School wood stove for 100 liter pot Tanzania 1989

At the school where I worked 1988-89 in northeastern Tanzania was a large group of students that lived at the school. The school buildings had been built by scandinavian missionaries who had a primary school there. Now the school was run by a coffee co-operative as a secondary school.

There was a kitchen where "uji", a thin maize porridge, was cooked in the morning and "ugali" thick maize porridge and beans served after school. It/they was/were prepared in 2 pots that were halves of a 200 liter oil barrel on "three stone" stoves. The kitchen was always smoky, the walls were black. Half of the same building was a dormitory which also was smoky since there were no ceilings. I could see that there was an unused chimney on one of the walls of the kitchen. I thought it was a shame not to use that to take the smoke out. I decided to try to make a stove I could connect to the chimney.

I talked to the cooks about what I thought might be a good stove. I thought it should be close to the chimney for many reasons: easier/cheaper to build, easier to get the draft flowing, stable placement, out of the way. It turned out that they didn't like that when they would be making ugali (which is a daily staple food). When you make ugali you boil water then mix in the maize flour while mixing it with a big wooden paddle. The fire is continuing and if you don't keep working the thick mass it will burn against the walls of the barrel. The cooks wanted to be able to move all the way around the pot while intensively pressing the paddle between the pot walls and the ugali. This also meant that they didn't want the stove to be higher, because then they wouldn't get good leverage on the paddle.

Another adjustment to local practice was making a relatively long firewood magazine. Wood was usually collected by hand or with a "panga" (machete) and they didn't usually have access to a saw to be able to easily make short pieces. It had a sheet iron lid which covered all but the hot end. There the lid was brick and mortar. I encouraged them to keep some kindling piled there (for drying) which made firestarting easier.

I decided to make a stove that would surround a greater part of the pot and have tight seal at the top. To make sure it would be durable in spite of the violent ugali kneading I cast a reinforced cement ring for the top of the stove with an internal diameter equal to the diameter of the ridges of the barrel-pots. (The reinforcement was probably several circles of heavy wire. I don't remember.) To form a seal and support the pot we had a welding shop in town weld on a approx. 2½-3cm iron ring (probably 1½-3mm thick) on the crest of the barrelridge. We also had some rebars welded to form a simple grate. These were the only things that were done off-site. The rest of the material was local bricks with redclay "mortar" finished off with a cement-sand mortar plaster.

Before starting the masonry for the stove, we knocked out a hole in the floor which would be the air entrance, ash pit. (This was to meet the cooks wish that the stove be as low as possible.) See the drawing for the construction.

When used, care is needed to get the draft flowing up the chimney. When the system was cold a small fire of paper or leaves was put in the ash cleanout hole at the base of the chimney. Once there was a certain amount of upward flow there, a well prepared fire was ignited in the fire box of the stove and the chimneys ash-clean-out hatch closed. It could take several minutes before the draft was really going well. If one closed up the stove well after the days cooking and closed a damper in the chimney, the residual warmth made starting up the next morning easy and fast.

The major goal of cleaning up the kitchen was achieved very well. The cook said "Now I can wear a tie when I work!" (I don't think I ever saw him with a tie, but his eyes were less bloodshot. I previously could stay in the kitchen only minutes at a time without watering eyes and coughing. I can't find any notes on the efficiency. The cooks said it used ½ as much wood and cooked 30-50% faster. I had some notes on how long it took to boil an amount of water but never measured how long it took on a "3 stones" stove. The stove was used daily for the remaining 1 year that I was there. The only problem was that they often didn't take out the ashes regularly. A cooling air draft from below can make the grate last longer. So I feared that the grate would perhaps have to be replaced unnecessarily early. Though not very expensive, for a school which often was 3 months late with teachers salaries, this could lead to the stove ceasing to function when the grate burns out. I believe that I heard that the stove was still operational in 1995.

I decided to try making something based on the traditional kind of oven that is used in Northern Sweden for baking "tunnbröd" (thin bread). In these stoves you make a fire in the arched oven space for a few hours which heats up the bricks surrounding it. When it is hot enough (usually guaged by throwing some salt in), you sweep out the ashes and embers (or sweep the embers to the edges) and put in your bread. The traditional thin bread is quickly baked while the oven is very hot. People have special "pizza"-sized thin "boards" on long handles which are used to quickly lay in and lift out the thin bread. When the oven has cooled down one could do thicker breads and finally large loaves.

I had used this kind of oven and knew how they worked. But I wanted to try adding a few other features. One thing I wanted to do was be able to use some of the heat that was unused in the smoke from the stove. So I laid out a system of ducts in the masonry of the oven which could be used to send the gases from the stove through on their way to the chimney. (This is similar to the principle of the traditional space heating "kakelugn" where heat is magazined in brick and tiles.) When cooking beans which take a long time but need low power once the water is boiling, the strong draft of the chimney can be disadvantageous and lead to unnecessary wood consumption and burning of the food. Therefore the circuitous route through the oven could help control the power/wood consumption of the stove. When starting the stove and needing high power (=strong draft) some dampers would direct the smoke directly to the chimney. Once the food was boiling the dampers could be changed so the smoke is redirected through the oven. This would give the mass of the oven an elevated temperature. This would make heating up for baking both faster and more economical. Even without extra fire, it would also make the oven useful for: predrying firewood, drying/dehydrating foodstuffs (grain, coffee, fruits, vegetables, fish). By directing the smoke through the central oven space it could "smoke" the produce, esp. for preserving fish or meat.

Design features: 2 horizontal rebars to brace sidepressure from arched roof. Notches and easily removable fitted bars for a second level in the oven to increase production capacity. (The oven was made with a higher arch than the "thin bread ovens") Doors seated by gravity against 80 degree sloping sides, many ash-removal openings and dampers for regulating gas flow (and stopping flow to preserve heat), woodash insulation, an extra fire box below the baking chamber for maintaining heat for possible longer work cycles.

Thermal tensions made cracks in the outer face of the oven, but the arch construction remained solid. The oven worked fine for baking, but the project didn't. The bread producers that we were to compete with had access to free "aid" flour so they were selling at a price which was equal to our material costs (flour, yeast, etc) (without even counting the value of firewood or investment in oven and transport costs). My contract was over and I never connected the oven to the stove duct so the other uses for the stove were never tried.

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