Clay brick making machine-Brick machine-Baoshen brick making machine

Comparation of the energy performance of VSBK with that of fixed chimney kilns

It is interesting to compare the energy performance of VSBK with that of fixed chimney kilns. The superiority of VSBK in terms of energy, over fixed chimney kilns, is primarily due to following reasons:

a) Compared to surface heat losses equivalent to 0.15 -0.4 MJ/ kg of fired brick for fixed chimney kilns , the surface heat loss in VSBK is of the order of only 0.05 MJ/kg of fired brick. The area to be insulated in a VSBK is only 1/5th of the area to be insulated in a fixed chimney kiln for same level of production. Thus it is possible to provide good insulation in a VSBK. A VSBK is insulated by 2 m thick sand fill, along with a layer of firebricks on the inside of the shaft. b) Fixed chimney kiln also suffers from poor insulation at the base. Generally the floor of fixed chimney kilns is unpaved or paved with a single layer of ordinary bricks. An accurate determination of the heat loss to the ground is difficult but it is estimated to be in the range of 0.1 -0.3 MJ/kg of fired brick, the ground heat loss is highest at the start of the brick making season and goes on decreasing as the season progresses. In comparison ground heat loss in VSBK is almost negligible, because the contact area of the kiln with ground is small and also the high temperature zone of the kiln is not in direct contact with the ground. c) Fixed chimney kiln is a moving fire kiln, in which fire moves round the kiln while the bricks remain stationary. Typically 15 to 30 days are required to complete firing of one complete circuit. Due to the movement of fire, the kiln structure and ground gets heated and cooled periodically and never attains a steady state. The periodic heating and cooling of the kiln mass results in significant energy loss. In comparison, in a VSBK, firing zone is stationary and hence the kiln mass attains a steady state temperature profile few days after the initiation of fire.

VSBK has many similarities with modern tunnel kilns, which are extensively used for firing bricks in developed countries. In fact, VSBK can be considered as a vertical tunnel kiln.. However unlike a modern tunnel kiln, VSBK being a natural draught kiln, very precise control of air flow (hence precise control of combustion and heat transfer processes) is not possible in a VSBK.

Some other advantages of VSBK are: a) very less area requirement (15 -20 % compared to fixed chimney kilns for same production capacity); b) modularity, it is possible to increase the capacity in small increments by constructing new shafts; c) covered by roof and well insulated therefore not affected by changes in weather; d) the kiln can be started and stopped at very short notice and kiln stabilises in a very short time (3-4 days); e) better working conditions as dust pollution and exposure to high temperature is very less compared to fixed chimney kilns.

As the firing operation in VSBK is very fast, good supervision and control over firing process is required. If proper precautions are not taken then problems such as formation of cracks due to rapid cooling of fired bricks, incomplete combustion of internal fuel in bricks etc. can occur. The firing process has to be tailored according to the properties of the clay and the quality of product desired.