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
4.3.5.3 Evaporators with Vertical Internally Rifled Tubes
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)
130 4 Steam Power Stations for Electricity and Heat Generation Fig. 4.44 Wall tubing of a
single-pass boiler with vertical tubes in the furnace section (Source: Alstom Power)
and shutdowns, which would have a positive effect as regards both the fatigue of ele- ments and the fuel consumption, because start-up and shutdown losses are avoided.
In addition, the plant could do without a circulating system for low load.
Lower allowable mass flow densities also entail operational advantages. Inter- nally rifled tubes have higher pressure losses per metre tube length than plain tubes at the same mass flow and the same dimensions. The pressure losses of a steam-generating system equipped with internally rifled tubes decrease consider- ably though, because of the low mass flow density and the shorter tube length.
Whereas conventional evaporators involve pressure losses between 25 and 10 bar, it is possible to achieve levels of pressure loss as low as 5 bar by using vertical internally rifled evaporator tubes (Franke et al. 1995).
Lower mass flow densities and vertically mounted tubes improve the buoyancy conditions in a once-through steam-generating system. The outcome is a so-called natural-circulation characteristic, where extra heating typically results in a better cooling of the tube, similar to a drum boiler.
The pressure gradient along the tube evolves through fluid friction and geodetic or hydrostatic pressure of the steam column. If the fluid friction, or friction loss, along the pressure gradient predominates at high mass flow densities, the additional heating leads to an increased steam fraction in the boiling water flow, to a higher flow velocity and to a rise of the pressure loss. Yet since the pressure difference is the same in all parallel tubes, the throughput of the more strongly heated tubes decreases.
4.3 Design of a Condensation Power Plant 131 Fig. 4.45 Throughput
characteristic of a tube with 25% extra heating (Wittchow 1995)
If the geodetic pressure loss predominates, the additional heating leads to higher mass flow densities. Due to the increased steam formation, the geodetic pressure loss of a tube with constant mass flow diminishes, because the steam column becomes lighter. The decrease of the geodetic pressure drop is higher than the rise of the friction loss. The pressure loss being given, however, the mass flow through the additionally heated tube rises (natural-circulation characteristic, see Fig. 4.45). The impact of the extra heating on the steam temperatures at the evaporator outlet is minimised by the self-regulating effect. This can be an advantage for the application of higher steam conditions, since the difference between the fluid temperature in the evaporator and the allowable material temperature may be smaller (Franke et al.
1993, 1995; Wittchow 1995). On the other hand, the counterbalance of the heating by the helically wound tubes does not apply.
Though one might expect a higher price for the tubes, financial benefits of the steam generator of up to 10% have been found, because the evaporator can be designed as a self-supporting construction (Wittchow 1995). Also, manufacturing and mounting are simpler than for helically wound tubing, which may be an advan- tage if the manufacturing is to be done in newly industrialised countries. Investi- gations in large-scale industrial plants with a test configuration of several vertical internally finned tubes mounted in parallel with helically wound tubes confirm the advantages of this concept (Franke et al. 1995; Kral et al. 1993).
In circulation steam generator construction, the more economical vertical tubes are used. The maximum heat flow density of about 0.4 MW/m2 common in coal- fired furnaces requires mass flow densities in the evaporator of around 600 kg/m2s, which have to be controlled by the natural circulation. Since the circulation ratio decreases with rising pressure in natural circulation, limits of approximately 185 bar arise for the maximum pressure in the evaporator, which corresponds to a live steam pressure before the turbine of about 175 bar. In a forced-circulation system, the circulation mass flow of 1,000–2,000 kg/m2s is controlled by the circulating pump (Strauß 2006).
132 4 Steam Power Stations for Electricity and Heat Generation 4.3.5.4 Evaporator Stability
Different operating modes of and uneven fuel flows to the burners of a burner group cause asymmetric firing conditions and non-uniform heat fluxes to the furnace walls.
Given their great lengths and temperature rises, evaporator tubes of forced once- through steam generators react to heating differences with differing temperatures in the tube wall and at the evaporator outlet. The helical winding still ensures a good heating balance because each of the parallel tubes runs along all four walls of the furnace (Franke et al. 1993).
The design of a steam generator has to ensure an even flow through all the parallel tubes of the evaporator as well. Impacts of additional heating on the flow conditions in the evaporator tubes depend on the characteristic response of the evaporator. If the extra heating of a tube causes the flow through it to diminish (once-through char- acteristic), the possible consequence is that the temperatures exceed the allowable limit for the material. For the previously described natural-circulation characteristic, a temperature rise through extra heating is counterbalanced by the self-regulating rise of the boiling water vapour flow in the tube in question. This characteristic depends on the mass flow density and the fluid friction of the fluid involved. Low mass flow densities (below 1,000 kg/m2s) favour the operator-preferred natural- circulation characteristic response (see Fig. 4.45) (Wittchow 1995).
One option for checking whether a stable and even flow in the evaporator has been achieved is to consult the characteristic curves of the evaporator (Baehr 1985).
Figure 4.46 shows the correlation between pressure loss and steam mass flow with the heating as a parameter. While the characteristics of tubes filled with a water flow
Fig. 4.46 Characteristic curves of the evaporator (Baehr 1985)
4.3 Design of a Condensation Power Plant 133 correspond to a second-order parabola, tubes which are filled by a flowing two-phase mixture, i.e. boiling water and steam, give a third-order curve.
An unstable flow occurs if the curve has a saddle-like behaviour, the consequence of which can be that three different mass flows evolve for the same pressure gradient.
If a mass flow has a lower rate than needed for cooling the tubes, the effect can be damage to the tubes. The stability of steam generators and measures to raise the stability are dealt with in detail in Doleˇzal (1990).