Superheaters consist of a bank of tubes located within the boiler setting, through which saturated steam flows from the steam drum and is superheated by the same flue gas that generates steam in the boiler. They may be of the radiant design, convection design, or a combination of both, depending on the manner in which heat is transferred from the heater gases to steam.
Superheaters may have tubes in hairpin loops connected in parallel to inlet and outlet headers. They may also be of the continuous tube design in which each element has tube loops in series between inlet and outlet headers. In either case, they may be designed for drainage of condensate or may be in non-drainable pendent arrangements.
Non-drainable or pendant arrangements are very susceptible to tube failure due to overheating on start-up. Water collected in the pendent must be slowly vaporized to assure a flow path for the steam. If the boiler is heated too rapidly, some pendants will not clear of liquid; therefore, steam will not flow and the tube will overheat and fail. Special start-up instructions should be taken into consideration with this type of arrangement. Both straight and pendent arrangement superheaters are susceptible to failure due to steam impurities. When steam is used in processing operations, superheated steam may be required to obtain the desired process temperature. Most of the large-capacity, high-pressure steam generators, especially those used for power production, are equipped with superheaters. Superheated steam is also necessary for the most efficient production of power, especially when used in high-pressure, high-speed steam turbine drives.