|Boiling Point No. 04 - March 1983 (ITDG Boiling Point, 1983)|
by Stephen Joseph
ITDG Stoves Project
To date we have discussed how ceramic stoves crack due to thermal stress, but stoves also crack because of poor structural design.
Fired clay is a very brittle material that has a relatively low strength and a low resistance to impact, and thus cannot bear heavy loads. Ceramic stoves must therefore be carefully designed to reduce the likelihood of fracture.
To understand how to design structurally sound stoves it is useful to have some understanding of the theory of crack formation and propagation.
Most ceramic materials have imperfections present in the clay; impurities such as small stones. But they can also have imperfections which are incorporated in the manufacturing process, for example, badly joined pieces or uneven thicknesses of clay will subject the object to further stress on firing. It is the ready propagation of cracks from these defects that give ceramics their characteristic low shock resistance.
When a material is subjected to a load (eg putting a pot on a stove) the stress (force per unit area) at the imperfections is locally very much greater than in the body of the material. Cracks will start to grow and if the applied load is greater than some critical value the crack will rapidly grow larger, resulting in the failure of the material. Cracks are more likely to propagate from right-angled joins, where there is a concentration of stress. Stoves are not only subjected to this mechanical loading but also to thermal loading. Stresses are established because one part of the stove is hotter than another, and the greater the local temperature difference, the higher the stress. (For a more detailed discussion on the theory of crack propagation see "STRUCTURES" by J E Gordon, published by Penguin Books 1978.)
Some general rules governing the design of stoves emerge from an understanding of the concepts of stress concentration, and the formation and propagation of cracks.
1. Avoid putting sharp corners in your stove. For example:
- Adjoining tunnels and pieces make sure joined pieces are gently tapered and rounded, avoiding right-angled joins
2. Never place the air entrance and flue gas exit on the same side of the stove - place them opposite each other. The narrow space left is particularly susceptible to stress.
3. Always try to build a stove out of cylinders to avoid the high stresses that result from bending in flat shapes: ie exploit the natural strength of curved shapes (arches or domes)
4. When the food in the pot is likely to be stirred vigorously during cooking, or if large pots are used, it is essential to reinforce the ceramic, or remove the load from the ceramic in some other way. This can be done in a number of ways - (a) to (d) illustrate some well used methods.
a) Place mud around the ceramic and sit the pot on the mud
b) Use a metal frame
c) Use a double walled stove
In both (b) and (c), avoid a 'wedging' situation between the pot and the edges of the pot opening, otherwise problems could occur with differential expansion between the pot and the pot seat.
d) Use an outer metal casing
5. Reject stoves that have black marks, after kiln firing, on the
inside or outside of the firebox. The black mark indicates a spot that has been
reduced in the firing. Localised reduction may weaken the clay at this
6. Avoid features that give rise to high temperature gradients, for example, baffles under the second pot seat.
With this issue of Boiling Point you should also receive a copy of Technical Notes ho 2 'Optimisation of Chimney Stoves' and No 3 'Testing of the Zip Ztove'. (No 1 in the series 'Comparative Performances of Kenyan Charcoal Stoves', mailed with BP No 3 was called a Technical Paper, but it was pointed out that ITDG Publications Ltd is issuing a series of Technical Papers hence the change of name.