Clay brick manufacturing process Part five
The fire is made to move for ward by “taking on” a row of fire holes at the front and dropping a row at the back, every 2 to 4 hours in an average sized kiln. In this way the fire moves right around the kiln every 10 to 14 days. The hot gases from the firing zone are drawn forward to preheat and dry out the green bricks, while the fired bricks are cooled down by the flow of air passing from the open wickets behind the firing zone.
The tunnel kiln is also a continual kiln, but the fire is stationar y while the bricks move past it on kiln cars. As in the T.V.A. kiln, the unfired bricks are preheated by the spent combustion gases. After the fire, heat released by the cooling bricks may be drawn off for use in the associated driers. With this interchange of heat, the tunnel kiln uses less fuel than the intermittent type of downdraught kiln. It has several other advantages. For example, cars can be loaded and unloaded in the open factory, and always at the same loading points, so that handling problems are simplified; and the kiln car acts as a conveyor belt so the bricks are fired as they pass through the firing zone.
In clamp kilns, some fuel is placed into the body of each brick. The bricks are packed into a pyramidshaped formation. The clamp has a layer of coal, equivalent to two courses of bricks, packed at the bottom. This layer (scintle) is set alight, it ignites the fuel in the base layer of bricks and progressively, each brick in the pack catches alight.
Clamp kiln firing can take up to three weeks and although the bricks might have finished burning in that time, it may take longer before they are cool enough to be sorted. Temperatures can be as high as 1 400˚C in the centre of the clamp.