Bull's trench Kiln and Hoffman's Kiln
- These are commonly used in India.
- Usually oval-shaped in a plan, but these may also be circular or rectangular in plan.
- It is an underground kiln, constructed in a trench. The inner & outer walls are constructed in bricks, providing openings in the outer walls of the kiln to act like a-holes.
- Fig shows Bull's kiln with two sets of sections with the tentative arrangement of the section at any instant.
- This requires two gangs of laborers and two sets of twin chimneys for the continuous operation of the kiln.
- The arrow in the plan indicates the direction of the movement of chimneys. Iron plate dampers, placed by the sides of openings divide the kiln into suitable sections or chambers.
- Each section of a trench of 5.5m width contains about 10000 bricks & requires 1.6 m' of coal to burn.
- Iron plate dampers are placed by the sides of the openings to divide the kiln into suitable sections or chambers.
- The trench is generally 2m deep.
- If full depth cannot be dug, a portion of it can be made above the ground with banks of earth.
- The length L may vary from 40 to 60m & the width b may vary from 5 to 7m.
- In the various zones of the kiln trench, all the different operations ie loading, unloading, cooling, burning & heating of bricks goes on simultaneously.
- Openings are generally provided in the outer walls to act as flue holes.
- The movable chimneys are kept in advance of the section being fired.
In the trench, bricks are arranged in sections. They are arranged in such a way that flues are formed at the bottom. At the top, flues are provided with fuel holes at 1m intervals, to charge the fuel from time to time during the firing process. Coal is used as fuel.
Bricks are stacked in sections of about 3.5m lengths, each section being separated by a gap of 0.15 m over which two movable iron chimneys on four wheels are placed.
In order to prevent heat loss by radiation, the top surface of the loaded bricks is covered with earth & coal ashes.
When one section is fired, the outgoing gases from that section are led to the next section for warming the stacked bricks, before they escape out of the chimney.
Each section requires about one day to burn.
After the burning of one section is complete, the flue holes of that section are closed & it is allowed to cool gradually.
The fresh air is preheated by the cooling bricks.
The next section is then fired & the chimneys are moved forward by one section. Thus, as the fire advances to the next sections, the previous burned sections are cooled, these cooled sections are then unloaded & air-dried raw bricks are again stacked.
The smallest size kiln of 20 chambers & 5.5 m width one chimney can turn about 10000 bricks daily.
A larger size kiln of 24 chambers worked by two sets of chimneys will be economical to work & cheaper in construction.
- It is an overground continuous kiln, which has a permanent roof due to which it can function even during the rainy season.
- Circular in plan & is divided into a number of chambers (usually 12 in number) each provided with a door at the outside face for loading & unloading & a flue with damper for controlling the inflow of air & another flue on the inside connected to the central chimney. The door could be closed with a dry brick wall having sand packing. The flue could be closed or opened as desired by operating a metal plate known as a damper. Fuel in the form of cold dust is fed through the holes provided in the kiln top.
- In this kiln also, all the operations like loading, drying, heating, burning, cooling & unloading take place simultaneously.
- At any instant ten compartments (say nos 3 to 12) would be full but compartments 1 & 2 would be in the process of loading raw molded bricks & emptying cooled-down burned bricks.
- Thus the functioning of all 12 chambers may be as follows
- Cold air enters through open doors of chamber 1 (loading) & chamber 2 (emptying)
- Air enters into cooling chambers 3,4,5,6 and in that process, the air gets heated.
- The heated air then enters into burning chambers Nos 7 & 8.
- The hot flue gases from chambers 7 & 8 passes in continuity to chambers 9,10,11 & 12 for pre-heating the raw bricks. These gases are then obstructed by the iron shutter & led through a flue into the chimney from where these escape into the atmosphere.
