PRINCIPLES OF OPERATION OF HOFFMAN KILN 2
Once the required temperature has been reached, caps over the openings in the roof are opened permitting ‘easing’ of bricks by allowing cold air into the chamber. Coal is subsequently fed through openings in order to achieve the necessary soaking time (30 hours at 930-960oC). After firing the bricks are allowed to cool in the chamber before they are removed. This cooling process provides the heat to drive the drying process of the next batch. New ‘green bricks’ are then placed in the chamber and the process cycle starts again. Each cycle takes 10 days to complete.
The kiln operates by natural draught with the buoyancy of the exhaust gases providing the motive force, with maximum exhaust temperatures at 150oC. The draught has to be sufficient to overcome the pressure drop through the kiln and to ensure that the chambers are always under negative pressure. The driving pressure in the chimney is maintained at around 25mm water gauge.
The brick making process attempts to minimise the use of fuel. Therefore the mass throughput of gases through the flue system is kept to a minimum which ensures removal of the drying effluents and the combustion products used to maintain the required temperature. Gas velocities in the chambers are low and not conducive to high turbulence.