| Boiling Point No. 01 - January 1982 |
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.