1612 K Street, NW
Washington, DC 20006
|Stoves Archive for October 2002|
|236 messages, last added Tue Nov 26 17:31:57 2002|
[Date Index][Thread Index]
RE: Ethiopian Woks
Crispin, Margaret, Stovers, Bob van Buskirk
A bit more than a year ago, I sent a message to the list related to your
query - as Dan Kammen was looking for good stoves research.
I referenced a web site maintained by a US researcher - who I think is not
a member of this list - Bob van Buskirk - who maintains the following web
site - which I strongly recommend before your trip:
(I recommend this to everyone trying to heat big flat surfaces.)
Some more comments below.
From: Crispin [mailto:firstname.lastname@example.org]
Sent: Monday, October 14, 2002 4:39 PM
To: Stoves; Margaret Pemberton-Pigott
Subject: Ethiopian Woks
Dear Stovers, especially those with experience in Ethiopia
(RWL1) I have spent about 8-10 months of my life in Ethiopia - a part on
stoves. I urge you to also think about Eritrea for your research (which is
where Bob's work took place) - as they (or at least those in the north)
speak the same language - Tigrigna.
Robert van der Plas is here in Swaziland and we managed to get together at
least for one evening, at which we discussed a little about the BTG project
he has going in Addis Ababa. He got to the workshop today with tired kids
in town having looked at animals all day (and it was pretty hot) so we
didn't light anything.
It is clear that people there are not using pots as much, or I could say, as
universally as they do here. They are using something that is about like a
650mm diameter shallow clay wok. This is put on three supports. I would
like to take a stove up with me next week to Addis to see if the clay
tray/wok can be effectively heated by the Shisa Stove, and what local fuels
it burns well.
(RWL2): I question the term "shallow" - I only remember flat units -
called a mogogo. But the 65 cm size and clay (low fired) material you
mention are the same. van Buskirk describes how inefficient these are for
cooking. A wok shape would probably not work - as the enjira cooked on the
mogogo starts off with a "heavy cream" consistency. - and the preference is
strongly for enjira of uniform thickness.
I will try to meet with his man-on-the-ground who arrives today and we will
make an attempt to do one or two cooking tests during the following week.
What would be useful to know is people's estimate of how the wok can be
supported on the stove. As it is not going to fit inside the 285mm diameter
top of the can, it is going to have to sit on top of a special support.
Pots normally sit inside the heat shield. I safely presume the cooking
surface is not pushed around much in the way that a three-legged pot is when
something is being stirred.
(RWL3): My recollection is that the historic method was three stones -
with wood coming in from three directions in between. But all the improved
designs seem to be ringed with bricks (hopefully as insulating as possible -
to keep the breezes out and heat in).
I started my work on the charcoal-making design as you have suggested -
with a 30 cm diameter. It was awful - very hot in the center and not hot
enough on the outside. But when I shifted to a conical flame section from
the 30 cm fuel chamber - up to the 65 cm diameter mogogo, the temperature
turned out to be quite uniform. I tried many means of supporting the mogogo
in the conical piece (made out of a semicircular - the final top
circumference being the same as the initial half-circumference) - the bottom
circumference being 30*pi cm. (the top-lit fuel can come right up to this
secondary air inlet level) One that seemed to work moderately well was
using steel wool pads - that had a nice cushioning effect and could
withstand the hot exhaust gases. One big problem was making sure that the
mogogo stays level - so that the "heavy cream" batter doesn't run off one
side of the mogogo. Cooking is very rapid - I recall about 2 minutes per
enjira. You don't see any pushing around. The enjira is pretty delicate.
The secret of success is getting the mogogo well oiled/seasoned.
Are these clay woks fragile? Can they sit on three relatively sharp points,
like the ends of three pieces of 12mm round bar? I could make a triangle
with three legs which sit on the 'top deck' of the stove (which is 150mm
below the top lip) and protrude just above the lip. The wok would then be
held up about 20mm above the lip of the 25 litre can. This would allow the
gases to leave the stove at the lip and travel out to the edge of the wok
heating the undersurface.
(RWL4): The mogogos are not so much fragile (being about 3 cm thick) - but
they don't last very long - mostly due to thermal stresses I suppose. They
are for sale in every marketplace - I think only costing a $ or $2. From my
experience, the geometry you describe is never seen and will not give a
sufficiently uniform surface temperature, as I said earlier. In the
traditional approach, the cook maintains surface uniformity by moving new
sticks of flaming wood (or moves the charcoal from earlier sticks) to below
that part of the mogogo that seemed coolest when cooking the previous
enjira. With your design (and mine) you have to have it right the first
time (although they can probably start pouring the batter first where the
mogogo is coolest).
