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Dear Paul,
I just sent two stoves to Colorado State U which cost $23 for
shipping. These stoves are either made from sheet metal or the new insulative
ceramic. When the 5 Aprovecho folks return from Central America in a month there
will be more people power to make stoves for those interested in the design
principles. How does $20 per stove sound plus shipping? I'll go lower for those
who can't afford this...The design book costs $8.
I guess that the Rocket elbow is the most distinguishing part
of Larry's stoves but here are the design principles copied from an
article:
Design principles resulting in improved fuel efficiency
Insulate everywhere around the fire and hearth except where heat touches
the pot(s) – the next section describes what materials make good
insulators. (not earth!)
Force the hot gases to rub against the outside of the pot(s) by creating
small channels with narrow gaps that follow the shape of the pot(s) –
otherwise the flames will just warm the escaping air.
Get the pot near to the hot flames. Intense heat is much better at
heating food than moderate heat.
Use materials which allow hot air to pass through quickly and easily to
the food, so if possible, use metal pots rather than ceramic.
Increase the speed of the hot gases as they hit the pots. Faster hot
flue gases punch through the still air that surrounds the
pot(s).
Rocket stoves are based on a combination of simple principles:
- The combustion chamber is insulated in order to keep the fire hot (above
650 oC ) to burn the wood more completely, thereby reducing smoke
(which is fuel that has not burnt completely).
- The combustion chamber and interior stove parts are as insulative and
lightweight as possible. Heavy materials in contact with hot flue gases
absorb heat that could have been used for cooking.
The 'elbow' is an important part of the Rocket family of stoves. Made in the
shape of the letter L, or U in a downdraft stove, fuel is fed into a combustion
chamber, placed under a short internal chimney.
Wood is burned at the tips and is pushed in towards the fire as it
burns. Pushing the sticks of fuel into the combustion chamber as it is burnt
ensures that the fuel is fed into the stove at the correct rate, creating
cleaner combustion, reducing smoke. A small opening is provided for incoming
air. Too much air just cools the fire. As the air moves through the opening
it is heated, which helps to keep the fire above 650oC assisting
more complete combustion.
A 'skirt' surrounds the pot on all sides. A small gap (about 7mm)
between the skirt and the pot forces hot flue gases to scrape against the
sides of the pot, which greatly increases heat transfer. The hot gases
contacts the sides of the pot as well as the bottom. Putting insulation
around the skirt decreases heat loss even more.
Rocket stove with internal chimney and combustion chamber
- A shelf placed under the sticks of wood in the entrance to the fuel
magazine allows the sticks of wood to remain separate rather than rolling
together. Creating a grate from sticks entering the fire helps to diminish
smoke. The feed magazine (horizontal pipe through which the fuel is
fed) protects the fire from wind and creates some preheating of air which
rushes in under the sticks as the air is pulled up the internal
chimney.
- The short insulated interior chimney, placed above the fire, creates a
strong draft that helps the wood to burn fiercely and cleanly. It also makes
the stove easier to light and to use, so even a small fire will continue
burning. (Eight inches in height is fuel efficient but higher chimneys
create more secondary combustion. A twenty inch high chimney smokes much
less but it is much less fuel efficient.)
- Adding an external chimney to a simple Rocket stove is accomplished by
partially submerging the pot into the body of the stove. The top of the
stove is formed to allow the pot to be placed snugly inside, usually down to
the level of the handles on the pot. The gap between the pot and the stove
body is about 6mm. The external chimney exits from the side of the stove
body which also surrounds the Rocket elbow. The Rocket elbow is surrounded
with insulation, usually wood ash. Using insulation, like wood ash, pumice
rock, vermiculite, or perlite also seals cracks in the combustion
areas.
Anyway, this gives you an idea of the principles. Rocket stoves are simple
one pot stoves, griddle stoves, bread ovens, incinerators, heating stoves, water
heaters, etc. The principles stay the same. Larry has written up the latest
version of the Golden Rules that will be published soon as part of the
proceedings of the ETHOS stove conference. I'll give details when it happens.
Capturing Heat explains all this in detail, available from Aprovecho: www.efn.org/~apro
People now a days see sidefeed most often in a Rocket stove but
downdraft/downfeed used to be as popular.
We like to see credit given to Tio Larry but a long time ago he decided to
give all design work away: our attitude is "Please Steal This Book!",
(we've already read it...).
Best,
Dean
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