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Boilers are equipment in which a fluid, normally water, is heated and usually evaporated and superheated to produce steam for power generation and/or heating purposes. The energy required is transferred from a heating fluid by combinations of radiation, convection and conduction by one of the following means: either the heating fluid is passed through tubes mounted within a drum which contains the water (a shell boiler or fire-tube boiler) or the water is contained in arrangements of tubes over which the heating fluid is constrained to flow (a Water-Tube Boiler). The heating fluid may be a product of combustion (as in Fossil Fuel-Fired Boilers), a chemical reaction or have been used to cool plant like a nuclear reactor or equivalent device, such as a chemical reaction vessel (as in Heat Recovery Boilers). Heat Recovery Boilers are also commonly found in combination with gas turbines where they are used to extract valuable energy from the exhaust gas.

An important class of equipment in which the heated fluid is not usually water is Reboiler, which is generally found in chemical plants in association with distillation columns.

The classification of boilers is often based on their application, viz., Industrial (for steam generation, heat and/or power where the upper limit is usually considered to be about 150 Te/hr or 50 MW(e)), or utility (for electricity generation where the size is usually in excess of 200 MW(e)).

Boilers may also be subdivided according to the type of fuel being burned or the type of firing system used. In furnace-fired boilers, fuel — either coal, oil or gas — is fired through a number of burners mounted in the walls of the furnace, whereas solid fuels can also be fired on grates or stokers at the base of the furnace, or they may be burned in fluidized beds. Similarly, waste materials may be burned on grates, in rotating drums or fluidized beds.

Boilers are also classified as recirculation boilers or Once-Through Boilers, depending on whether the heated fluid is partially evaporated or completely evaporated and superheated during a single pass through the unit.

Applications of boilers range from domestic heating units with capacities of a few kilowatts to utility boilers for power generation, which have capacities of up to 3000 megawatts of heat.

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