|Boiling Point No. 01 - January 1982 (ITDG Boiling Point, 1982)|
IT Intermediate Technology
Over the last three years there has been a great deal of effort in the Third World, Europe and North America to design and introduce improved cookstoves.
At a meeting held in Reading, we discussed the issue of how better to communicate the accumulated knowledge of stove researchers, designers and programme managers. Ianto Evans presented a draft of a networking newsletter that was being produced by Aprovecho. I.T.D.G. also informed the meeting that it was preparing a technical newsletter along similar lines. In order to avoid duplication, it was decided that the I.T.D.G. newsletter should contain more in-depth technical articles covering areas pertaining to design, laboratory and field testing of stoves; design and testing of ceramics for use in stove fabrication; and methods of extension, including training and methodology. As well as these articles, we have agreed with our collaborators to include details of the work they are undertaking. It was also felt that we should publish interesting technical enquiries and our replies to these enquiries where appropriate.
We are keen to receive technical articles for inclusion in future issues. These should not be longer than 700 words and may cover areas related to theory as well as practice. We would appreciate any comments on the aims, layout and material in this first issue.
Produced by the I.T.D.G. Stoves Team
Intermediate Technology Development Group Applied Research Section, Shinfield Shinfield Road Reading, Berks. U.K.
The Tungku Lowon stove was designed by Desa Desa, an Indonesian AT organisation which is based in central Java. The stove was developed from the high mass Lorena stoves originally introduced in this area. These stoves were fairly rapidly found to be unsuitable for local cooking practices and the broken up stoves were remade several times, each time getting smaller. The present Tungku Lowon is similar in many ways to traditional stoves and is thus able to satisfy the same requirements of the villagers as the traditional stove. ITDG was requested by Dian Desa to evaluate the stove under laboratory conditions.
Testing the Tungku Lowon in Reading began in July 1981 appropriately with the new stove being given a new user. I was pleased to find that there were no problems with starting the stove, even from cold, using our standard wood, Jelutong. First in line was a series of tests to find out how repeatable the results were. This proved to be good, particularly when the tests involved boiling both pots rather than just one. Shorter tests are more prone to inaccuracy of measurements and starting a fire is the time when outside factors matter most.
unnumbered = standard
1. 3 sticks
2. Small diameter
5. Central flue blocked
6. Airholes blocked
8. 25% moisture
9. 48% moisture
10. Forced airflow
11. 63% moisture
Our calculations i of percentage heat utilised (PHU - defined in our Interim Report No.1, 1980) showed that in standard tests, 18 - 22% of the energy available was used in heating or evaporating water. This was an encouraging start as it agreed with the Indonesian test results. Time to boil 2 kg of water- in the first pot varied between ,12 and 19 minutes. Time to boil 2 kg of water in each of two pots varied between 21 and 25 minutes. Much of the variation in time to boil was due to the heat output from the stove which varied between 17 and 29 g wood burnt per minute (approximately 5 - 8 kW). These times are comparable with other relatively efficient low- powered stoves. The weight of wood used during the tests varied considerably - between 290 and 390 g to boil the first pot; 400 - 460 g to boil both pots.
Having completed the standard tests, we carried out a series of tests to find the effects of using wet wood and of partly blocking up some of the air and flue gas passages. On a good stove, minor variations from standard operating conditions shouldn't make much difference to the performance, so we reduced the central flue size by one third and blocked the airholes completely. PHU was reduced from 22% to 16% for blocked flues and from 22% to 19% for blocked air holes. One of the big problems with these changes in the configuration of the stove was that it produced lots of smoke and made the boiling times very long - up to 41 minutes for pot 1 with wood with a 63% moisture content.
Because of the slit at the front of this stove, smoke tended to escape through the front. We combatted this by slightly raising the back by 3 - 4 cm which reduced smoke without changing the PHU.
The overall conclusion was that we were testing a basically good stove. Modifications might be made to improve "user friendliness" but that it would be difficult to improve the efficiency without creating a new and different stove. Also I liked it!
This is the first of a series of articles introducing the subject of constructing stoves out of fired clay. We will look at where clay comes from and the two main types of clay most commonly found. Each article will have a small glossary of words commonly used in clay technology.
