[Stoves] Wood-ash as insulator

[Gerrie Baker]

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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