After moulding and drying of raw bricks, it is then burnt in the kiln. There are two methods of burning raw bricks. The first one is the clamp burning of bricks and the second one is the kiln burning of bricks. In this post, we will learn in detail about the difference between these two.
Clamp Burning of Bricks
In this type of clamp burning, a temporary structure of bricks and fuel are placed in alternate layers. The lowermost layer contains more fuel and the quantity of fuel is subsequently reduced in the upper portions. Each brick tier consists of 4 to 5 layers of raw bricks.
Bricks are spaced not too close to each other, some space is left so that fume gases could be circulated freely. When around 30 % of the loading is completed lowermost layer of the fuel is fired and then loading of subsequent layers of bricks is carried out.
This method of burning can produce 2 to 3 lacs bricks in six months. When loading is completed the kiln is then covered with mud and cow dunk. This process yields about 60 % of the first-class bricks.
Then next set of bricks and fuels are stacked after taking out the burnt bricks. Thus, this type of kiln can’t meet the continuous demand. Another major disadvantage of this method is that we can’t control the fire. Also, more fuel is used in this type of burning thus affecting the economy.
Process of Clamp Burning
A portion of the ground usually in trapezoidal shape is selected. The floor of the kiln is made in such a way that the wider edge of the trapezoidal is raised at an angle of 15 degrees from the ground.
After making the ground, the layer of fuel of about 70-80 cm thickness is placed. The fuel consists of cow dunk, husk or rice, and grass, etc. More superior fuel like wood and coal dust can also be used.
A stack of brick containing 4 to 5 layers is placed over this fuel leaving small spaces in between for the circulation of air. The second layer of fuel is placed over it and then another layer of brick is placed. The total height of the clamp is about 3 to 5 meters.
Either it is fired during the stacking process or after the completion of the stacking. The top and sides of the kiln are then covered with mud to prevent the escape of heat. Burning to take place for about 2 to 3 months and additional 2 to 3 months are required for cooling the burnt bricks.
Advantage of clamp burning
- The burning and cooling of bricks are slow in clamps. Slow-burning produces tough and strong bricks.
- It is a very cheap method of burning.
- No skilled labors and supervision are necessary for this type of kiln burning.
- There is a saving of fuel if grass etc is used in place of coal dust.
- This method of brick burning produces a low amount of good quality bricks.
- The process is very slow.
- Not possible to control fire in the clamp.
- Quantity of irregularly shaped bricks is produced more.
Kiln burning of bricks
This type of brick burning is superior to the burning in clamps. These types of brick burning methods consist of a permanent structure. These kilns are of two types. If the process of burning bricks takes place continuously with no break in-between then it is called a continuous kiln.
Bull trench’s and Hoffman kilns are continuous kilns. If there is a break in the burning process then it is called an intermittent kiln. The kiln can be underground like Bull’s trench kiln or overground like Hoffman’s kiln.
In this type of kiln after loading raw bricks, it is fired, cooled, and unloaded. Only then next set of bricks is again loaded and a similar process continues.
They have a major disadvantage over continuous kiln that after unloading the bricks, the kiln gets cooled. Thus have to be reheated again and in doing so there is a wastage of fuel. This type of kiln is usually provided overground and is rectangular in shape. These kilns are of two types
Intermittent up drought kilns
These kilns are rectangular in shape and consist of thick outer walls. Bricks are stacked in rectangular shapes and some space is left between the stacks for the movement of flue gases.
In these kilns, flue gases move horizontally while in down drought kilns flue gases move vertically towards the roof. Bricks are burnt for 48 to 60 hours during this time strong fire is maintained.
Intermittent down drought kilns
These kilns are more or less similar to the up-drought kilns and are rectangular or circular in shape.
They are also provided with permanent thick walls and roofs. The only difference is that flue gases are carried vertically up to the `roof and then these gases move downward by the chimney drought and in doing so they burn the bricks.
Bull’s trench kiln and Hoffman’s kilns are examples of the continuous kiln. These types of kilns are divided into different chambers. The bricks are heated differently in different chambers. When the brick in one chamber is fired, the bricks in the next chamber are dried and preheated.
Bricks in some chambers are loaded and in some chambers, it is cooled. So the brick burning process is continuous and the kiln always remains heated.
When one chamber is heated the hot flue gases from this chamber moves to another chamber and bricks placed on this chamber get preheated which will save the amount of fuel for burning these bricks. Kiln burnt bricks yield about 80-90 % of the first-class bricks
Comparison between Clamp burning and kiln burning of bricks
|Item||Clamp burning||Kiln burning of bricks|
|Capacity||About 20,000 to 1 lacs of bricks can be burnt.||About 25000 bricks can be burnt but the process takes very little time.|
|Cost of fuel||Cost is low because cow dust and grass etc. can be used.||The cost of fuel is more because powdered coal is used.|
|Initial cost||Practically nil because no permanent structure is built.||High because the permanent structure is required to be constructed.|
|Quality of bricks||The yield of good quality brick is about 50-60%||Kiln burning of bricks yields, about 80-90%. of good quality bricks.|
|Regulation of fire||Fire can’t be controlled much.||There is good control over the fire.|
|Skilled supervision.||Technical knowhow is not required.||Skilled labours are required.|
|Structure||No permanent structure.||Kiln burning of bricks requires a permanent structure.|
|Suitable for||As yield is low thus suitable for small scale.||Large-scale production of bricks is possible.|
|Time take,||Burning and colling can take 2 to 6 months.||Burning takes 1 day and cooling takes 12-15 days.|
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