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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link=blue vlink=purple><div class=WordSection1><p class=MsoPlainText>Dear François and Xavier<o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>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.<o:p></o:p></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>Stainless steel <i>can</i> 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. <o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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!<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>Regards<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Crispin<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p></div></body></html>