High draught kiln
In India zig-zag firing concept was first introduced in the from of High Draught (H. D) kiln by CBRI during 1970s (figure 9). HD kiln has several similarities with the Habla kiln. This kiln consists of a rectangular gallery which is divided into 24 chambers by providing temporary partition walls with green bricks. The wall of each chamber runs along the width of the gallery except one end, wherein a space of 60 to 65 cm is left for communication to the next chamber. Draught is created by an induced draught fan with a 15 hp motor for proper combustion of fuel. Depending on the design capacity of a kiln, a chamber can hold 7,500 to 15,000 bricks. Normally two chambers are burnt per day and output of 15,000 to 30,000 bricks per day can be obtained. When brought to full firing, the kiln operates on a draught of 50 mm WG. Several problems were encountered in the original H D kiln [Majumdar, 1986]: i) bricks remains too hot for handling at unloading point, due to this reason the number of chambers was increased to 28 or 32. ii) dampers in the flues provided in the inner wall communicating with the main central flue being too close to the firing floor, were exposed to high heat resulting in rapid deterioration. To solve this problem dampers made up of a heat-resistant alloy were employed. In some kilns a brick wall was constructed to protect the dampers from direct heat. iii) As the draught and hence the negative pressure in the kiln is several times more than that observed in BTKs and fixed chimney kilns, the H D kiln is also more susceptible to air leakage. Most of the leakage takes place through the wicket walls and through leaking valves and dampers. The problem of leakage through wicket gates is solved by providing a cavity wall at each wicket, the cavity being loosely packed with ash.