Bull’s Trench Kiln
Bull’s trench kiln is usually of rectangular, circular, or oval plan shape. These are constructed below the ground level by excavating a trench of the required width for the given capacity of brick manufacturing. There are 12 chambers in the trench constructed to enable the production of two cycles of brick burning at the same time for a larger production. The heat is conserved to a large extent since the structure is underground and is more efficient because of that. When the fire is started it constantly travels from one chamber to the other chamber. The operations like preheating, burning, cooling, unloading, loading take place simultaneously.
The construction of these Bull’s trench kilns is in such a way that they have a manufacturing capacity of about 20,000 bricks per day. The manufacture of bricks in the monsoon season is not possible in these kilns because they don’t have a permanent roof. It has a movable metal chimney which is placed on the brick setting and was moved as the firing progresses.
Bull’s Trench Kiln Diagram
Difference Between Bull’s Trench Kiln And Hoffman’s Kiln
|Bull’s Trench Kiln||Hoffman’s Kiln|
|Mostly rectangular, circular, or oval in plan shape||Mostly circular in plan shape|
|Constructed below the ground||Constructed over the ground|
|Do not have a permanent roof||Have a permanent roof|
|Cannot be used during the monsoon season||Can be used in all the 12 months of the year|
|Divided into 12 chambers and the entire process takes place simultaneously||Divided into 12 chambers and the entire process takes place simultaneously.|
In Hoffman’s Kiln, the fire moves forward continuously through the stacked bricks in the circular, elliptical, or rectangular-shaped closed circuit with an arched roof and burns them. A chimney or Fan provides a draught that causes fire movement. In the year 1858, Friedrich Hoffman in Germany developed and patented this kiln. In India, it was introduced in the Malabar coastal region.
The first Hoffman Kiln had a circular circuit built around a central chimney but later it was modified with time and now Hoffman Kilns with elliptical or rectangular shapes are mostly used.
Also Read: Drying and Burning of Bricks
Hoffman’s Kiln Burning of Bricks
In Hoffman’s Kiln, the fire moves through the stacked bricks which are covered with an arched roof. The Kiln is protected from rains by the arched roof. The firing movement is caused by the draught provided by the Chimney. In some cases, a fan is also used to augment the draught.
The central flue duct of the kiln is connected to the chimney through an underground duct. In the Hoffman’s kiln, there are three distinct zones for operation namely the Brick firing zone where the fuel is fed and combustion is happening, the Brick preheating zone where green bricks are stacked and being pre-heated by the flue gases, and the Brick cooling zone where fired bricks are cooled by the cold air flowing into the Kiln.
The back end of the cooling zone is kept open to allow air entrance. Whereas to guide the flue gas to the chimney through the flue gas duct system the front end of the preheating zone is sealed. Through openings provided in the inner wall of the kiln, the kiln is connected to the central flue duct.
The openings to the kiln before the seal are kept open to allow the entrance of flue gases from the kiln to the central flue duct. For feeding of fuels feed holes are provided in the kiln roof. A single fireman standing on the roof of the kiln fed the solid fuels from the feed holes.
At an interval of 15-20 minutes, the fuel is fed and the fuel feeding lasts for 5-10 minutes. The fire inside the kiln travels around 10m in 24 hours and fires 10,000 to 20,000 bricks. From the back end of the brick cooling zone daily the fired bricks are unloaded and the same batch of green bricks are loaded ahead of the brick preheating zone.
Also Read: Different Tests on Bricks
Fixed Chimney Bull’s Trench Kiln
It is a modified version of the Bull’s Trench Kiln, which has more efficient and less polluting. Fixed Chimney Bulls’ Trench Kiln is one of the most widely used brick firing technology. It consists of a continuously moving fire kiln in which the fire is always burning and moving forward in the direction of airflow due to the draught provided by a chimney. In the different parts of the kiln, the bricks are warmed, fired, and cooled simultaneously.
It’s operational during the dry season. Across the cross-section of the kiln, there is a non-uniform temperature and that may cause under-fired bricks at the sidewalls and top corners and causes a difference in product quality.
Advantages of Bull’s Trench Kiln
The Pros of Bull’s Trench Kiln are:
- All the processes happen simultaneously since the structure is divided into 12 chambers.
- The heat remains in the structure for a long period as it is located underground.
- It manufactures a continuous supply of bricks.
- It produces a high percentage of First-class bricks.
- Drying space is saved as even raw bricks can be loaded in it which are dried before being burnt by hot air coming through the chimneys.
Disadvantages of Bull’s Trench Kiln
The Cons of Bull’s Trench Kiln are:-
- It doesn’t have any permanent roof and so it cannot be used during the monsoon seasons.
- High Initial Cost.
- Needs constantly skilled supervision.
Also Read: Moulding of bricks