Improved institutional stoves for Sudan schools

by Mohammed E. Abdelrazing: Sudan Ireland Development Co-operation Program, Rufaa, Sudan

Background

The total consumption of wood in the Sudan during 1994 was estimated as 16 million cubic metres, 90% of which was by the household sector (14 million cubic metres).

The efficiency of the traditional stove used in Sudan was estimated to be 12% so more efficient stoves could save thousands of feddans (acres) of forests.

The El Nabti Quranic School has an improved stove which cooks for 1000 students. The main dish for the three daily meals is asida, a thick porridge made from sorghum. This dish is similar to the Kenyan I Ugali and Zimbabwean Sasda. The food is cooked in large pans with curved bottoms. This causes high heat losses when a three stone fire is used. The new stove saves two cubic metres of wood per day.

In Rufaa secondary school in North Sudan (300 students), three charcoal stoves were built to replace the traditional stoves which consumed three bags per day (40kg bag) of charcoal. Using the new stoves, the consumption dropped to half a bag daily. The new stoves saved charcoal, reduced overall cooking times by five hours and retained enough stored heat to keep the food warm till supper (see Figure 1).

The stoves can be used inside the kitchen or out of doors, but the chimney should always be outside the kitchen. The site of the stove should be in a safe, clean and convenient place where disturbance by the wind is slight.

There are two main types of cooking pots which are usually used in Sudan; cylindrical pots with capacity from forty to eighty litters which are made of aluminum and imported: and pots with curved bases with capacities from 80 to 250 litres. These pots are manufactured in Sudan by blacksmiths and they use heavy gauge 1.5-3mm sheet.

In prisons the stove is used for making a sort of pancake from sorghum on a flat rectangular heavy gauge plate. In oil factories, the boiler shape is square or rectangular.

Improved woodstove

General Construction

The firewood stove is constructed of bricks and mortar and it is not more than 0.8 metres high. It has a chimney to produce draught for combustion and to remove the smoke from the kitchen, assisted with a bottle for blocking flue gas. There are two inlets, the upper one through which the fuel is fed and the bottom one to draw the preheated air into the chamber. The fuel inlet has a door to control the entry of cold air.

The method for constructing the stoves is always the same. The shape of the pot holes in the top of the stove is dictated by the shape of the cooking pot, which is usually round, but in the case of the soap factory shown in Figure 2, rectangular containers are used for the oils used in soap manufacture.

Internal Construction:

The grate is made from steel reinforcing rods, 12 or 16mm in diameter with 12-14mm spacing The gaps allow sufficient air to get in and the ash to fall through. To ensure good combustion and heat transfer, the distance from the fuel bed to the pan bottom is critical.

The pot should be a good fit in the hole in the top of the combustion chamber in order to make a seal and ensure that the hot gases go through the passage to the chimney. The small gap between the vertical wall of the combustion chamber and the pot sides is designed to suit the pot to be used so as to give maximum heat transfer.

The passage to take the hot gases from the combustion chamber to the chimney must be the right size and shape, lead up to the chimney and must be kept clear of ash and debris.

The charcoal stove

The four main differences between the charcoal stove and the wood stove are:

· The absence of a chimney in the charcoal stove (see Figure 4);

· The grate is a punched metal sheet instead of the steel bars in the firewood stove;

· The channel between the stove walls and the pot sides must be wide enough to allow the hot gases to escape at the top whilst giving maximum heat transfer to the pot [normally 3-8mm, ed.]

· There are inlets for secondary air.

There have been no laboratory tests carried out to determine the efficiency of the stove, but according to users, the fuel saving is 60-80%.

The stoves have an expected life of more than two years, if they are well constructed and maintained regularly.

Figure 3. Woodfuel burning stove for institutions

Woodstove and char coal stove construction and maintenance

· The dimensions of the pot (diameter and depth) must be measured correctly.

· The construction should follow the dimensions in the design.

· To prevent flue gas leakage's, seal brick layers with mortar.

· Insulate the bottom of the stove with ash mixed with salt and clay.

· The optimum chimney height is about 2.5m but this may need changing when the stove is first tested.

· Pay great attention to the distance between the pot bottom and the grate; for the normal pot (60cm), this should be 18cm for a firewood stove and 13cm for a charcoal stove

· In the firewood stove, the area of the grate can usually he taken as a quarter or a third of the area of the pot bottom Occasionally it will he as little as one fifth.

· The gap between the grate bars should be kept between 12mm and 14mm The bars will need replacing when burnt through.

· Plaster the external stove body with cement after two or three days.

· Ensure smoke passages do not become blocked.