It is an overground continuous kiln. In a tunnel kiln, the fire remains stationary while the bricks move slowly through the tunnel.
The fire is situated in the center and the brick slowly proceeds through the tunnel on a train or a conveyor belt, ao that bricks which are at the beginning of the tunnel are slowly being preheated by the fire further on, and bricks which have passed the fire are allowed to cool down gradually.
It is made in a circular or oval shape.