Book - School of Science and Technology

270 Boilers and firing equipmentFlueFlowGassupplyReturnBurnerFigure 10.7 Cast iron boiler with atmospheric gas burner (Broag)Steel ± sectionalThe place once held by wrought iron as a favourite material for boiler construction, due toits ductility and ability to resist corrosion, has largely been taken by mild steel as a resultof the disappearance of the manual skills in puddling. Steel is homogeneous in structure, incontrast to the more laminar nature of wrought iron, and is very liable to attack bysulphurous corrosion products.For service to hot water systems, mild steel sectional boilers were obtained in a varietyof forms to cover a wide range of duties from about 30 kW to 1.5 MW. However, steelsectional boilers have generally gone out of favour.Steel shell ± reverse flowFor long life, a mild steel water boiler requires that the system which it serves be designedto operate at a temperature outside the region where severe corrosive attack may beanticipated. Given these conditions, steel has advantages over cast iron in that it is moreversatile: the more modern developments of the last 40 years demonstrate this versatilityas shown in Figure 10.9.