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a component of a boiler unit that superheats steam, that is, heats steam above its saturation temperature. A superheater consists of parallel-mounted steel tubes, with internal diameters of 20 to 60 mm, connected directly to a boiler drum or to input, output, and intermediate headers. Depending on the direction in which the steam moves relative to the flue gases, superheaters are said to have parallel flow (uniflow), counterflow, or mixed flow. They are classified as radiant, screen (semiradiant), or convection according to the location of the superheater in the boiler and the type of heat exchange that results.
Radiant superheaters are placed on the crown or walls of a furnace chamber, often between the tubes of waterwalls. The superheaters and waterwalls take up heat radiated by the burning fuel. Screen superheaters, which are separate flat screens of tubes connected in parallel, are mounted in the outlet from the furnace ahead of the convective part of the boiler; heat is transferred by both radiation and convection. Convection superheaters are placed in the flue of a boiler unit, usually after the screens or beyond the furnace; they consist of multirow banks of coils.
Superheaters composed entirely of convection stages are usually installed in average- or low-pressure boiler units having superheated steam no hotter than 510°C. High-pressure boiler units with considerable steam superheating use composite superheaters, which include convection, screen, and sometimes radiant sections.
The boiler units of steam electric power plants always have superheaters, because increased steam temperature means higher thermal efficiency for the plant. When steam pressure is 14 meganewtons per sq m (140 kilograms-force per sq cm) and higher, secondary (intermediate) superheaters are usually used along with basic (primary) superheaters in order to superheat steam that has been partly used up in a turbine.
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