Clay is an abundant and cheap material which does not need major processing to make it useful. There are few places in the world where it cannot be found and it has been used as a building material for centuries. The fund of knowledge built up by local peoples about its properties and uses, plus its cheapness and ready availability make it a most suitable material for the construction of stoves.
The chief constituents of all clays are alumina and silica, the latter always being in excess of the former. All clays may be regarded as consisting of a mixture of one or more hydrous alumino-silicates with free silica and non-plastic minerals or rock granules. The chemical properties of the clay are dependent on the nature and proportion of these accessory ingredients.
Clay falls into two general categories:
1. Primary Clays: those which have been formed on the site of their parent rock by the weathering of feldspathic or volcanic rock. The rocks are broken down by water seeping through the strata, leaving the clay in irregular pockets. The water action is fairly gentle and so particle size remains large and varied. Primary clays are usually non-plastic. They contain little or no contaminating materials and are recognisable by their purity and whiteness. Kaolin is an example of a primary clay.
2. Secondary Clays: These have been transported by water, glaciers or occasionally wind, to a site far from the parent rock. They are usually composed of a complex mix of different clays and minerals picked up on the way. The action of the water breaks down the particle size and in the process of sedimentation the coarse and fine particles are separated. This means that secondary clays are plastic and hardly ever pure. The colour in - the raw clay indicates the presence of either iron oxides, or carbonaceous matter. Iron which is present as haematite will produce the red, limonite gives yellow; ferrous iron gives gray/green and black. Iron in the clay lowers the vitrification point in firing, so all red clays are low firing or earthenware clays.
Clay used for the construction of cooking pots and earthenware vessels in the Third World is almost always the red secondary clay. It moulds easily into large thin shapes and will harden into biscuit ware at low temperatures (600-800 degrees C) in primitive bonfire firings.
Sometimes the clay is not as plastic as is required and potters have various ways of dealing with this. Some dig the clay, let it dry, crush it, remove stones and unwanted debris, then leave the clay to soak again in pits. Others soak the clay for many months, then dry it, crush it and reconstitute it again for use. This soaking or "ageing" of the clay improves its plasticity because bacteria grow in the clay and produce acid residues. These aid the formation of gels which break down the particles and improve plasticity. Potters of China used to use clay prepared for them by their grandparents. They in their turn prepared clay for their grandchildren to use.
Some Voltaic potters add millet husks, charcoal, grog and donkey dung to their clay. - Why? Find out next issue....
Grog Clay which has been fired and then ground into granules.
Plasticity: The property of a material enabling it to be shaped and to hold its shape. Plasticity in clay depends on particle size. Clay particles are plate-shaped and slide against each other on a film of water. The smaller the clay particle size, the more plastic the clay will be.
Vitrify: to fire to the point of glassification.
Much of the literature that has been published on stoves has focussed on improving their performance. Until recently very few stove researchers and field workers have acknowledged that a stove is part of the cooking system. The performance of this system depends upon the type of pots used and how they are positioned on or in the stove, the method of cooking and the method of operation.
As early as 1963, Singer reported that by lowering pots below the upper surface of the hole, a dramatic increase took place in the heat transferred to the pots.
Dr. G. de Lepeleire, (1981) carried out some simple heat transfer calculations to show that a round based pot received less radiant energy from the fire than a flat based pot (given that the bases of both pots are the same distance from the fire). The theoretical work of Keith Bennett (1980) also indicated that exposing more of the pots' surface area to the fire and to the hot flue gases was one of the few practical ways of increasing the percentage of heat utilized in the cooking process. Tests carried out at Shinfield and at the Open University also indicate that clay pots use more wood than aluminium pots to heat and evaporate the same quantity of water. The amount of wood used depends on the porosity and thickness of the clay pots. It has also been found that, for aluminium pots, heat transfer is greatest when only a very thin layer of soot covers the bottom of the pot. (Eindhoven University, 1980). However, designing stoves which allow more of the pots' surface to be exposed to the hot gases is not an easy task. Waclaw Micuta of Bellerive Foundation is, at present, the only person who has designed, constructed and introduced stoves whose designs incorporate pots.
