Bull’s Trench Kiln

It is a continuous type of kiln. This type of kiln is generally oval in plan. A trench is excavated below ground level to a depth of 1.5 m to 2.5 m and length varying between 50 m to 100 m while breadth is kept between 4 m to 10 m. It is divided into 8 to 12 parts or sections keeping one passage in each section.

The dried bricks stacked in the kiln with spaces in between them for circulation of the hot gases and forming fuel galleries and over these fuel galleries, vertical shafts or holes at 60 cm to 90 cm interval are provided to provide fuel when burning is in progress. The bricks are stacked in sections of about 3.60 m length having 20 to 30 thousands bricks in the section. About clear space of 15 cm is left between two sections. The top two layers in the kiln are loaded with kaccha bricks and the section is covered with a layer of about 20 cm of earth or ash. The doorways are also closed. With under burnt bricks and mud-plastered to avoid any heat losses.

In the kiln when the section is being brunt, the hot gases are allowed to pass through the next section before they escape through the chimney at the top which saves the fuel cost. Sheet metal dampers are fixed across the section beyond the chimneys. Bricks in each section take about 24 hours to burn and temperature varies between 800° c to 1000° c. On completion of burning in one section, all the flue holes and feed holes are closed and section is allowed to cool. In this kiln, while one section is being unloaded, the next two sections are left for cooling. One section shall be under fire and one under pre-heating, unloading remains continuous in different sections of the kiln. But it is difficult for the kiln to work in the monsoon season hence it is called semi-continuous and is most commonly used in all parts of India giving an output of about 30000 bricks a day.

It gives good quality of bricks at a low cost continuously. But its initial cost is high and constant supervision and skilled labor is required in it.