3.1.5 Regenerative feedheating
To complete the cycle development of the steam cycle to that of Fig 1.21, the inclusion of regenerative feedheating must be discussed. Physically, a proportion of the steam is bled from various points on the turbine, which is then condensed to heat feedwater on its return to the boiler.
Figure 1.28 illustrates the improvement in thermal efficiency for a simple Rankine cycle (Fig 1.29), by virtue of the bled-steam releasing all of its heat to the feedwater, and little or none to the condenser. There will be a small loss of work available from the bled-steam not expanding in the turbine; however, this loss is outweighed by the gain in cycle efficiency.
The greater the number of feedheaters installed, the greater the improvement in thermal efficiency. However, the incremental gain for each additional feedheater reduces as the number of heaters increases. The methods of optimising the number of heaters for a particular size of plant are covered in Section 4.2. of this chapter.
The cycle has now been developed to the modern plant cycle that was first considered. Before going on to discuss this in detail, the T-S diagram for a reheat cycle with regenerative feedheating is given in Fig 1.30.