He maintains that a cast iron heating plate and cast iron pots that fit exactly into the holes on the heating plate must be used if large amounts of wood are to be saved. This heating plate can then be incorporated into a mud brick or cement mud stove. The author and Tim Wood (VITA) have carried out tests on these stoves and have found that they are far superior to all other existing designs in Upper Volta. The only drawback is the cost of this heating plate (greater than $20). In Upper Volta, to cut costs and use locally available materials, the heating plate and pots were made from cast aluminium at a cost of approximately $30. However, the author noted that with these improved stoves the rate of corrosion of aluminium pots seemed to be greater than open fires, possibly due to the higher flame temperatures. It would appear that these aluminium pots will need replacing once a year! In many countries people will not be able to afford the initial capital cost of the metal plates nor the replacement costs. Thus some type of subsidy may be necessary. Attempts have been made by Gern et al (1980) and the author, to make cheap single pot stoves from mud and pottery where the pots sit inside the stove.
The results of the tests on these stoves have not shown the type of fuelwood savings and decrease in cooking time that is found with the Micuta stove. A small metal stove with a grate and primary and secondary airholes has been built and tested at Eindhoven. Prasad et al (1981) concluded that "this simple design provides the highest efficiency (over 50%) among nearly 400 experiments done by-the group over different types of stoves.') The one drawback of this design is that wood must be cut into small pieces.
An alternative to sitting the pot into the stove is to produce a cheap pressure cooker. Bhatt and Reddy have developed and tested such a pot. They give the following description and method of using the pot.
"Any aluminium vessel can be converted into a low cost pressure cooker by fitting a small spout to it. In the case of mud vessels, it has to be made anew with a small modification. The spout consists of an aluminium tube 12 x 2 x 50 mm. It is threaded to a 30 mm length on the outside. The other accessories are two collars 16 x 5 x 5mm, threaded on the inside and two washers, 16 x 5 x 2 mm. The aluminium vessel is first drilled and the spout is fitted as in Fig 5. Another big vessel is placed over this vessel. Both these vessels together constitute the pressure cooker.
Operation: -to start with, the food to be cooked, say dal, is put into the smaller vessel (vessel 1)- and placed inside the larger vessel (vessel 2). About half a litre of water is added to vessel 2 for steam generation. Vessel 3 is then placed on the top of vessel 2 and about two litres of water is put into it. The arrangement is now placed over the stove. After sufficient quantities of steam come out of the vessel, the spout is closed by a rubber cork. The chamber inside vessel 2 now experiences a pressure cooker action. The water and steam particles hit the bottom of vessel 3 and forced convection heat transfer takes place to that vessel."
Cookirg tests were carried out at I.I.S., Bangalore, on three indigenous brick and mud stoves and one charcoal stove. A meal of dal, vegetables and rice was cooked on the four stoves, both with the pressure cooker and aluminium pans. The weight of wood used, water evaporated and the time taken was-measured. The energy input was calculated using measured calorific values of the fuel and the energy output was calculated based on the energy required to bring water to the boil, evaporate water and cook food. Using this pot, it was found that the average overall efficiency increased from 11% to 19%.
Extended field trials of the stoves designed by Gern and Micuta are now in progress. We will monitor the progress of these trials and write a follow-up next year. Careful tests will be necessay to determine the lifetime and safety of any low-cost pressure cookers such as described above.
Bennet, 1980. Woodfuel Combustion and Heat Transfer in Cookstoves. I.C.London.
Bhatt and Reddy, 1981. Low Cost Pressure Cookers. Indian Institute of Science.
Gern et al, 1980. Improved Cookstoves in Upper Volta.
Prasad et al, 1980. Some performance tests on open fires and the family cooker. Technische Hogeschool, Eindhoven.
Prasad et al, 1981. A Woodstove Compendium. Technische Hogeschool Eindhoven.
Micuta, 1981. Modern Stoves for All. Bellerive Foundation.
Singer, 1961. Improvement of Fuelwood Cooking Stoves and Economy in Fuelwood Consumption. FAO.
SAWDUST RICE HUSK COOKER
J.A. Hazbun, WHO Solomon Is.
In the Solomon Islands, both rice husks and sawdust from the mills are burned as waste. This is not only wasteful, but causes a nuisance to neighbouring communities. The WHO Environmental Health Workshop has tried to develop a stove to make use of these fuels.
