. "The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace through a superheater to become superheated. Superheated steam is defined as steam that is heated above the boiling point at a given pressure. Superheated steam is a dry gas and therefore used to drive turbines, since water droplets can severely damage turbine blades. Cool water at the bottom of the steam drum returns to the feedwater drum via large-bore 'downcomer tubes', where it pre-heats the feedwater supply. (In large utility boilers, the feedwater is supplied to the steam drum and the downcomers supply water to the bottom of the waterwalls). To increase economy of the boiler, exhaust gases are also used to pre-heat the air blown into the furnace and warm the feedwater supply. Such watertube boilers in thermal power station are also called steam generating units. The older fire-tube boiler design \u2013 in which the water surrounds the heat source and the gases from combustion pass through tubes through the water space \u2013 is a much weaker structure and is rarely used for pressures above psi (MPa). A significant advantage of the watertube boiler is that there is less chance of a catastrophic failure: there is not a large volume of water in the boiler nor are there large mechanical elements subject to failure."@en . . "Water-tube boiler"@en . . "The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace through a superheater to become superheated. Superheated steam is defined as steam that is heated above the boiling point at a given pressure. Superheated steam is a dry gas and therefore used to drive turbines, since water droplets can severely damage turbine blades."@en . . . . . . .