[Stoves] Wood-ash as insulator

[Crispin Pemberton-Pigott]

Dear François and Xavier

 

X Says >I strongly recommend you not to use stainless steel for the
combustion chamber, nor metal. It won't last long. We saw it with our
stoves, we used conventional stainless steel. The chambers didn't last 2
months, they were deformed and ripped-off by the fire intensity.

 

Stainless steel is 'stainless' mostly because it is not heated. It is not
technically 'heat resistant'. When the temperature rises the chromium atoms
cluster together ('condense') as soon as they are mobile enough and form
clumps. As their role was to occupy 'oxidation sites' on the surface of the
iron, so once they are gone the oxidation sites are available once more, and
sure enough Oxygen attaches to them causing corrosion (iron oxide) to form.

 

Stainless steel can be used in a high temperature fire as long as it is
continuously cooled by (usually) air passing behind it. Putting insulation
behind metal is a guarantee that the chemistry will get completely messed up
when hot, unless you are using a high temperature steel (sometimes called a
temperature resistant steel) which is invariably very (seriously) expensive.


 

This type of use of a cooling jacket was patented in the UK in 1947 when an
engineer made a hot water boiler with a 2mm wall in the combustion chamber!

 

So, if you put the heat coming through the wall to work as a preheater
(especially for secondary air) you can in fact use steel as a liner.

 

If you are building a Rocket-style stove with no separate secondary air
provision you are best off to use the hardest, densest, lowest thermal
expansion ceramic you can find for the combustion chamber of an
institutional stove. There is a slight gain in combustion efficiency in the
late fire when using high mass bricks.

 

If you find that the heat loss is excessive (as a % of the total heat
produced) then consider insulating the hardened chamber, but not until then
and only then. Put the money into air control instead.

 

Usually the idea of 'heat being lost' from the combustion chamber is both
overstated and uncalculated. Usually system efficiency is not improved much
at all by preventing some heat loss from the combustion area. If that
'prevention' takes place at a lifetime cost (reduced lifetime) then it is
not worth it as Xavier says, for stoves costing a couple of hundred Dollars.

 

Do not guess what the heat loss prevention is. Calculate it. Painting it
silver on the outside might drop losses by 30%.  If you find that the
difference in heat loss is 200 or 300 watts on a 15 kw stove, be very clear
that it is FAR easier to transfer a few hundred watts extra to the pot by
controlling excess air than it is to prevent its loss from a hot exterior of
a combustor.

 

Most stoves of the Rocket style have high excess air (due to no control of
air) so have a clear perspective on how much that EA costs the system. If EA
drops the system efficiency 10%, then that little bit of heat lost to the
surroundings low down is 'peanuts'. Get a handle on the air supply. You can
choke it going in or choke it passing the pot (with  tight skirt). The
effect is the same. The latter pressurises the stove so it is more dangerous
in a sealed room.

 

So for any system you intend to work for a long time, first seek strength
and thermal shock resistance, then look at the thermal conductivity. Really
hard bricks have a lousy conductivity anyway and low density bricks are not
all that much better as insulators. The low density ones usually have a very
poor thermal shock resistance because they are not dealing with the
mechanical deformation. The ones used industrially in high temperature
applications are usually cycled very slowly. A domestic or institutional
stove actually has a very difficult environment because the poor suffering
bricks are heated and cooled several times a day across a huge temperature
range.

 

Regards

Crispin

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20110925/8ac4c19c/attachment.html>