Steam Power Stations for Electricity and Heat Generation
4.2 Steam Generators
4.2.2 Evaporator Configurations
4.2.2.3 Once-Through Systems
In once-through systems, evaporation and a slight superheating take place in one stage in the evaporator tube. In contrast to circulation systems, the liquid – vapour phase transition point in the evaporator tube changes its position depending on the load or, for control processes, along with the change of the fuel-to-feed water flow ratio.
Well-known once-through steam generators are the Benson and the Sulzer boil- ers, or in Russia the Ramsin boiler. However, they are only rarely utilised now in their original design. The Benson boiler shown in Fig. 4.16 had an evaporator consisting of several vertical tubes with upward flow, mounted in series-connected banks, which at the same time defined the furnace perimeter. The liquid – vapour phase transition point was in the so-called final evaporative bank which, for salt deposit considerations, had been installed after the furnace in the convective heat exchanger range, with low heat transfer rates. In the Sulzer boiler, several parallel evaporator tubes meandering through the furnace formed the evaporator (see Fig. 4.17) (Doleˇzal 1990; Wauschkuhn 2001).
The difference to the Benson boiler was that this way each tube ran the entire length of the evaporator. Typical features of the Sulzer boiler were the wet operat- ing regime of the evaporator and the following downstream water separator, which was designed to separate a residual water content of 5%. The mineral-containing residual water was disposed of as boiler blowdown. The differences between the Sulzer and Benson boilers have vanished as the development of the boiler systems has advanced – modern once-through boilers are largely identical.
Once-through steam generators usually operate with circulating devices, which in the lower load range ensure flow stability in the evaporator and sufficient cooling
4.2 Steam Generators 91 Fig. 4.16 Benson boiler
(Doleˇzal 1990)
of the evaporator tubes (see Fig. 4.13). Water at the end of the evaporator that has not vaporised is separated as residual water, collected and recirculated. This circulation fixes the liquid – vapour phase transition point, in particular in the low load range.
In the upper load range, the evaporator is operated in once-through mode without the circulating pump and without water separation. In this case, the liquid – vapour phase transition point migrates, occurring after, or near the end of, the furnace, i.e.
Fig. 4.17 Sulzer boiler (Doleˇzal 1990)
92 4 Steam Power Stations for Electricity and Heat Generation in areas of low heat flux density. This is usually in the area where convective heat transfer has started (Adrian et al. 1986; Baehr 1985).
Evaporators of once-through steam generators today are made of tightly welded membrane tube walls. The mass flow density in the evaporator tubes has to be set such that excessive tube wall temperatures are avoided even with low heat trans- fer coefficients of the inside tube walls. Helically mounted evaporator tubes are a measure to ensure that the mass flow densities required for cooling are also suitable for large radiant heat fluxes in the furnace. Heating differences due to high heat fluxes in the wall centre and relatively low fluxes in the furnace corners, as well as unbalanced combustion, are compensated by each of the tubes running through all the walls. Another possible measure is internally finned evaporator tubes.
Membrane tube walls in a helically wound pattern, however, are not able to carry, without additional support, the weight of the furnace, the structural bracings, the insulation and the water contained within it, as well as the possible fouling and slagging deposits. Nor can they sustain tubular offsets, caused by changes in the furnace pressure. The load is borne by vertical sling straps, which are welded on.
Tubular offsets of the membrane walls caused by (furnace) pressure forces work- ing vertically against the tube walls are limited by structural sling straps mounted horizontally around the perimeter at different heights. In the upper section of the furnace, it is possible to change to vertical tubing under lower heat flux densities (see Fig. 4.18) (Franke et al. 1993, 1995; Wittchow 1995).
In Germany, all large-scale steam generators since 1960 have been designed as once-through systems. Disadvantages of such systems are the more complex con- struction of the furnace walls, due to the wound pattern of the tubes; the higher auxiliary energy demand for the feed water pressure increase; and the higher control requirements. Despite lesser material requirements, the more complicated manufac- turing and assembly result in higher costs compared to drum boilers with vertical tubing. The fact, however, that once-through systems have no thick-walled compo- nents, such as the drum in circulation systems, offers advantages such as the ability to operate with a sliding pressure, faster start-up and a greater flexibility to make fast load changes. This is important, especially for large coal-fired power stations
Fig. 4.18 Evaporators with wound-pattern furnace walls and with vertical tubing for once-through steam generators (Wittchow 1995)
Furnace with vertical internally finned tubes Furnace with wound-pattern
walls and girders
4.2 Steam Generators 93 which are used for medium load operation, where some stations are started up and shut down daily. In contrast to circulation systems, once-through systems can also be applied in advanced steam generators with higher steam parameters, because it is the only system suited to supercritical pressures. Higher steam temperatures, though, can also be used in circulation systems (Stultz and Kitto 1992; Doleˇzal 1990; Strauß 2006; Wittchow 1982).
A further development of once-through steam generator technology is the use of internally finned tubes. The more intensive cooling of these tubes allows lower mass flow densities in the evaporator, thus making it possible to use vertical evaporator tubes. In contrast to the state-of-the-art wound-pattern walls, this new construction involves both lower costs and a number of additional operational advantages, which will be discussed in Sect. 4.3.5.3 in the context of evaporator design. For future power stations with advanced steam parameters, the forced once-through circulation concept has many advantages (Wittchow 1995; Lehmann et al. 1996).