High Draught Kilns

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High Draught Kilns

The Zig-Zag, Habla or the High Draught Kiln is similar firing technologies with essential variations in brick setting patterns where the kiln basic structure remains same. The principle followed in this family of kiln types is to prolong the travel path of fire. This high thermal efficiency of the kiln reduces fuel consumption by 25%. The earliest type of this technology was termed as Zig-Zag kiln developed in the early 1900s. Various reforms were also built which were relatively inexpensive variants adopted by small brick manufacturers. The design of this type of kiln ensures continuous operation of the kiln.

One of the more developed variant was The Habla Kiln.
It was essentially an arch less zig-zag kiln patented in 1920 by British Engineer A. Habla. This had very wide and continuous open firing channels. Unlike zig-zag variants there were no permanently built dividing walls. Thus, the energy savings were higher. However the most important aspect of this kiln was that bricks could be loaded into the firing spaces through vehicular movement.

In Asian countries of India, Nepal, Bangladesh and Pakistan the High Draught or HD Kiln is the most popular technology of Zig-Zag firing. Their difference with European variation is the absence of any dividing walls. In the European variety of HD kilns, the length of the kiln gallery is increased by zigzagging the chambers and the fire follows a zig-zag path instead of a straight path followed in BTK. In the early 1900s, these types of kilns were widely used in developed countries particularly in Germany and Australia. In Europe, the interior cross-section of the kiln used to be small in original zig-zag kilns (7.5 ft wide x 7.5 ft high) and the kiln used to have 16-20 chambers each 20-25 ft long. Fan draught was provided and the kiln operated on high draught at a very fast rate of fire travel (50-100 ft per day).

The High Draught or HD Kiln in India also follows the Zig-Zag firing concept. The Indian variation has several similarities with the Habla kiln. The kiln consists of a rectangular gallery which is divided into 24 chambers by providing temporary partition walls with green bricks. The wall of each chamber runs along the width of the gallery except one end, wherein a space of 60 to 65 cm is left for communication to next chamber. Draught is created by an induced draught fan with a 20-35 HP motor for proper combustion of fuel. Depending on the design capacity of a kiln, a chamber can hold 7,500 to 15,000 bricks. Normally, two chambers are fired per day and an output of 15,000 to 30,000 bricks per day can be obtained. When brought to full firing, the kiln operates on a draught of 50 mm hg.

Coal feeding process
in HD Kiln

Green brick setting
in HD kiln

Disadvantages being faced in Asian countries

Several problems are being encountered in the HD kiln in different Asian countries which are as follows:

•  Bricks remain too hot for handling at unloading point.
•  Dampers in the flues provided in the inner wall communicating with the main central flue being too close to the firing floor,
   are exposed to high heat resulting in rapid deterioration.
•  As the draught and hence the negative pressure in the kiln is several times more than that observed in fixed chimney BTK’s,
   the HD kiln is also more susceptible to air leakage. Most of the leakage takes place through wicket walls and
   through leaking valves and dampers.

Specific energy consumption of around 1.35 MJ/kg of fired bricks has been achieved in HD kilns being operated under full capacity. However it is generally observed that due to shortage of trained manpower and lack of exposure to proper operating practices the performance of HD kilns are much below the expected level of performance. Moreover, in most of the rural areas either electricity supply is not available at brick kiln sites or the supply is not reliable. Therefore installation of a diesel generator set for electricity generation becomes essential with a HD kiln which further adds to the complexity of the problem.

In recent years a new variant has also been developed as an alternative to the High Draught Kiln technology.

Unlike similar variants this kiln technology does not use any forced draught ; thereby relying only on natural draught. The structure is essentially similar to the fixed chimney Bull’s Trench Kiln. The loading pattern of green bricks has been changed to a zig-zag pattern to increase the travel path of the fire and thus utilize the heat more efficiently. However the feasibility of the same is yet to be established.