[Stoves] ash layer beneath fire

[Crispin Pemberton-Pigott]

Dear Richard and Vincent

 

Jambo.

 

I see just about everyone found that adding a grate to a flat-bottomed stove
improves the performance.

 

RS>.I would advise use of round bars as Crispin but I would suggest you
grind any round bars down to result in effectively a series of had cylinders
along their length with the flat side up. 

 

I would advise the opposite and the metric to use in evaluating the
difference is the mass fraction of charcoal produced by the fire. We found
(for reasons that are not yet known) having a flat top definitely produces
more char thus losing more heat. It may have something to do with the
thermal contact the char piece makes with the metal, conducting the heat
away. This lowers the temperature of the 'coal' and causes it to burn poorly
or go out. There was a large mass difference for flat and round bars - more
than double.  The thermal contact between a coal and the round bar is much
smaller.

 

That is the only 'guess' I have so far. We will look into this further at
YDD this year. The two things tried were round bars from mild steel
(multiple welded pieces) and cast iron with a flat top (single piece). Cast
iron conducts less heat than mild steel but it still consistently produced
much more char. That is not useful as fuel in most homes and it is tossed
with the ash. Indar reported the round bar grate produced virtually no char
at all, finishing its burn at virtually the same time as the wood. That was
amazing. 

 

You may need to restrict the air entry area to limit the air level below the
combustion chamber. If it is 'right' you will see large flames wandering
around in a low velocity environment below the pot - no flapping arms of
flame.  You should limit the pot gap (by shaping those tongues of clay) to
about 7mm for all pot sizes. This means trimming the slanted ones and
lowering the top ones (in most stoves). 

 

The combination gives you the right balance of primary and secondary air. 

 

Nzuri kuhusu

Crispin

 

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