Vertical arrangement Boilerconsists of a cylindrical shell with tubes inside. At the bottom of the firebox, is a grate. The firebox is provided with two or more inclined cross tubes F, F. When fuel is placed in the furnace and burns then hot flue gas is generated. Then these hot flue gases pass through the fire tube and heat surrounding water. When water becomes heated then it converted in saturated steam. You can call this saturated steam as primary steam. This saturated steam is stored at the steam chamber.All hot burn flue gases, smoke, and ash are release out from the boiler by a chimney. Boiler chimney creates a natural draft to increase boiler efficiency