Steam Power Stations for Electricity and Heat Generation
4.3 Design of a Condensation Power Plant
4.3.5 Design of the Steam Generator and of the Heating Surfaces
4.3.5.2 Design of the Evaporator
In the furnace, the radiant heat transferred to the evaporator wall determines, via the heat flux distribution, the mass flow density necessary for cooling the evaporator tubes. At the furnace wall, at about the height of the burners, the highest heat flux densities occur. They decrease towards furnace end, falling further afterwards, in the area of the convective heating surfaces (see Fig. 4.38). Increasingly, the heat is transferred by convection, which also occurs in the flue gas duct walls.
For the design of a steam generator, it is necessary to know beforehand the max- imum tube wall temperature, which is a function of the gas-side heat flux density (about 300–350 kW for hard coal firing systems), and the mass flow density of the steam – water mix. This is in order to avoid the allowable material temperatures being exceeded where the boiling crisis occurs. In once-through steam generators, the water/steam mass flow used for cooling decreases with the load, whereas the heat flux densities in the burner area decrease only to a minor extent, so it is the partial-load condition that determines the design. In general, the mass flow density of an evaporator with plain tubes lies between 700 and 800 kg/m2s at a minimum output of 30–40% (Franke et al. 1993). The mass flow density at the rated power lies between 2,000 and 2,500 kg/m2s.
4.3 Design of a Condensation Power Plant 127 Fig. 4.41 Inside wall
temperatures of a heated plain tube (Franke et al. 1993)
Figure 4.41 shows the inside wall temperatures as a function of the steam quality for plain tubes at different heat and mass flux densities. At a strong heating density of 450 kW/m2, a too-low mass flow density of 900 kg/m2s causes a strong rising of the tube wall temperatures. With reduced heat flux densities, such as occur in partial-load conditions, the temperature rise is less dramatic (Franke et al. 1993).
For a forced once-through steam generator with plain tubes, the high heat flux densities inside the furnace require a helically wound furnace wall (see also Sect. 4.2.2.3). The number n of the welded parallel tubes depends on the mass flow densityΦrequired for cooling at the partial load a, the inner tube diameter diand the steam flow ˙ms(Strauß 2006):
am˙s=n·πdi2
4 Φ (4.16)
or expressed in terms of n:
n = am˙s π
4di2Φ (4.17)
If the tubes, with the tube pitch tP, are welded together in parallel into a band, the equation for the bandwidth b of the helically wound wall applies, shown in Fig. 4.42:
b=ntp=tp· am˙s
π
4di2Φ (4.18)
Both the tube diameter and the tube pitch cannot be chosen freely. The tube diameter is confined at the upper limit by the heat transfer and the tube weight and at the lower limit by the pressure loss. Large steam generators have tube diameters usually between 30 and 50 mm. The tube pitch is influenced by the fin-bar width as well as the tube diameter. The allowable fin-bar width is between about 12 and
128 4 Steam Power Stations for Electricity and Heat Generation Fig. 4.42 Schematic drawing
of the helical winding (Doleˇzal 1990)
15 mm. The upper limit of the fin-bar width is given by the hazard of scaling of the fin bar, the lower one is given by constructive and economic points of view.
The bandwidth b is smaller than the furnace perimeter PF, so the band has to be wound helically around the furnace to completely line the furnace wall. The helix angleβ can be calculated as
β =arcsin (b/PF) (4.19)
The helix angle increases with the boiler size.
In the upper area of the steam generator, before the convective area begins, the helical winding transforms into vertical tubing. Because vertical tubing is more economical than helical winding, it should be designed to begin at the lowest pos- sible furnace height. The helical winding and vertical tubing are joined by clevises (see Fig. 4.43). In the vertical tubing, the mass flow density of the working fluid, then in a vapour state, is diminished by increasing the number of tubes by a factor of 3–4. If the transition to vertical tubing is carried out at a furnace height which is too low, it is possible that, with high gas-side heat flux densities and low mass flow densities of the cooling fluid, excessive tube wall temperatures arise in the vertical tubes. In contrast, when the helical winding is too high, non-uniform heating can have stronger effects due to the longer tubes of the helix, thus also causing the tube wall temperatures to exceed the allowable limit (Kefer et al. 1990).
Commonly, the helical tube winding finishes with the furnace, while the verti- cal tubing begins in the convective section. Where heat flux densities decrease, for instance when flue gas temperatures fall to between 750 and 800◦C, the number of tubes is diminished. It becomes possible to double the tube pitch, because the fin-bar temperatures are below the scaling temperature of the material. The larger tube pitch of the membrane walls facilitates the insertion of the superheaters and reheaters, which have narrowing tube pitches. The tube pitch in the upper section of the vertical pass – commonly 100–120 mm – defines the tube pitch in all wall areas.
Figure 4.43 shows the tube pitches of the reference power plant.
4.3 Design of a Condensation Power Plant 129 Fig. 4.43 Wall tubing of a
single-pass boiler with helical winding in the furnace section (Source:
Alstom Power)