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Dear Kirk, et al
I would say that if, in an "improved" cook stove, a
big pot full of cold water is lowered near the flames more smoke can easily be
produced. Any cold mass can lower the efficiency of combustion that's why I'm
such a fan of natural insulation, like wood ash, pumice rock, etc.
This has been a pretty freewheeling conversation and I very
much appreciate that we are discussing this important topic. The experience on
the List spans decades. I hope that we can turn this knowledge into a consensus
that will help create a world of better stoves.
In my opinion, if the designer moves the cold mass of the pot
closer to the flames or decreases the gaps around the pot, then it's necessary
to clean up combustion first. The box type combustion chamber just isn't clean
burning enough.
One can clean up combustion by metering fuel, raising
combustion temperatures, increasing draft, turbulence, etc. and then once the
heat is clean enough the designer can push heat transfer strategies without
creating a greater percentage of emissions. A good stove has to achieve clean
combustion first then we can go after getting more heat into pots.
In the Rocket stove the design compromise centers around the
height of the internal chimney. If is very tall, say fourteen inches, then
combustion is clean but as we all know the Delta T drops and a smaller percent
of heat cooks food. Alternatively, a eight inch internal insulated chimney above
the fire really smacks the pot with hot flue gases. Heat transfer efficiency is
much improved but more harmful emissions escape.
So, we tend to use higher internal chimneys with unvented
stoves and slightly shorter ones when the clients are rich enough to afford
chimneys or when fuel scarcity is the big issue.
If I were to play devil's advocate I would be tempted to
suggest that massive, box type combustion chambers that encourage loading with
too much fuel are obsolete and since they are not clean burning should not be
matched up with heat transfer strategies that require better systems of
combustion. A box made from thermal mass is just not a modern combustion
chamber.
Best,
Dean
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