| Boiling Point No. 02 - Special Edition April 1991 |
by W Micuta & E Haas of REDI (Renewable Energy Development Institute), Geneva 1987
Abridged by Editor
Stoves cannot function properly without good chimneys, which are an inherent part of the combustion and cooking systems. Yet, it seems to the authors that practical knowledge and experience of the building, functioning and maintenance of efficient chimneys is far from satisfactory in developing countries. This has become a serious hindrance to the introduction of efficient and economical stoves, and thus to economy of fuel.
Apart from allowing for the evacuation of gases into the atmosphere, the main function of the chimney is to promote an adequate and steady draught, which may be thought of as a 'motor' of combustion. In technical language, a draught may be defined as the differential between the density of hot gases in the combustion chamber and the ambient air above the chimney. The chimney draught during the combustion process will in general be determined by the height of the chimney and the temperature of the flue gases.
If the differential is too small, which means that the temperature of the gases in the chimney is too low, the stove will cease to function correctly. In this respect stove designers are faced with a frustrating dilemma. On the one hand, it is important to retain the maximum heat in the stove, around the cooking vessels, and any heat which escapes to the chimney is lost for this purpose. On the other hand, it is necessary to sacrifice a certain amount of hot gases in order to maintain adequate draught and to avoid vapour condensation in the gases, and the accumulation of soot and tar. In household stove chimneys the temperature of the gases at high power rates should not rise much above 150°C while at normal rates of combustion the temperature should not fall below 80° C. This statement is not valid for industrial or high, apartment house chimneys. In countries or regions with particularly hot climates, it is necessary to maintain a somewhat higher temperature in order to ensure optimal draught.
If the differential is too large it causes wastage of heat, and thus of fuel. If combustion becomes too intense and raises the temperature of gases leaving the stove to an excessive level it is necessary either:
• to diminish the volume of fuel
• to diminish inflow of air into the stove,
• to diminish the flow of gases in the chimney ea. by means of a damper, or
• to reduce the height of the chimney
In household stoves, the best regulation of the draught seems to be by volume of fuel and by inflow of air into stoves. This underlines the importance of well-designed stove doors and the ability of the operators to use them correctly. If a chimney damper is used, it must be designed never to close the chimney section completely but must always leave space for evacuation of gases, which may include highly toxic carbon monoxide (CO).
The chimney should be provided with an arrangement enabling the removal of soot during regular periodical sweeping. It is usually in the form of an opening, at the base of the chimney, through which soot can be removed. It is important that, after cleaning, the opening is well closed. If this is not the case, there will be no draught in the stove.
The inside surface of a chimney should be heat-resistant and as smooth as possible to diminish gas flow friction and accumulation of soot. The chimney channel should be straight and entry of gases from the stove into the chimney streamlined. If bends are unavoidable, particularly in metal pipe chimneys, the elbows should also ensure an easy flow of gases.
Stove chimneys are usually made of mud, ceramics, cement or sheet steel.
To avoid cooling of gases, particularly in regions with occasional cold weather, it is advisable to build chimneys inside houses. An advantage of such chimneys is heat storage in the chimney's walls, which warm the house even when the fire has subsided.
The chimney is an inherent part of an internal stove design, closely related to other parts such as the door, combustion chamber, inside flow of gases, and the size of the opening from the stove into the chimney. It is therefore not possible to give chimney dimensions that could be applied to different stoves. One has to consider also the fuel used for combustion, altitude, prevailing winds and climatic conditions. When all those factors are duly taken into account the chimney may Still not work correctly because of local, and unfortunately frequent, air turbulence due, among other factors, to the surrounding buildings or trees.
It is, of course, possible to calculate the technical characteristics of a given chimney such as volume flow of gases, velocity, friction, coefficients, pressure losses and others as is done for industrial chimneys. Unfortunately' such calculations are not always of great help in the building of simple household chimneys in developing countries and may even be misleading, as they give a performance standard which is far too high. In this respect, the best method is still trial and error, applied to each region and sometimes even to each locality.
Dimensions of household chimneys given below are based on the experience gained in household stove building in many countries. They should be treated as general indications and corrected in view of the prevailing conditions.
Metal sheet pipes used for rural household chimneys should, in general, not exceed a diameter of 10 cm which gives a section of about 80 cm2. The inner section of square brick chimneys of 10 x 10 cm is quite adequate. The authors consider that the dimensions should not exceed 15 x 15 cm. The velocity of gases flowing in the chimneys of larger sections is lower. They diminish draught, cool gases, create more vapour condensation, waste heat.
In a simple household chimney, the gas velocity should be about 0.5 to 1.0 m/sec. In cold conditions a large column of cold air may clog the chimney and make it difficult to start a fire. Cold air may also descend into the premises and cool the ambient temperature. The flow of gases depends also on the total system's flow resistance coefficient. It is therefore important to keep inner walls of the chimney smooth and, in the case of metal chimneys, minimise the number of elbows.
The chimney should extend about half a metre above the roof ridge. If the chimney comes out through the slope of the roof it should also be about half a metre above the highest point of the roof and, to avoid the wind turbulence which is always present close to the edge, about two or three metres from the ridge. In the case of small portable metal stoves, half a metre is usually sufficient.
Experienced stove makers test the chimney's draught when the stove is cold. They light a piece of newspaper and keep it close to the opening of the stove into the chimney. If the burning paper and ashes fly up the chimney, the draught is satisfactory. For domestic stoves, this test indicates that proper draught does not depend only on the temperature of the passing gases but first of all on a properly-designed and well-built chimney.
A chimney fire is a serious danger. During the period of combustion, some unburned particles settle on the inner surface of the chimney. They are highly inflammable and, if not removed in time, may burst into strong flames and destroy the house. Therefore chimneys must be periodically brushed or swept. This can be done as shown in Figure 1.
Another danger is blockage of a chimney, preventing easy evacuation of gases. In such a case noxious gases such as carbon monoxide may enter the house and poison the inhabitants. Quantity production of steel chimney pipes with bends and dampers and cowls etc made from recycled sheet will facilitate the use of chimney stoves and can be a good business for a sheet metal workshop with some simple equipment. Where it is necessary to put the pipe through the roof, additional accessories such as storm collars and roof flashings are needed (see Fig 4).
Suggestions for efficient, yet simple chimney cowls which could be applied in developing countries:
Decisions on which kind of cowl should be chosen for a given province or region should be preceded by appropriate investigations. The authors suggest two models of cowl, both of the same conception, but one made of metal sheets and the other of ceramic.
The metal sheet model consists of two or three metal sheet cones of appropriate diameter. The cones and the covers are held together by means of three pieces of flat iron fashioned in such a way that all pans can be easily riveted to them. The pieces of flat iron descend below the lowest cone by 20 to 30 cm and create 'legs' for the cowl. The 'legs' are curved and thus are 'springy'. When pressed together by hand and put into a chimney they press against the internal walls of the chimney and keep the cowl firmly in the right position. If the cowl is to function correctly, the bottom of the lowest cone should be about I cm below the top of the chimney. This position prevents the lateral wind entering below the cone and cutting the draught. When the cowl is installed, the wind or local air turbulence, from any direction, cannot disturb the draught but, to the contrary, keeps it strong and steady.
The authors suggest that a simple cover be riveted to the top of the three brackets. The cover should be placed about 20 cm above the top of the highest cone to leave enough space for gases to leave the cowl and thus to avoid condensation of vapour on the cover's inside surface. If two cones are not found to be effective, a third may be added.