Thanks for this Crispin. Your answers to Francois and Xavier have
helped me in my thought process with the project I envision here. Love
always, G
Regards, Gerrie Baker, aka The Worm Lady
Dedicated to delivering organic waste solutions through education and
demonstrations of worm composting habitats indoors and outside. Focused on
converting garbage to gardens and encouraging people to grow their own healthy
nutritious food and beautiful edible flowers.
The Worm Factory
874 Grady Road, Foley Mountain
Westport, ON K0G 1X0
613-273-7595
www.thewormfactory.ca
On 25/09/2011 11:40 AM, Crispin Pemberton-Pigott wrote:
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
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