If the wok is not strong enough to be used when sitting on three points, I
could make a ring about 275mm in diameter and put that onto 3 legs with the
same approximate result in gas flow.
(RWL5a): I think you will find that you can use a three point support -
but I don't believe support will be the issue. van Buskirk's paper
describes the much greater efficiency possible with a metal rather than clay
mogogo. Then there is no question of lifetime - and that is the route I
would go - especially if you can get the right surface treatment.
(RWL5b): Concerning thermal uniformity, Rogerio Miranda has described
putting an extra thickness of metal where the cooking surface was hottest.
This might help somewhat, but I still predict a need to have something like
the conical gas combustion section I described above. Gas flow modelers
could probably give us a better shape - but I was not unhappy with the cone.
I have, through my wife Margaret who is there already, located a potential
manufacturer of small stoves so I will either leave it with him or else with
the BTG project for them to test in the field. Apparently about 90% of the
population uses wood for fuel. Biomass is used in the form of leaves, wood,
charcoal and dung, all of which can be burned reasonably in a Shisa Stove.
It seems there would be an advantage to using leaves in this stove compared
with using an open fire because it has some measurable air control. The
morning meal is cooked over a low heat and top lighting a pack of leaves
that burn primarily as a gas might prove to be just the ticket.
(RWL6) There is nothing wrong with the circular shaped enjira - but the
Ethiopians and Eritreans never serve it that way. Rather they cut it into
strips of about 10-12 cm width which are served rolled up into a fat cigar
shape - of diameter maybe 4-5 cm. I have often wondered if it wouldn't be
culturally permissible to cook something more like that shape - as long as
you had some cooking advantage that way (and I don't off-hand know of one -
unless you start talking about a solar device). The electric mogogos one
can buy are usually of smaller diameter - maybe 45-50 cm (and I have seen
smaller). These come with a traditional clay material or steel or aluminum.
(RWL7): To keep the heat losses down and keep the cooking humidity in the
right level, the enjira cooks for about half of the time under a "hood" -
made from dung by local specialists. The electrics always have an aluminum
"hood" - so I would plan that in advance.
(RWL8): What we need also is something like an automatic controller - that
turns down the heat during the "lengthy" time that the hood is off and the
enjira is being "pulled" from the mogogo skillet and the time necessary to
reload it for another enjira. These enjira are cooked one at a time - with
a typical batch taking more than an hour or even two (every several days)
depending on the family size. In a factory setting, I think one cook can
keep two or even three mogogos going at one time - but one family (wife)
never does that I think. This form of heat control is of course not
possible with clay because of the clay's poor thermal conductivity - but it
might make sense with a thin metal mogogo. But there may be a possible way
to cook enjira something like we cook pizzas - by sliding a thin round (or
square) plate with the "wet" enjira "cream" into and out of (when ready) a
more thermally insulating "oven". Now we are talking real cultural change -
and you won't have time to do that while in Addis.
(RWL9): There is an excellent private (formerly government-sponsored)
stove development group out near the airport. I urge you to look at these
Addis Ababa stove expertise leads early this year at:
http://www.repp.org/discussion/stoves/200201/msg00115.html, where there are
phone numbers, etc.
(RWL10): I certainly like the idea of using leaves, etc. I believe the
can of straw you mentioned today can be further developed - but you better
plan on a cooking time of more than an hour - and probably well over 5 kW -
not easy to do without some densification.
I do believe that your design with air control, preheating primary and
secondary air, etc could help a lot on a most difficult national cooking
(RWL11): When I first showed something to a local leader in Tigray - and
she could see that it burned pretty cleanly - she asked for one that could
fold up - as space is at a premium inside most homes. (Much cooking is
outside - but also often inside.) Then I started hanging the cone and
mogogo from a tripod arrangement - but I didn't solve this space-saving,
folding problem as I ran out of time.
Best of luck - anxious to hear what happens. Say hello to Margaret and hope
you have a good time. Let me know if you need any assistance in Addis - as
I have some other friends who I believe would want to help. Ron
Stoves List Archives and Website:
Stoves List Moderators:
Ron Larson, email@example.com
Elsen L. Karstad, firstname.lastname@example.org www.chardust.com
Other Biomass Stoves Events and Information:
For information about CHAMBERS STOVES