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These boilers, in contrast to recirculation or natural circulation units, are characterized by continuous flow paths from the evaporator inlet to the superheater outlet without a separation drum in the circuit. They are almost exclusively used for steam production in connection with utility electricity generation, and have been the most popular design in Europe and Scandinavia for many years.

A number of different detailed designs are offered by Siemens KWU (owners of the Benson Boiler patent), Sulzer and, in recent years, Japanese and American companies. All once-through boilers incorporate relatively small bore evaporator tubes (usually about 25 mm bore) which are generally arranged in a spiral fashion to form the furnace envelope. Two principal options are adopted for the section of the boiler after the furnace. Either a two-pass arrangement with a vestibule cage and downpass may be used (as shown in Figure 1) or a tower concept with the superheat, reheat and economizer sections above the furnace (as shown in Figure 2). The walls and roof of the boiler may consist of tubes forming part of the evaporator or superheater circuits and inside these are mounted banks of tubes acting as the principal superheat, reheater and economizer circuits.

Once-through boilers are generally associated with high pressure operation and the feed water enters at high sub-critical (>180 bar) or supercritical pressure whilst superheated steam leaves at a pressure some 20–30 bar lower. Sliding pressure operation is adopted to accommodate requirements of part-load running.

A typical Babcock two-pass once-through utility boiler. (Courtesy of Mitsui Babcock Energy Ltd.)

Figure 1. A typical Babcock two-pass once-through utility boiler. (Courtesy of Mitsui Babcock Energy Ltd.)

A typical Babcock once-through tower boiler. (Courtesy of Babcock Energy Ltd.)

Figure 2. A typical Babcock once-through tower boiler. (Courtesy of Babcock Energy Ltd.)

Furnaces may be fired by burners mounted in the front wall or by opposed firing with burners, normally, in the front and rear walls or by tangential firing achieved by slot-burners mounted in the corners to create a circulating flow which is claimed to have advantages for suppressing pollutant formation.

Because the water is evaporated to high quality in a once-through boiler, it is particularly important to guard against dryout occurring in high heat flux zones or to take other precautions against the phenomenon being associated with burnout. In spiral furnace boilers operating at sub-critical pressures, where dryout would take place at about 40% quality, it is possible to arrange for this position to coincide with an area of reduced heat flux above the primary combustion zone, such that the temperature rise in the tubing at dryout is limited. An alternative solution is to use rifled-bore tubing which, by creating centrifugal forces, causes more of the liquid phase to remain in contact with the tube wall, thus delaying dryout to higher qualities and/or enabling lower water velocities to be adopted.

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