[Stoves] Fwd: Re: Building a Better Fire

[dstill at epud.net]

Dear Stoves,

Friends working in Darfur asked "What is a good fire" and I thought I'd forward
my thoughts to the List as related to discussions of insulating pots, etc. See
lots of you at ETHOS next weekend!

In my opinion, a good fire that cooks using the least wood is characterized by:

The size of the fire is matched to the job. The job is determined by how good a
heat exchanger the pot is. Pots loose heat mostly by evaporation. Losses
through
the sides are about 20%, evaporation 80%. Attention paid to deceasing evaporation
means a smaller fire, less wood used. Savings during bringing to boil are less
but during simmering savings are big.

Heat gets into the pot mostly through the bottom. We want lots of heat in,
least
heat lost. So big bottom, small top. Decreasing the water surface area
decreases
evaporation.

And a lid is very helpful.

The job of the fire is to bring the water to boil in a time period that pleases
the cook (usually fast!) and then if the pot is covered then we need less than
1/4 of the energy to simmer. Big fire first, then much smaller fire.

A well tended open fire takes about 1200 grams of dry (10% moisture content) wood to
boil 5 liters of water in a normal straight sided 7 liter pot and simmer for 45
minutes. More than half the wood is used to boil.

The pot can be 5" above the ground over the fire. Sticks should be burned at the
tip and pushed
into the fire as consumed. Folks usually know this. A grate under the fire
helps
to keep the fire going. Good to let air under the fire. If a lot of charcoal is
made then dirt can be pushed to seal space between pot and outside air, leaving
a very small hole to keep the charcoal burning. If the pot is covered it only
takes about 80 grams of charcoal to simmer food for 45 minutes.

Great 3 stone fire makers put a 2" layer of ash under the fire. Insulates the
fire.

For the 5 liters of water in the 7 liter pot high power should be approximately
4-6,000 watts. Too big a fire wastes energy. Fire size is dependent on pot size.


Using a skirt saves 1/3 of the energy to boil and simmer water.

Unfortunately a fire that is 1,000W to 1,500W can only be made with really
small
sized wood, like 3/8" or so, and it requires constant tending. Bigger sticks go
out. So cooks tend to
make much bigger fires for simmering than necessary because the wood is too
big.
Wastes wood.

I think that these are the highlights...probably missed some. 

Best,

Dean