The surface area for heat-exchange with the gas flame (5A,5B) is increased by column-type bodies (7).The column-type bodies form a labyrinth-type path for the hot gases (6) travelling upwards, and also slows them down. Peripheral passages can be provided, preferably at the top, for the collection and transport of the gases, particularly in a shaft-type firing system. A trough for frying oil can be formed at the top by welding
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