Thursday, August 22, 2013

Drying, Burning of Bricks- Bull's Trench Kiln, Hoffman's kiln


The Manufacturing of bricks is consists of  following processes:

  1.  Preparation of brick clay
  2.  Moulding of Bricks
  3.  Drying
  4. Burning of bricks
First two operations are already discussed in the previous two parts of the article in the same blog. In this blog I am going to discuss with you the Drying and the Burning of the bricks:

  • Drying Of Bricks: 

Drying of the bricks at lower temperature is done to strengthen the bricks to have sufficient strength so that they do not crack or crumble while holding and carrying them in hand.
Finally bricks are to be stacked in the kiln, but they must possess the sufficient strength to maintain their shape while the transportation and loading operations, so drying of the bricks is important process. Drying of the bricks can be done in two ways:

(a) Natural Drying: Natural drying is done by putting the bricks in the under the air circulation, generally avoided to direct Sun-light.

(b) Artificial Drying: In artificial burning bricks are put under a temperature of about 120 degrees Celsius in a tunnel like structure where arrangements are there to maintain the temperature.

 These tunnel kilns can be period or continuous. In periodic kilns the bricks are dried periodically but in the continuous they are put continuously at one end and dried bricks are taken out at the other end.

  • Burning of Bricks: 

Bricks are burned at high temperature to gain the strength, durability, density and red color appearance.
All the water is removed at the temperature of 650 degrees but they are burnt at an temperature of about 1100 degrees because the fusing of sand and lime takes place at this temperature and chemical bonding takes between these materials after the temperature is cooled down resulting in the hard and dense mass.

 Bricks are not burnt above this temperature because it will result in the melting of the bricks and will result in a distorted shape and a very hard mass when cooled which will not be workable while brickwork. Bricks can be burnt using the following methods:
(a) Clamp Burning
(b) Kiln Burning

  •   Clamp Burning: 

Clamp is a temporary structure generally constructed over the ground with a height of about 4 to 6 m. It is employed when the demand of the bricks is lower scale and when it is not a monsoon season.

 This is generally trapezoidal in plan whose shorter edge among the parallel sides is below the ground and then the surface raising constantly at about 15 degrees to reach the other parallel edge over the ground.

 A vertical brick and mud wall is constructed at the lower edge to support the stack of the brick. First layer of fuel is laid as the bottom most layer with the coal, wood and other locally available material like cow dung and husk.

Another layer of about 4 to 5 rows of bricks is laid and then again a fuel layer is laid over it. The thickness of the fuel layer goes on with the height of the clamp.

After these alternate layers of the bricks and fuel the top surface is covered with the mud so as to preserve the heat.
Fire is ignited at the bottom, once fire is started it is kept under fire by itself for one or two months and same time period is needed for the cooling of the bricks. 

  • Disadvantages of Clamp burning: 

  1. Bricks at the bottom are over-burnt while at the top are under-burnt. 
  2. Bricks loose their shape, and reason may be their descending downward once the fuel layer is burnt.
  3. This method can not employed for the manufacturing of large number of bricks and it is costly in terms of fuel because large amount of heat is wasted.
  4.  It can not be employed in monsoon season.

  • Kiln Burning: 

Kiln is a large oven used for the burning of bricks. Generally coal and other locally available materials like wood, cow dung etc can be used as fuel. They are of two types:
(a) Intermittent  Kilns.
(b) Continuous Kilns.

  • Intermittent Kilns 

are also the periodic kind of kilns, because in such kilns only one process can take place at one time. Various major processes which takes place in the kilns are:

Loading, unloading, Cooling, and Burning of bricks.

There are two kind of intermittent kilns:
(i) Up-draught Intermittent Kilns
(ii) Down draught Intermittent Kilns

Down draught kilns are more efficient because the heat is utilized more by moving the hot gases in the larger area of the kiln. In up draught kilns the hot gases are released after they rise up to chimney entrance.

  • Continuous Kilns: 

These kilns are called continuous because all the processes of loading, unloading, cooling,  Heating, pre-heating take place simultaneously. They are used when the bricks are demanded in larger scale and in short time. Bricks burning is completed in one day, so it is a fast method of burning.
There are two well known continuous kilns:

  • Bull's Trench Kiln: 

Bull's trench kiln consist of a rectangular, circular or oval plan shape. They are constructed below the ground level by excavating a trench of the required width for the given capacity of brick manufacturing.

This Trench is divided generally in 12 chambers so that 2 numbers of cycles of brick burning can take place at the same time for the larger production of the bricks. Or it may happen that one cycle is carried out at one time in all the 12 chambers by using a single process in the 2-3 chambers at the same time.

The structure is under-ground so the heat is conserved to a large extent so it is more efficient.  Once fire is started it constantly travels from one chamber to the other chamber, while other operations like loading, unloading, cooling, burning and preheating taking place simultaneously.

Such kilns are generally constructed to have a manufacturing capacity of about 20,000 bricks per day. The drawback of this kiln is that there is not a permanent roof, so it is not easy to manufacture the bricks in the monsoon seasons.

  • Hoffman's Kiln:

The main difference between the Bull's trench kiln and the Hoffman kilns are:

  1.  Hoffman's kiln is an over the ground structure while Bull's Trench Kiln is an underground structure.
  2. Hoffman's kiln have a permanent roof while Bull's trench Kiln do not have so it former can be used in 12 months a year to manufacture bricks but later is stopped in the monsoon season.

Layout Plan
Hoffman's kiln is generally circular in plan, and is constructed over the ground. The whole structure is divided into the 12 chambers and all the processes takes place simultaneously like in Bull's trench Kiln.

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