal (I'm guessing from values given for nichrome and similiar alloys)
the temperature difference needed to radiate the 2.54 watts /cm sq to a kiln
at 1200 deg C is 35 deg, i. e. the wire needs to be at 1235 deg to radiate
the power. If the emissivety is less the temperature difference needs to be
higher. If the wire is covered with a coating that doubles its diameter the
radiating area is doubled, so the temperature difference for radiation is
half as much, if the coating has the same emmisivety as the wire. A
temperature difference will also be needed to drive the heat flow through
the coating, which for a coating heat conductivety of 0.08 cal/cm-deg which
might be reasonable for aluminum oxide or Magnesium oxide, the temperature
difference to drive the heat flow throuogh the coating works out to be 5.25
deg. For this idealized case the coating has reduced the wire temperature
from 1235 deg to 1223 deg. We have to assume that the ITC people have
succeeded in formulating the coating so that a high emmissivety is attained
and a high thermal conductivety.
This analysis suggests that coatings can reduce the wire operating
temperature, but they should not be too thick, since as the coating gets
thicker its hast flow resistance becomes dominating.
Stuart Ridgway
>----------------------------Original message----------------------------
>At 10:18 AM 17/10/98 EDT, you wrote:
>>----------------------------Original message----------------------------
>>The Skutt rep told me that the ITC coating acts as insulation.
>>Thus preventing the radiation of heat out into the main area of the kiln
>>holding it in the groove and element.
>
>This is both true and false. Here's how it goes:
>
>The element heats up because of current passing through it. So long as the
>resistance of the element remains constant (it actually rises slightly with
>temperature) and the current does also (it actually falls slightly, owing
>to the rise in resistance with temperature: we will ignore these small
>effects), the power dissipated from the element remains constant. No
>matter what you do to the element surface, this will remain true. There
>are only two things that can happen with this heat: it can be stored
>internally by a rise in temperature, or it can escape from the surface,
>mostly by radiation.
>
>Suppose you coat the surface with a substance that makes it harder for heat
>to radiate from the element (e.g., ITC 100 or 296A). What happens then?
>The element gets hotter until the heat radiated once again balances the
>resistive heat. So, ITC coated elements are presumably hotter than
>elements not coated. Is this a good or a bad thing?
>
>In general, the hotter the element, the more reactive and susceptible to O2
>diffusion it is. So excess temperature is undesirable. However, if the
>excess temperature is accompanied by an increase in the impermeability of
>the surface to O2 and to a lesser extent N2, then the net effect may be
>good (i.e., longer element life).
>
>So, if the ITC 100 or 296A is forming a thin barrier layer which resists
>the passage of O2 and other vapors and gasses, then the net effect could be
>beneficial, provided the temperature rise does not compromise the element
>mechanical stability. Most pottery kilns use fully supported coil elements
>which are not sensitive to increased creep, so this is not an issue in most
>cases.
>
>The second part of the issue is whether the coating on the lip of the slot
>is a significant barrier to the radiation of heat from the element. The
>result is the same: if there is an increased barrier, then the element
>will get hotter.
>
>I don't know what ITC is made of, but a guess is a combination of zirconia
>and alumina with minor constituents to modify the flow, adhesion, etc. The
>several formulations are probably optimized for low emissivity at kiln
>temperatures (that's the part that makes it a kiln insulator even in thin
>layers) as well as for thermal expansion matching to various substrates
>(that's the part that resists flaking and spalling). The zirconia and
>alumina make it very refractory and very resistant to kiln atmospheres.
>They also make it quite expensive.
>
>The transfer of heat from a surface is proportional to the 4th power of the
>difference in temperature between the hot body (the element) and the cold
>body (the rest of the kiln), and directly proportional to the product of
>their emissivities. A normal element can be considered to be a pure
>alumina coating over iron alloy. An ITC coated element is not very
>different: it is an alumina-zirconia coating over iron alloy. This is
>probably not a great difference. The rest of the kiln, in the case of an
>uncoated brick kiln, is a combination of clays and glazes, including the
>ware. The coated kiln is pretty much the same, from the element's point of
>view, except for the kiln wall and groove in which it sits. The wall
>behind it will reflect more (i.e., remain cooler relative to the element),
>and so will the lip in from of it. So the heat transfer to the brick will
>be less efficient, which will tend to raise the element temperature, while
>the heat transfer to the ware will remain much the same. In net terms, the
>element temperature will probably rise slightly, and the heat transfer to
>the ware will also rise slightly. The effect will be to increase the kiln
>efficiency.
>
>The above is for a thin coating of ITC. For a thicker coating, the element
>will look more like pure alumina-zirconia, and less like a thin
>(transparent) coating over iron alloy, so the emissivity will fall, and the
>element temperature will rise. There will also be a slight temperature
>drop over the thick coating, so the element metal will become that much
>hotter. The thicker the coat, the hotter the element, until all the good
>effects of coating are "used up" by the adverse effects of the temperature
>rise. So a thickly coated element may fail prematurely.
>
>Hope this helps,
>
>Gavin
>
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