| Boiling Point No. 17 - December 1988 |
Future editions of Boiling Point will carry detailed profiles of improved stoves now being promoted. Some are reproduced from ITDG's "Practitioner's Manual" If any readers have improved the design or construction of any of these stoves we should like to hear from them and publish details.
Extract from "Improved Wood, Waste and Charcoal Burning Stoves" IT Pubs Ltd
The stove is made from pottery and is surrounded with mud when installed, the finished stove being approximately 80cm x 45cm x 20cm. The pottery sections can be transported in a 60cm x 35cm x 20cm package. An average-sized stove weighs 8kg and costs US$ 2.45. The mud surround may weigh up to 60kg. There are two pot-holes on household models. The chimneys are either made of 30 cm long, 10 cm diameter pottery sections (US$ 0.80/m) or asbestos cement pipe with cap (US$ 6.5() for 3m chimney with cap). There is a sheet-metal grate, no door or dampers and a baffle beneath the second pot-hole.
History and field experience This stove was developed in the early 1950s at the Gandhiniketan Ashram in T Kallupatti (Madurai, Tamil Nadu, India 626702) as an improvement on the original mud-mix Magan Chula developed at Maganwadi in north India. The original design had three pot-holes but by the 1970s all the stoves had only two. The stoves produced at the Ashram are sold through their co-operative outlets, which are all within 60km. By 1987, 17,000 stoves had been produced and sold by the Ashram as a profit-making activity; of these 5,000 had been sold to the Rural Development Office as part of the National Programme. Other potters in the region have copied the design and produce additional stoves. The stoves made at the Ashram can last from four to ten years if treated with reasonable care, but lower quality stoves from other producers often last less than three years.
Limited research carried out in 1983 by ITDG showed that the stove was purchased and accepted where smoke removal and cooking convenience were the main priorities. Where smoke removal was not important, or where there was fear of fire from the chimney, people did not buy the stoves. Broken baffles were often found in installed stoves, and where these stoves were still in use, fuel consumption and cooking time increased to equal or exceed that of traditional stoves. Repair of the baffle improved performance considerably. It is reported that stoves with a properly built baffle account for fuel savings that are more than a third of that used by traditional stoves.
The stove is now disseminated through the National Project on Improved Chulas and the Department of Non Conventional Energies provides a subsidy of US$ 3.44 to institutions that install it. A few chulas are available in local shops at US$ 4.91..
Construction, installation and maintenance
The stove is complex to construct and requires considerable pottery skills; only a few south Indian potters have been successful in making it. The clay mixing and processing has to be carried out meticulously. The raw clay is dug from near-by tanks and allowed to dry in the sun. This is later crushed and mixed with water in purpose-built tanks to form a slurry. The slurry is allowed to settle so that the sand and stones may separate from the clay. The clay is then scooped out of the tank and mixed with a more uniform and finer sand in an adjoining tank. This slurry may the be removed and placed on to hessian cloth to dry out to a plastic-type consistency. Before forming, the clay is extensively wedged, using a traditional technique with the feet.
All the stove components are thrown on potters' wheels with the help of measuring sticks to check the diameters and heights of cylinders. As many as 10 to 20 sets of components may be thrown in one session at the wheel. (Each set consists of 5 cylinders). It is usually the following day that the component cylinders are joined together using a slip. Where components are prone to slumping they are supported underneath with bricks. The drying process take about one week during which time shrinking causes many cracks. These are filled with a special paste made up of slip and paper, to form a type of papier mache with clay. After further drying the stoves are fired in an open bonfire using acacia shrub and rice straw as fuel.
The stoves are installed very quickly, and providing the materials are at hand, this may only take an hour. The stove is placed in the required position which may either be at ground level or on a purpose-built platform. Fired bricks are then built up in a rectangle around the stove, using mud and sand as mortar. The gaps around the stove are then filled in with sand. The mud and sand mixture is then used to render over the bricks and across the top of the stove, ensuring the primary air and fuel entrances do not become blocked. The same mixture is also used to build up the baffle inside the second pot position.
The chimney consists of pottery tubes with a cap at one end so that it makes fitting easier. They are joined together and to the stove with a sand and cement mixture. The mud and sand mixture is usually not strong enough for this application. The number of chimney sections required often depends on the position of the roof but it is recommended that a minimum of eight be used, otherwise there may be insufficient draft. The chimney should be taken through the roof of the house and often requires a tile to be removed. The gap between the roof and chimney then needs to be filled properly with sand and cement, otherwise it often leaks water. Finally a cap is fitted to the chimney to reduce the effect from down drafts caused by wind turbulence.
Use of the Stove
The stove must always be used with two cooking pots that provide adequate sealing to prevent the escape of smoke. The Magan Chula burns a large range of fuels because of its grate, but when using residues such as acacia twigs, rice straw or coconut husks the grate must be checked regularly to prevent clogging. It is best not to overload the stove with fuel as this tends to restrict the air. As with all chimney stoves the chimney must be checked regularly, and in South India it has been found necessary to clean it every six to twelve months. This is done by putting a stick down from the top after removing the chimney cap. The soot can be scraped away from the base of the chimney through its entrance to the second pot. Users have not found it any more convenient to have a special entrance at the base and this was also found inconvenient to construct.