Such watertube boilers in thermal power stations are also called steam generating units. To increase economy of the boiler, exhaust gases are also used to pre-heat combustion air blown into the burners, and to warm the feedwater supply in an economizer. (In large utility boilers, the feedwater is supplied to the steam drum and the downcomers supply water to the bottom of the waterwalls). Saturated water at the bottom of the steam drum returns to the lower drum via large-bore 'downcomer tubes', where it pre-heats the feedwater supply. Superheated steam is a dry gas and therefore is typically used to drive turbines, since water droplets can severely damage turbine blades. Superheated steam is defined as steam that is heated above the boiling point at a given pressure. In some services, the steam passes through tubes in the hot gas path, (a superheater) to become superheated. Here, saturated steam is drawn off the top of the drum. The heated water/steam mixture then rises into the steam drum.
In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam. Fuel is burned inside the furnace, creating hot gas which boils water in the steam-generating tubes. Type of furnace generating steam Schematic diagram of a marine-type watertube boilerĪ high pressure watertube boiler (also spelled water-tube and water tube) is a type of boiler in which water circulates in tubes heated externally by the fire.
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