The size of refractory bricks can differ widely between one country and another. For this reason, only approximate dimensions are given, and the instructions are written in terms of the number of bricks in each layer, to avoid unnecessary cutting of bricks. The small variations in the size of the completed incinerator will have no effect on its performance. It follows that the steelwork also must be made to the measured dimensions of the brickwork so that the steel and brick components fit together.
Sealing issues and air flow
The fuel and load should burn only after they reach the level of the air holes. Thereafter the flames should be drawn down to the grate and through the transfer hole to the secondary combustion chamber and up the chimney. This means that the loading door must be completely airtight except when it is opened to add more fuel.
It is also important that the firebricks of the primary combustion chamber are sealed with fireclay or fire cement to the top plate to prevent the burning gases or flames being drawn up the primary combustion chamber and directly into the chimney. If for any reason the sand seals should distort during operation, incorrect burning will occur and the maximum incineration temperature will not be attained. Should this happen, the joint between the sand seals and the brickwork should be again sealed with fireclay.
The air holes have been calculated to allow in the correct amount of air for efficient combustion at high temperature. Some tolerance is allowable so that air pipes can be made from materials available locally, but it is not good practice to leave the ash door open during operation, and it is incorrect to leave a gap beneath the ash door. Air should enter the combustion chamber only through the air holes.
The chimney should be at least 4 metres high, and can be extended if it is thought necessary to take the gases above any nearby buildings. Its diameter can be between 100mm and 150mm, but the spigot on which it rests must be adjusted so that the chimney is a loose fit.
There are a number of ways of maintaining the chimney in a vertical position. If a roof is built over the incinerator area, it may be possible to attach the chimney to a roof truss. Otherwise four steel cables can be attached to the chimney at a point about two-thirds up the chimney and the other ends of the cables anchored to the ground at least two metres from the chimney. If the chimney weight is above average, either because it is made of heavy gauge steel or because it is much longer than 4 metres, the plate supporting the chimney spigot must be doubled in thickness.
Most users are usually somewhat puzzled by the fact that there are no precise measurements provided and that in the plans one talks in "bricks" and not centimetres or inches.
The reason behind this choice is due to the fact that brick dimensions tend to vary from one country to another and that it is simpler to adjust the overall size of the incinerator to the available bricks than to have to cut bricks to an exact dimension.