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|Stoves Archive for September 2002|
|189 messages, last added Tue Nov 26 17:31:51 2002|
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RE: Stoves at the Summit
Dear All and Sundry
Well, I have been back for a few minutes from the Summit and am wading
through the junk-filled inbox to sort out the wheat which definitely
includes the messages below.
>I'd love to see Crispin's stove.
Well, go the website www.newdawn-engineering.com to the Products section.
Look under stoves for "Stove burning, boiling and efficiency tests" and then
click on "Basintuthu Stove Tests" to arrive at
<stove\Tests\basitests\basitest1.htm> (that is not the whole URL).
The one in the top picture is exactly what Dr Agnes Klingshurn from GTZ has
taken back to Zimbabwe today for pretty comprehensive testing. It is not
perfect (which David Hancock was quite insistent it should be) but it will
give an indication of what a hand-crafted one will do. Don't be put off by
the fact it has a small 10 litre can under it. There are two versions that
work the same way and one has an ash can under it. The stove has partial
primary air pre-heating. It is a 4 litre pot. The bakelite handles have
been removed and a wire loop put on so it can be lifted easily.
Dr Agnes and Marliss (head of the RSA wood stove program who brought Peter
Scott in) were justifiably sceptical of the stove when they saw and heard
about it. It breaks a lot of 'rules' being taught hither and yon. An hour
of cooking was pretty convincing I guess. At a very low power it is very
efficient though I will wait for them to say what they think it is. The
pre-heating of the air can keep a tiny fire going. I am formulating a plan
to do some comprehensive improvement testing of some hardware
changes/versions to get the primary air firmly under control. That plan
should be ready for scrutiny in 90 days.
>1) I don't think air preheat is necessary or even desirable as
>it doesn't mix as well with hot gases as dense cold air and
It allows us to burn pretty crummy fuel, and to pyrolize wood to charcoal
with a very small fire. It also obviates the need for insulation or ceramic
pipes and so on. Most 'leaking' heat is re-directed into the fire. At low
power the bottom half of the stove doesn't reach 60 on the outside inspite
of being over 600C in the centre.
>2) The good thick steel for combustion zone soaks up heat before
>it can deliver it. I have been amazed that tin cans (tincanium)seem
>to last forever, are widely available and easy to machine.
Ron's report is slightly misleading. The grate is indeed strong and very
stiff but it is only 1.2mm thick. The material is a titanium stabilized
low-chrome stainless steel called 3CR12 which is made in Middleburg, a town
between Swaziland and Johannesburg. It is very strong but not very
malleable - almost brittle. It is a little less than 1/2 the price of
stainless (304L etc). It is shiny (reflective) with an 'as rolled' finish
called 2B. It discolours with high heat and eventually discolours over time
anyway. It is welded with a rod called 309L which we use in 1.2mm wire form
on a MIG welder using an Argon-CO2 blend with a tiny amount of Oxygen in it.
It will spot-weld.
>It is the best I have seen for sale ($26 now - likely to go lower).
This is indeed the present price and we will try hard to get it down to $20.
>This looks like a good application for thin rolled iron someday in the
The temperatures involved will not allow thin unprotected steel to last
long. It will rust through and especially at the welded points where the
chemistry is a little wonky. The parts of the stove that do not get touched
by fire are made from epoxy-coated tin-plated steel. The 25 litre container
(285mm dia at the top) with a rolled top lip, seam welded with a water tight
rolled-seal bottom costs only $2 and I am not going to treat the material at
all. It is impossible to make a can of that size and quality for that price
by any normal means. We will have to live with what is available unless we
want to invest literally millions in tooling.
The "... upper outer baffle for getting higher efficiency for the convective
transfer to the pot. [Ron]" can be made from something better than the 0,6mm
galv sheet I used but that was just because I had some around the shop. It
is very malleable which means making a square lip on a ring is a cinch. One
problem we have seen is that at high power the galvanizing melts and runs
down like wet paint.
Ron sez: > 4. Controllable primary air (and power out)...
The variability of power out is about 1:7 or 1:5 depending on fuel load and
conditions. When turning the power down there is a delay during which the
combustion lower down decreases but gassification continues and the flames
move up and/or it smokes a little. After a time (1-3 minutes) the internal
temps drop and it stabilizes at a lower output. Separately controllable
primary and secondary air would probably fix that. Opening the air vent
increases the power immediately.
Dan sez: > I am concerned that this pail might be galvanized.
I hope to get it galvanized but that is a difficult option. Zinc, when and
if it burns off, I understand, is positively beneficial to people with lung
infections. The olden day treatment for TB was to get a job in a galvanizing
plant. Breathing the zinc fumes would eventually cure or ameliorate the
condition. We have terrible TB problems in this region of the world. The
problem with zinc protection is that it is sacrificial, not real protection.
Cadmium plating is a terrible danger in a stove and must be avoided
My conclusion : Thin, uncoated steel will not last in this device.
Ron sez: > 6. Looked very clean burning after short time in startup.
In the test he saw it took about 30 seconds to 'go secondary' which is
better than average. The reason was the tiny amount of fuel in it. A
couple of hardwood sticks and two square briquettes (90gms total) boiled 1.5
litres of water and kept it simmering (bubbling) for about 50 minutes. I
found the fuel to be a bit smokey initially though it was certainly hard.
The firebox was a little over filled height-wise (contributing to smoke) but
no one had a saw and it was too strong to break. The second demo on
Saturday(?) used damp newspaper and rained-on wood but apart from taking
longer to get going it was just fine.
>The stove went, after a high output pyrolyzing period, into
>a charcoal burning mode.
When the gassification is finished the air vents are opened up to allow for
a faster charcoal burn to maintain the heat output.
>I haven't yet convinced him that this is worth saving
>Can Paul Anderson's, or another version of the removable charcoal
>holding cartridge be used to remove the char and replace with
The present configuration is such that the grate can be lifted out, the
charcoal removed and some coals swept back inside, more fuel loaded and the
grate replaced. This is done with a long wire hook (5mm nail wire). The
Basintuthu Baking Stove comes with a hook for this purpose. The grate in
the baking stove has a loop over the top for lifting it out. If it is
important I can put one on the single pot stove grate as well. The baking
stove is run for longer at higher power so there is a real chance of the
grate getting plugged up. It is occasionally necessary to remove the grate,
tip it over and remove the unwanted material. It takes about 1 minutes to
do this. Also, it allows one to see that it is well lit before returning it
to the stove. It is removed through the larger pot hole directly over the
I apologize for the website not being up to date with the pictures. There
are actually pictures of the Tsotso stove on most of the pages.
Regards to all
Crispin back home in the Ezulwini Valley
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