It is now usual in public utilities to have only one boiler per turbine. This has made it possible to build even the largest power plant in unit design thus simplifying the piping systems and facilitating boiler and turbine control, especi~ in plants using steam reheating.
The appearance of water-cooled furnace walls, called water walls, eventually led to the integration of furnace, economiser, boiler, superheater, reheater, and air preheater into the modem steam generator. Water cooling is also used for superheater and economiser compartment walls and various other components, such as screens, dividing walls, etc.
Three design concepts of water tube boilers are illustrated in Fig. 6.16. Type A is a boiler with natural circulation as is type (a). Heat transfer to the water tubes around the walls is mostly by radiation from the fuel flame and less by convection from flue gases. Natural circulation is used up to steam pressures of approximately 180 bar, with separation of the steam from the water taking place in the boiler drum. Boilers with forced circulation by a special pump, originally known as La Mont boilers, are shown schematically as type B and also (b) in Fig. 6.16. They offer a certain amount of freedom in the arrangement of evaporator tubes and the boiler drum. Such boilers can be adapted to limitations in height and space. They are suitable for steam pressures up to 200 bar. Boilers operating at subcritical pressures «221.2 bar) which rely on a drum and recirculation, either natural or forced, are commonly known as drum boilers
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