The central part of a shell boiler is the furnace where the flame burns out by releasing extensive energy. Corrugated furnaces are used for operating pressures of approx. 12 bar and more. Due to its shape the corrugated furnace is more stable than the plain furnace and thus can withstand the operating parameters of a high-pressure boiler. Corrugated furnaces made by Arvin Bokhar are also used in boilers made by other manufacturers.