Today, in this article you will learn about construction and working mechanism of Bull’s trench kiln and Hoffman’s kiln:
First, we will shed light on construction of bull’s trench kiln and then we will try to explain about the construction of Hoffman’s kiln:
1. Construction of 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.
2. Working of Bull’s Trench Kiln
The dried bricks stacked in the kiln with space in between them for circulation of the hot gases and forming fuel galleries an over 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 to30 thousands bricks in a section. About clear space of 15 cm is left between two sections. The top two layers in the kiln are loaded with kacha bricks and the section is covered with a layer of about 20 cm of earth or ash. The door ways are also closed. With under burnt and mud plastered to avoid any heat losses.
In the kiln when section is being burnt, the hot gases are allowed to pass through the next section before they escape through chimney at 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 about 24 hours to burn and temperature varies between 800℃ to 1000℃. After 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 the other one in smoking while last two sections are under loading. Thus the process of burning, loading and unloading remains continuous in different sections of the kiln. But it difficult for 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 a 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.
3. Construction of Hoffman’s kiln
This type of kiln is generally circular in shape but may also be rectangular and is covered with roof. It is constructed above ground level. It has 8 t0 24 chambers with a chimney at the center. Each chamber has a doorway which can be closed. Every chamber is also connected by means of the flue, which can be opened or closed, leading to central chimney. At the top of each chamber fire holes with covers are provided to feed fuel.
4. Working of Hoffman’s kiln
It is a continuous kiln which works in all season. But it involves very high initial cost so it is rarely used in India.
When chamber 1 is loaded with raw bricks, the burnt up bricks in chamber 2 are unloaded. The fresh air entering through the opened door of chamber 1 and 2 gets heated by the bricks cooling in chamber 3 to 6 while the fuel in chamber 7 and 8 is burning.
The hot gases coming from the burning chambers pass through the raw bricks loaded in chamber 9 to 12 and helps in drying them before burning. An iron gate is fixed at the end of chamber 12 and hot gases thus pass through the flue into the chimney. Usually this type of kiln has a compartment with sizes of 12 m length, 5 m width, and 2.5 m height, which can produce about 20000 bricks a day.
This type of kiln has a low fuel costs and having more percentage of good bricks. This kiln works through out year and it saves time and space for drying because raw bricks are directly fed into the chamber. It produces no smoke. But it has initial cost and requires skilled labor and careful supervision. In the kiln we get 80% to 90% ist class bricks.
A standard brick of size 19cm × 9cm × 9cm should have 3.2kg of weight approximately.