[Stoves] Lion Cub Stove and Rocket Barbecue

[Crispin Pemberton-Pigott]

Dear Steve'n'All

> No, the angle was about 40 deg to the horizontal, and there's no sign of
back draught.

That is basically the VITA stove from the late 70's. I feel there is a
generic problem with it (down-angled self-feeding wood): if the excess air
flow is high enough there will be no backdraft up the fuel hopper, but this
condition is only sustainable with high excess air.

In every case I can think of, if the excess air is brought down to a
reasonable level like 100-125%, there is far too much heat rising in the
fuel at the burning end to be carried away by the measly air flow even is
100% of it directed through the fuel. The buoyancy of the gases in the
burning zone overcome the downdraft.

Two solutions seem to have been worked out (excluding the Rocket stove which
was developed in response to exactly this problem). They are a) run the
stove with high excess air, sweeping the heat and gases and flames into the
combustion zone and b) sealing the fuel supply gas tight to allow it to be
treated as a self-pyrolysing bottom-burning fire with air admitted at the
bottom somewhere. This is done with coal boilers and some pellet stoves and
the GTZ/GIZ 7-series coal stoves. The fuel is replenished periodically.

In fact the GIZ-7 stove is slightly different in that it has a crossdraft
fire under the hopper (a pyrolysing zone) which leads to a combustion
chamber that is not directly over that zone. The hopper is vertical. But
these are details. The principle is the same: 'inverted fuel', fire at the
bottom, and air entering somewhere near the fire (only) as a response the
issues created if the excess air is brought down to a reasonable level.

My prediction is this: if you limit the air moving past the pot to the point
where there is low EA and good heat transfer efficiency, the flames will
rise from the fire up the angled fuel hopper. It is a fundamental problem.

David Hancock experimented with other at Aprovecho in 1982 using weights and
string to make an 'auto-feeding' tray that moved wood horizontally into the
fire as it burned. Not too successful apparently. The idea was to use
David's idea of a horizontal shelf with air coming from below to control the
backdraft. Larry W developed this into the metal shelf with air passing
under it in a compact and simple package (original Rocket stove). The result
was a lower excess air ratio without backdraft, giving higher heat transfer
efficiency than the VITA stove. However it did not solve completely two
other problems: a) it was not self-feeding, requiring a significant amount
of attention to be efficient and even in heat production and b) the excess
air ratio was still too high. I might add a third which only emerged later:
the flame runs to the back of the chamber if all the air comes from one
side. This is what Andrew Parker has not observed with his new 16 brick
stove when he adopted the Lion stove air supply arrangement at the back to
counter-flow the primary air.

Variations on the theme have attempted to limit the airflow and provide a
proper balance between primary and secondary supplies to reduce the PM and
incomplete gas combustion.

And that, ladies and gentlemen, is, over a 30 year period, a (very) Short
History of Stoves.

Regards

Crispin

 

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