4.2.1 Feedheating plant stages — superheat cycles
The optimum number of feedheating stages, in strict terms of cycle efficiency, could be as high as the number of steam expansion stages in the associated turbine cylinders. But it is not practical, in engineering terms, to provide steam extraction points at each stage in the cylinders, because of the casting/casing complexity that would arise and the additional and excessive length of the complete turbine, with correspondingly long rotors.
As with most engineering designs, a compromise between the academic optimum design and a design which is practical; producible and commercially viable is necessary.
The type of power plant with which the turbine plant is associated also influences the number of feed-heating stages because the steam-raising plant economics interact with the turbine plant.
For electricity generation in the UK, the following steam-raising plant sources have featured in recent years:
- Advanced gas-cooled reactors (AGRs).
- Pressurised water reactors (PWRs).
Coal-fired and oil-fired plants yield similar numbers of feedheating stages as optimum, about three LP stages, a heating/de-aerating stage and two HP stages, leading to a final feedwater temperature of about 250°C.
An optimisation technique is used to determine the number of feedwater heaters, with practicalities still dominating.
In a world of changing economics in the field of fuel supply, an upset in prices, upward or downward, such as experienced with oil supply prices can lead to a significant difference in turbine and turbine plant configuration. It is necessary at the inception of a power station project to revalue cycle efficiency in the light of prevailing fuel prices and best predictions.
AGR plants are found to be most economic with a moderate final feedwater temperature of about 150°C. This is because the lower temperature produces a wider 'approach temperature' to the reactor coolant gas, and a greater 'log mean temperature difference' (LMTD) for the steam generators associated with the reactor. This wider LMTD, produced by the feed-water temperature of 150°C, is beneficial in that the steam generators, encompassed within the concrete pressure vessel with the reactor, can be smaller, so the pressure vessel is smaller and lower in capital cost.
The final feedwater temperature is 'optimised' — i.e., the value of cycle efficiency, costed at a rate appropriate to the use of nuclear fuel over the life of the plant, is related to the capital cost variations of steam generators, etc. The lowest sum of lifetime operating cost and capital cost determines the optimum design, which includes the corresponding final feedwater temperature as a design parameter.
The outcome of this AGR plant optimisation is that three LP feedwater heaters are used, in conjunction with a fourth combined heating and de-aerating stage, to provide a final feedwater temperature of about 150°C to the complete exclusion of HP feedwater heaters. The boiler feed pumps draw their supply of water from an elevated tank, forming the combined heating and de-aerating stage, and deliver the water directly to the 'economiser' section of the reactor steam generators.
Economic systems evolved during the past two decades for both fossil-fired and AGR plants involved 'back-pressure turbines' (driving the boiler feed pump), with one or two steam extraction points for HP feed-water heaters.
The driving steam for the boiler feed pump turbine is taken from the exhaust of the HP cylinder of the main turbine, and is therefore steam which has been expanded and has lost part of its superheat (Fig 1.64).
Steam extracted from the boiler feed pump turbine is therefore relatively low in superheat and the heat to be surrendered to the feedwater is mainly the latent heat which is beneficial to cycle efficiency.
Various main plant manufacturers have evolved slightly different systems, such as either one or two extraction points on a boiler feed pump turbine, with a corresponding number of HP feedwater heaters, depending on their manufacturing costs and their relationship to the then prevailing worth of cycle efficiency. The final HP feedwater heater has, in every case, used steam exhausted from the HP cylinder of the main turbine (i.e., the same point in the cycle as the supply to the feed pump turbine).
For large capacity plant, HP feedwater heaters of the 'tubeplate' type (as illustrated later in Fig 1.69), cannot be made sufficiently large to perform the entire feedheating plant stage duty because of manufacturing limitations. These limitations involve tube plate diameter and thickness and tube hole drilling length.
The provision of heaters in pairs has proved necessary at each heating stage to permit the stage duty to be performed. Each 'line' (or bank, or 'string') of heaters can be by-passed in the event of a fault to permit continuity of feedwater flow to the boiler.
The heat transfer surface area for heaters (of all types), and therefore the overall size of the heater, is carefully optimised. The optimisation involves manufacturing cost versus the lifetime value of efficiency based on fuel cost and the predicted utilisation for the type of plant.
For relatively high fuel cost applications, an additional heat transfer section is incorporated in heaters as a 'drains cooling' section. The condensate of the heating steam is cooled to a temperature lower than saturation temperature by the ingoing feedwater, thereby increasing the effectiveness of the heater in the cycle.
Further features of feedheating plant — and suitable for any type of main plant — are those of 'pressure cascading' and 'drains pumping'. Heating steam, after being condensed in a heater, is led, as drains, to a lower pressure heater where it 'flashes-off and surrenders part of its remaining heat to the lower pressure heater. This principle can be applied to all heaters in a bank as a 'cascade'.
Drains pumping involves collecting drains after cascading and pumping them back into the condensate feed system at a point that closely corresponds to their temperature. Figure 1.65 illustrates both cascading and pumping for the two LP heaters.