PRINCIPLES OF OPERATION OF HOFFMAN KILN 1
The kiln is operated 24 hours a day (3 shifts) 365 days per year, producing 1.25 million bricks per week. Two fires ‘chase’ each other around the kiln, moving progressively from one chamber to another with the bricks undergoing drying, pre-heating, firing and cooling in turn. Once lit the kiln is not allowed go out and the whole process is controlled manually by manipulating dampers and sliders, which requires skill and experience. To assist the operator in controlling the process, chamber temperatures are monitored using a number of thermocouples linked to a PC.
The clay used for making the Fletton bricks is unusual in that it contains about 5% by weight of an organic lignite-like material which contributes significantly to the firing of the bricks and minimises the amount of support fuel needed. This organic material is formed from algae, spores, pollen, bacteria and other plant and animal cells.
Newly made ‘green’ bricks are set in a chamber and the entrance is bricked up and then sealed using an ash and clay based skim to prevent the ingress of air. Hot air conveyed from cooling bricks in one chamber is used to dry and pre-heat bricks in another. Drying is carried out relatively slowly to ensure that all the moisture is driven out of the bricks uniformly and that distortion does not occur. Once the bricks are dry then it is important to raise their temperature rapidly in order to maintain reducing conditions in the chamber. The organic material in the clays helps with this process.