| Boiling Point No. 05 - September 1983 |
Siddhartha Bhatt Central Power Research Institute, India and S Narvekar, R Kulkarni, & R Sunderesan Karnataka Regional Engineering College, India
It was decided to undertake cookstove studies in hotels because it is easier to bring in new stove technology in the commercial enterprise sector, rather than the domestic cooking sector. A survey was conducted in the Mangalore area. About 45 hotels were visited, and 130-140 stoves were studied in detail. These details, covering stove materials, construction, lifetime, operation, costs and fuel used, are available on request from ITDG or the author. The main findings, problem areas and recommendations for improvement are summarised.
45% of the hotels surveyed used stoves that were both inefficient and inconvenient to operate. m e conditions of the cooks operating these stoves were harrowing.
35% of the hotels had stoves constructed either by the cooks or by masons. They had in some cases tried to imitate improved designs and in other cases used their own ideas and experience. These stoves did not do much to improve efficiency or overcome pollution problems.
20% of the hotels had built stoves with chimneys, interconnections, fuel feed doors, air regulators, and exhaust hoods, etc. designed to simplify operation and reduce inconvenience, fire hazards, and pollution. In this respect they were superior but were not optimised from the stand point of energy conservation. Their overall efficiencies were low and they wasted most of their heat.
The Figures show a sample of the designs encountered in hotels of this region.
It was not possible to measure the efficiency by conventional tests (eg. PHU) because the owners did not allow intrusions. me owners had an idea of total daily fuel consumption, but not for each stove (no hotel had less than two stoves). m e efficiencies were assessed by noting the temperature rise of water (placed in a pan appropriate to the stove size) for 2 minutes. The input was determined by a rough figure of firewood consumption per day as reported by the operator.
The following sections refer to woodstoves, as these were the majority. The problems of charcoal and husk burning stoves are similar.
Pollution and Inconvenience
Some of the problems found were
1. Stoves were often located at ground level, so that the cooks had to bend down to place and remove vessels, collect ash, control the fire, and feed fuel.
2. No chimney, instead, vessel mounts are used and smoke enters the room causing irritation to eyes and lung problems for operators.
3. Stoves located in rooms so small that there is hardly any circulation of fresh air.
4. Exhaust fans or ventilators are not installed in some hotels.
5. Fuel feed openings too big in most stoves, the radiation causing uncomfortably hot conditions to the workers.
6. Where vessel mounts are used, it also causes uncomfortable heat and releases carbon monoxide.
7. The vessel mounts also result in a greater area of vessel getting exposed to the air cooled flame, causing soot deposition.
Life and Repairs
1. Use of non-fire resistant materials, improperly tamped materials, non-optimum proportions of constituents of bonding materials, lack of sufficient curing of the structure before use.
2. In brick stoves, the life depends to a great extent on whether the bricks are hand- or machine-made, sun- or kiln-dried, laterite or mud. Stoves of machine made firebricks show maximum life.
3. Cement plastering is not desirable because it regularly cracks due to heat.
4. Many stoves are frequently damaged around the pot opening due to the weight of the vessel. Stoves which use iron frames or pottery pieces around the pot openings show reduced frequency of damage.
Performance and Efficiency
Some of the defects which affect the performance are
1. Vessel mounts for exhaust gas outlet reduce efficiency.
2. In stoves with chimneys, these were ill designed with respect to their diameter, length and height.
3. No grates for ash removal, no front dampers for control of air flow rate, and no back dampers for control of fuel burning rate.
4. Height of combustion chamber and log size did not match.
5. Volume of combustion chamber and fuel burning rate not matched.
6. Unnecessarily large fuel feed opening
7. No iron skeleton and hence increased heat transfer to the walls.
8. Pot size and pot opening not matched.
9. Absence of secondary pot openings which utilized the flue gases from the combustion chamber for heating additional vessels.
10. In stoves which had secondary pot openings, the distance between them and the main combustion chamber was too large.
11. Cookstoves in hotels need constant load control, because the rate at which customers come fluctutes and in the absence of such control, the efficiency drops to a low level.
Designs of improved stoves must aim at solving some of the above problems.
Optimum System Efficiency
Heat transfer to the vessel is of two types - radiant and convective. In the former there is direct contact between the flame radiation and the pot, while in the latter the flue gases transfer neat to the pan. Both these modes of heat transfer depend on the area of contact and the surface temperature of the heat source. This relationship gives rise to two opportunities for optimisation
1. Maximise the surface area, ie the area of contact between the vessel and the flame, or between the vessel and the flue gas,
2. Adjust the height between the vessel bottom and log top to such a value that the heat transfer is maximum.
Devices for Improving Performance
1. Baffles to direct the flow of hot exhaust gases.
2. Dampers to vary the burning rate by the control of air flow at the inlet, outlet, and intermediate points. Dampers also prevent radiation losses from the front.
3. Chimneys to vent away the exhaust gases and also induce draught which aids combustion.
4. Grates to remove ash from the combustion zone, to improve combustion efficiency, pre-heat air drawn from below, minimise bottom radiation losses, and keep the flame at the central part of the combustion chamber.
5. Use of hand operated blowers to further increase the fuel burning rate at times when excess heat is needed.
6. A metallic lining improves the performance of cookstoves (1). The lining increases the combustion chamber temperature by virtue of the air gap (bad heat conductor) between the lining and the stove wall.
(1) Jayaraman S 'Notes on High Efficiency Wood Burning Stove' 1982, Central Power Research Inst. Bangalore 560012, India