Superheaters and Reheaters
- They consist of a group of tubes made of special steel which may be carbon steel, carbon- molybdenum steel or stainless steel, depending upon whether the operating temperature is 950°F, 1050°F or 1200°F respectively. The outside diameter varies from 25 mm to 64 mm.
- Tube bundle location and arrangement is dictated by type of firing, required steam temperature and steam temperature characteristic. The superheater tubes are heated by the heat of combustion gases during their passage from the furnace to the chimney.
Superheaters are of two types
(i) Radiant superheaters
(ii) Convection superheaters
- Radiant type superheater is located in the furnace between furnace water walls. It absorbs heat from the burning fuel the superheat falls with the increase in steam output.
- Convection superheater is located well back in the boiler tube blank. It receives its heat entirely from flue gases through convection and gives rising characteristics. The temperature of the superheater increases with the increase in steam output.
- Convection superheaters are more commonly used. Desired control of characteristics is obtained by :
- Proportioning and locating surfaces in series.
- Using internal dampers on boiler gas side.
- Temperature rating by water OR
- Supplementary burners.
- Heat transfer rates of 10 to 12 B.T.U. per hour per square ft. per degree temperature difference are representative.
- The steam is superheated to the highest economical temperature not only to increase the efficiency but also to have following advantages :
Advantages of Superheated Steam in Thermal Power Plant
- Owing to its high internal energy, there is reduction in requirement of steam quantity for a given output of energy which reduces the turbine size.
- Superheated steam being dry, turbine blades remain dry so mechanical resistance to the flow over them is small, which increases the efficiency of plant.
- There is no corrosion and putting at the turbine blades due to dryness of steam.