The raw materials enter the furnace through one or two doghouses installed on the furnace sides, immediately next to the rear wall. This type of furnace can have sizes varying from 20 m² up to 180 m². Heating is done by two burner banks beneath the burner ports, located in the furnace rear wall. The combustion air is preheated up to a maximum of 1350°C in the regenerators, which are alternately flowed through by hot waste gases and combustion air. Therefore a periodical reversal of the firing side, typically between 20 and 30 minutes, is necessary. Due to the U‐shaped flame and waste gas path, the combustion gases in the furnace have a relatively long residence time, which produces good energy utilisation.