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edit this page A Breath of Fresh Air for Smoky Houses by Peter Young
[top] [end]Principles of hood design There is only one process by which a hood or chimney can discharge smoke outside the house effectively without the use of a fan: by employing the natural buoyancy or the hot gases produced during combustion. In the case of a fire' these hot gases also include the smoke. If a collection hood and chimney of sufficient size are mounted over the fire, the products of combustion will be removed. The driving force behind this principle is that the lower density of these hot gases produces a pressure drop up the chimney. For hoods to work effectively, there must be a certain minimum velocity of air across the face of the hood to prevent smoke spilling into the room. A mathematical model has been established to provide help mine hood-face areas for various sizes of chimneys and amounts of heat released (for more details, see Boiling Point Number 28, August 92). This model has been used in the design of a simple hood for use in Maasai housing in Kenya and its effectiveness has been compared to other methods of increasing room ventilation. Like all Maasai homes the walls and roof are made from wattle and daub. The interior is open plan and gives a sense of spaciousnesss, although the rooms are very small. Very small windows in each room let in a meagre amount of light, and an open tin can placed in the roof above the central fireplace provides a small amount of ventilation (see figure 1).  Figure 1: Plan of the traditional maasai house of Mama Keroka |
[top] [end]Indoor pollution survey Five Maasai houses with different smoke removal systems were selected for survey. Background carbon-monoxide (CO) in the cook's vicinity and personal exposure to Respirable Suspended Particulates (RSP) were monitored during the cooking of a traditional meal. By the end of each test, the amount of CO in the room atmosphere had increased greatly and was allowed to decay naturally The rate of decay was then used to calculate the air-exchange rates. Each test was repeated twice. [top] [end]Evaluating the results The smoke-extraction system for an open fire (see figure 2), as demonstrated at Kisembe Polong's house reduced RSP exposure to levels that could not be detected; and the average CO levels were reduced to negligible amounts of between 10 and 4 ppm. Some smoke emission was just noticeable to the naked eye, and was caused by air movements around the fire as people walked past. This suggests that the system is working at the optimum level of effectiveness. Any changes to lower the chimney height, decrease its diameter and or increase its face area will increase the rate of spillage. Other forms of intervention to increase the ventilation - such as raising the ceiling height, making windows and doors larger and constructing vents in the roof have helped to reduced CO levels by as much as a factor of two when compared to traditional housing. CO levels are still unacceptably high, however, and these approaches do not offer any obvious solution.
CO and RSP levels are unlikely to be diluted sufficiently through natural room ventilation because: - Large windows and doors would need to be fitted which would be incompatible with the Maasai's
- For such a system to work consistent climatic conditions and light prevailing winds are necessary.
Cooking in a small separate kitchen, such as Mbareya's, is probably worse than cooking in open-plan living quarters. If Mbareya's kitchen had not been fitted with a roof vent and a large window, the CO levels could have equalled those in a traditional house. There may be some small benefits, however, for household members sitting in other rooms!  Figure 2: Type of smoke removal hood used in Mrs Kisembe's house with an open fire |
From the user's point of view the positioning of a smoke-extraction hood system is critical. In Maasai housing, cooking is normally done to the left of the fireplace, which must be unrestricted. The front and the right side of the fire must also be unrestricted so that other household members can benefit from the radiated warmth. Kisembe found the hood position comfortable when cooking with small pots, but, when larger pots were being used, it was a bit cramped. The view of the fire - hence the heat radiated from the right - was a little restricted, but seemed acceptable. Access to the fire on both sides could be improved by moving the fireplace away from the centre of the room towards the rear wall. Even with the limitations of such a simple experiment, there is conclusive evidence that a hood with a chimney mounted above the fireplace can be a very effective method of removing smoke. Also, it appears that hoods are more reliable than chimneys; offer greater choice of cooking appliances; and are unlikely to suffer the same degree of user rejection as has plagued most low-cost chimney stoves. They are perhaps best introduced as part of a kitchen redesign.
Table 1: Comparing the CO levels, air exchange rates and particulate concentrations in five Massai houses. | House owner | Roofing material | Windows (four to six) per house | Ceiling height in cm | Ventilation | Housing design | Average CO level (in ppm) | Peak/dip CO level | Air exchange rates(room changes per hour) | Average RSP µg/m3 | | Keroka | Traditional (mud/ash) | 6cm in diameter | <150 | Tin can. roof vent | Traditional | 172 | 302/94 (windy) | 0.87-1.4 | 10926 | | Lekuta | Cement | 15 - 22cm | 70 | Small roof vent | Traditional (layout feels spacious) | 89 | 168/46 | 0.78 | 2181 | | Kisembe Polong | Cement | 6 - 10cm | >190 | Hood | Modern with guest rooms (spacious) | 7 | 20/2 | N/A | 0 | | Mama Saitoti | Cement | 6 - 10cm | >190 | Large roof vent | Modem with guest rooms | 88 | 144/54 | 1.3 | 4135 | | Mbareya | Cement | 6 - 10cm | >190 | Small roof vent | Cramped kitchen | 127 | 184/50 | 0.9 | 4953 |
[top] [end]Contents: Boiling Point 34: Smoke Removal  . |
Smoke in the Kitchen - Any Stove Will Smoke if You Dont Use it Correctly - Acute Respiratory Infection, Conjunctivitis and Accidental Burns - Exposure to Air Pollution From Transitional Household Fuels In A South African Population - Smoke Removal in Kenya - Chinese Chimneys - Indoor Air Pollution in Rural Tigray - Removing Smoke from Nepali Kitchens - A Breath of Fresh Air for Smoky Houses - Vietnames Kitchens - Reducing Greenhouse Gas Emissions and Improving Environmental Degradation - Indian Governments Stove Programme in Question - Cooking energy Efficiency in Indonesia - Phillipines Ricehull Stove - Stoves for Cafes and Food Stalls - Fuel from Three Stages of Pyrolysis - An Electric Metad - Crack Reduction in Clay Stoves
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