The stove is based on an oil drum with the top removed and a hole 5cm diameter cut in the bottom. A length of pipe is inserted into the hole horizontally, and a grill is placed on the top to support the cooking pots. A second pipe is inserted during filling of the stove with either rice husks or sawdust.
Initial trials showed that two litres of water could be boiled in 15 minutes. A cooker loaded with 20 litres of fuel will burn for 5 hours with virtually no smoke. This type of cooker has not been extensively tested for efficiency. It is intended to use these waste fuels instead of kerosene and thus save foreign exchange for the Solomon Islanders.
West Nepal Seminar
Ralph Bland ? a volunteer working in Pokhara, West Nepal attended a seminar for workers at village technician level on stoves. Several stoves were built and tested. Points raised by the seminar are as follows:
The stoves used less fuel than open fires and removed smoke from the kitchen. Less heat was lost into the room.
In the mountains, the reduction in space heating of the stoves was a disadvantage. Open fires would better suit cold climates and high altitudes. The smoke from the stove was not available for drying clothes and smoking meat and fish. The stoves required metal grates and dampers which were hard to obtain. The stoves were more complex to use than an open fire.
Most Appropriate Situation for Stoves
In view of the advantages and disadvantages, it was felt that stoves would prove most useful in hotels and restaurants where cooking and not space heating was required. The stoves were also better suited to warmer climates for the same reason.
Christopher O'Brien writes of his work in the Gambia:
"I've been introducing principally two types of stoves, both made from a "Lorena" type mix of clay and sand (usually a 1:4 ratio). The first type is for two pots since most Gambian meals are cooked with two pots; one for the rice or millet, and one for the sauce. The stove has a chimney made from a sheet of corrugated steel roofing. A baffle is built up across the flue under the second pot. I have built the stoves without dampers for the sake of simplicity, but the wood savings remain impressive. Women report saving 1/3 to 1/2 of the wood normally used with an open fire. The second type of stove is a simple one-pot chimneyless design developed recently in Senegal. The stove vaguely resembles a volcano, with the pot sitting in the "crater", and smoke escaping around the side of the pot. Women also report significant wood savings with these stoves and seem particularly impressed with the improved cooking speed.
I have tried introducing the stoves in a variety of situations. Most have been built in private kitchens, others in more public and visible locations -such as a training centre and a restaurant. Although most of the stoves built are in regular use, I've bee disappointed that they have not proliferated more dramatically; only a handful of stoves have been built independently. I'm hoping to improve the durability of the stoves by imbedding short lengths of mild steel rod across the cooking holes as supports for the pots, thus reducing the pressure and wear on the sides of the holes."
One Hole Louga Stove
Hanno Pilartz writes of his work with the Centre de Reintegration des Handicapes in Rwanda. The one-hole louga mud stove seems to fit their needs best. River clay and sand are readily available and the thick mud walls and massive bridge over the firebox are easy to construct. It is common to cook meals in a single pot beans are the chief food, requiring over two hours to cook.
He is working with three potters and two masons who generally try to build stoves they have designed themselves. These people have other jobs and are building stoves part-time in their own neighbourhoods. This encourages local interest in new stoves.
In the canteen at the centre the cooks wish to use waste burning stoves in place of butagas. They have a ready supply of free sawdust and coffee hurts which they can utilize.
Tim Wood, VITA Representative in Upper Volta writes of his - problems when employing local potters.
"I had a potter make a dozen sets of inserts and they were beautifully done, but lack of proper fuel for his kiln forced him to burn them in an open pit, and in the process every one was destroyed.
The same potter spent considerable time in late spring adapting the double-skinned stove to local cooking needs, and came up with some very good-looking models. The modifications were mostly around the rim, flaring it out more and providing three evenly spaced pot supports around the inside. The dimensions were such that the stove would accept almost any size of round-bottomed aluminium pot. Malheureusement, most of these stoves were also destroyed upon firing, although the earlier prototypes have survived. The potter is a skilled craftsman who has spent most of the past two years making a clever chicken waterer that is sold throughout West Africa. He plans to abandon that completely after the harvest this year and concentrate on stoves, where he sees a lucrative market. His firing technique is fairly advanced over the traditional method as long as he has the proper fuel (millet stalks ) .
WOOD CONSERVING COOKSTOVES - A Design Guide.
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