Principle of VSBK
The working of VSBK is explained with the help of figure 11. VSBK has a vertical shaft of rectangular or square cross-section. The kiln works as a counter current heat exchanger, with heat exchange taking place between the air moving up (continuous flow) and bricks moving down (intermittent movement). Green bricks are loaded from the top in batches, the bricks move down the shaft through brick pre-heating, firing and cooling zones and unloaded at the bottom. The combustion of powdered coal (put along with bricks at the top), takes place in the middle of the shaft. Air for combustion enters the shaft at the bottom, gets preheated by the hot fired bricks in the lower portion of the shaft before reaching the combustion zone. Hot combustion gases, preheats the green bricks in the upper portion of the shaft before exiting from the kiln through the shaft or chimney.
The brick setting in the kiln is supported on support bars at the bottom of the shaft. Brick unloading is carried out from the bottom of the shaft using a trolley. For unloading, the trolley is lifted (a screw mechanism or chain pulley blocks are used for lifting) till the rectangular beams placed on the trolley touches the bottom of the brick setting and the weight of bricks is transferred on to the trolley. The support bars now freed of the weight of bricks, are taken out. The trolley is then lowered by one batch (4 layers of bricks) — support bars are again put in place through the holes provided in the brick setting for the purpose. With slight downward movement the weight of the brick setting is again transferred back on the support bars. The trolley (with one batch of fired bricks on it) is further lowered till it touches the ground and is pulled out of the kiln on a pair of rails provided for the purpose. In general, the time between two unloading operations is 2-3 hours and the firing time (the time for which a brick resides in the kiln) varies from 20 -30 hours.
Two chimneys located diagonally opposite to each other are provided for the removal of flue gases out of the kiln. A shaft lid is also provided on the top opening of the shaft . The lid is kept closed during normal operation and hence flue gases do not pollute the working area on the top of the kiln. The provision of shaft lid, better ventilation of the working area on the top and higher and bigger chimneys are some of the improvements made in the VSBK technology during its pilot testing in India.