[Stoves] Stove Material Properties

[Crispin Pemberton-Pigott]

Dear Tae

You are almost completely correct.

You have to fight the problem of insulating the fire creating the very
problems that you want avoid.

It would be helpful if you were not think of ceramics as being 'insulative'.
They are for the most part very poor insulators and it would be better to
simply quantify the heat losses. They are not much and the term 'insulative'
is often a misnomer for the characteristics of what gets into a stove.

One reason to reconsider this is that if heat gets through a stove
component, that does not mean it is lost. Look up Lion Stove for a
description (on this site) of how to recapture the heat passing through the
fired clay bricks and feed it to the fire. This recirculating of the heat
obviates any need to be insulated. If you subtract 10,000,000 Joules of heat
through the walls and feed 10,000,000 back with preheated air, you have lost
nothing and that is better than even very good insulation.

>i am working with clay in particular and my understanding is that the  
>heat transfer coefficient is what determines the stove wall's  
>insulation performance 

Yes.

>and the thermal conductivity is what determines  
>the temperature difference between different parts of the stove wall.

Partly yes.  The conditions on the external wall determine the temperature
difference. You could have a material with a conductivity of 1.0 and put a
shiny coating on both sides. In the hot side that would keep heat out, and
on the cool side it would keep heat in, giving a much more even temperature
(low Delta T) than an analysis of the material would indicate.

>and the tensile strength is what determines when the stove will crack.

Not really. The cracking is made by the compression strength. When it
expands the material gets bigger.  The cooler material beside it tries to
hold it back. Either a high compressive strength or a low tensile strength
will see it fracture. The ratio is more important than the magnitude of the
numbers.
 
>from what i've learned, for clay stoves, the two major aspects are the
thermal
>performance and longevity of life(prevention of cracking).

Longevity is probably far more important than thermal performance. How cares
if it performs well if it only lasts a couple of months?

>for thermal performance:
>thermal insulation(low heat transfer coefficient) is important

No, as explained above. You can recycle heat in a number of ways.

>for prevention of cracking:
>high thermal conductivity(so that the temperature within the walls  
>will be relatively constant, preventing cracks from occuring due to  
>different thermal expansion in different areas)

That is a valid approach that has potential to work for a long time. Making
spongy ceramic is another way. Don't be distracted by the words 'refractory'
which does not mean 'can take high thermal shocks'. It means high
temperature. There is a world of difference.

>low thermal expansion (to prevent cracking)

To reduce stress, not cracks, actually.

>and high tensile strength to withstand thermal expansion

And (relatively) low compressive strength.

The question of how to produce such ceramics is an altogether different
subject! At least you are on the right track.

Good luck
Crispin