|Boiling Point No. 37 - June 1996 : Household Energy in Emergency Situations (Intermediate Technology Development Group , 1996)|
|Non - Theme Articles|
by Er L C Verma
Improved cookstove (chulha) technology in India has undergone swift changes during the last decade. The improved chulha program, a 'minimum needs programme' of the Government of India, is basically for rural women, and aims to improve their living conditions and create awareness about the saving of big-fuel. It builds on the development work which has taken place in many parts of India over a much larger period.
The program, started in December 1983, is supported by 17 Technical Backup Units (TBU), established in various states of the country, for development of appropriate models of improved chulhas, based primarily on studies of actual cooking needs of the region or groups of users. The programme now has more than 40 different types of improved stoves. At the time of launching most of the models used for field propagation had dampers and baffles.
The originally recommended,
improved, two-pot, mud stove had a chimney and was fitted with dampers and baffles. Dampers are thin metallic plates, and in a twopot mud stove with chimney the front damper is put across the mouth of the fire-box. It regulates the intake of primary air for the combustion process and controls its cooling effect on the stove. The 'chimney damper' is put across the flue passage between the second pot hole and chimney pipe. It controls the flow of hot gases out through the chimney. The dampers thus play an important role in achieving higher thermal efficiency. As a result of experience, improved chulhas with dampers were rejected by the users for the following reasons:
• The dampers quickly become hot and the users fear burns if touched during cooking.
• In the case of the vertically sliding type of damper, the grooves in the mud-body of the stove become worn and do not provide smooth sliding and fixing at desired levels.
• In the case of the horizontally operating type, the metal slides frequently get separated from the body and are not refixed, so the damper can not be used.
• Damper plates are easily removable from chulha body and lost or used by children as toys.
Failure to use the dampers or incorrect use results in uncontrolled and excessive intake of air and outlet of hot gases due to the 'chimney draught' and so in turn leads to loss of useful heat, and waste of fuel.
Baffles are the mounds of mud of various cross-sections put in the path of hot gases flowing through secondary pot holes. They retard the flow of hot gases under the pot and so increase the transfer of heat to the pot. This improves the thermal efficiency the chulha. Such multi-pot stoves should have improved thermal efficiency if the baffles are properly placed, used and maintained.
However, baffles are easily damaged during the course of repairs by the users. Surveys over the years have revealed that the majority of users are unable to maintain the designed form of the baffle, which becomes ineffective or detrimental to stove performance.
The foregoing description of 'improved' chulhas with dampers and baffles indicates how technologies from the laboratory may not work under field conditions. The two components which could enhance thermal efficiency were not accepted by the users for practical reasons. If such improved chulhas were to survive, development of 'darnperless' models became a necessity.
The problem was solved by the Technical Backup Units by making major changes in the cross-sections and placement of the flue passages. Air intake was controlled by reducing the size and angle of the flue passages such that secondary pots got more heat. The optimum distance between the bottom of the secondary pot and the floor of the pot seat was determined and fixed.
The improved chulha became not only simpler to make but also cheaper. Since 1989, the Government of India, under its National Programme on Improved Chulhas, has approved only damperless models for field propagation.
Improved chulhas with ceramic linings
The important parameters which determine the thermal efficiency of a chimneyless improved chulha are: design dimensions of the firebox, secondary-pot holes and flue passages. Most village stove makers and users are unable to maintain the designed size and shape of these components to the close tolerances needed.
The solution to this problem was the use of pottery (ceramic) linings which can be pre-fabricated at the village level and built into the mud stove bodies when installed in the kitchen. These can be carefully designed and accurately made, and are more resistant to wear.
Professional potters are selected and given training in making linings for the popular models of improved chulhas or those to be used in their area, and so can earn additional income from the sale of linings. Improved chulhas fitted with pottery linings not only stay with designed parameters, but also reduce repair work on the part of the user. The increase in the cost of the chulha is justified by these benefits.
'Unified' models of improved chulhas
New and more appropriate models of improved chulhas were developed to meet regional cooking needs. By 1990, the Government of India had approved more than 40 models for field propagation. A close study of various models indicated that although there were differences in designs to suit local tastes, most of the models had similar working principles.
Despite the different cooking needs and habits of rural masses in India, the traditional cooking methods are basically the same. To simplify the choice of most appropriate stove for promotion by field workers and to help production, it was felt necessary to reduce the overall numbers of models. Thus. the concept of a 'Unified' model was created.
• The model is damperless with a pottery liner in the fire-box, and secondary pot-holes and a heat recovery system.
• The fire-box has an appropriate grate for improved combustion and reduced smoke emission.
• It must be able to burn different biomass fuels available in the region with approved levels of thermal efficiency.
• The design should cater to the habitual way of cooking of the users of different states.
• The power output should be sufficient for the cooking needs of an average family.
The chimney, is an important component of an improved chulha as it extracts the products of combustion from the kitchen space and also creates a natural drought in the fire-box to provide the air needed for combustion.
However, field experience shows that the chimney-pipe is a major factor for non-adoption of improved chulhas for the following reasons:
• Most field workers are not skilled enough to make or safely install and properly seal a chimney pipe through the roof.
• Users do not clean the chimney regularly, and if broken cannot repair or replace it.
• In most of the areas of India (except cold and hilly regions) the chimney pipe used is made of asbestos-cement mixture. In some states this is easily available, but in others it needs to be transported over long distances. If the chimney pipe itself needs to be transported it is easily broken. The chimney pipe often costs more than the stove itself.
A chimneyless stove needs a completely different design to give optimum combustion and heat transfer, particularly in the combustion chamber and by addition of a grate. Some models have provision for a second pot hole for recovery of exhaust heat. In practise in the field the heat utilization may equal that of a stove with a chimney and smoke emission levels may be within safe limits. Chimneyless stoves are simpler, cheaper and easier to make and use, and so are promoted by the government and the TBUs.
Reproduced from GLOW, Vol 18, Dec 1995, a publication of the Asia Regional Cookstove Program