11.4 Reheat cycle
As has
been already mentioned in Secs. 11.1 and 11.2, if very
wet steam flows through a turbine, the hydrodynamic conditions for the turbine
blades and nozzles deteriorate sharply, causing a reduction of the internal
relative efficiency of the turbine, 
; this in turn leads to a
reduction of the effective (thermal) efficiency of the power plant as a whole.
For modern turbines the admissible dryness fraction of exhaust steam (at the
turbine exit) should be not less than x = 0.86 to 0.88.
As has already been
mentioned, one of the ways to reduce the wetness of exhaust steam at the
turbine exit is to superheat the steam in the boiler. Superheating leads to an
increase in the thermal efficiency of the cycle realized, and at the same
time, on the T-s diagram it shifts the point corresponding to the
conditions of exhaust steam to the right, into the region of greater dryness
fractions, as illustrated in Fig. 11.20a.
We have also found that
with the same superheat temperature the use of high pressures increases the
cycle areas ratio and, consequently, the thermal efficiency of the cycle, but
simultaneously a higher pressure diminishes the dryness fraction of the exhaust
steam and the internal relative efficiency of the turbine.
Fig. 11.20
One solution could be to
further increase the superheat temperature (the dotted line in Fig. 11.206). However, as was already mentioned, further temperature increases are
restricted by the properties of construction materials. The economic advantage
of this undertaking should also take into consideration increased investments
involved in building such a plant.
One way to reduce the final
wetness of exhaust steam is to reheat the steam. After the flow of steam,
performing work in the turbine, expands to some pressure p* > p2, it is extracted from the turbine and directed to flow into an additional
superheater, or reheater,
installed, for instance, in the boiler flue. In this reheater,
steam temperature rises to T*, and
then the steam flows back into the turbine, in which it expands to the pressure
p2. As
can be seen from the T-s diagram, shown in Fig. 11.20c, the final
wetness of steam diminishes.
The diagram of a power
plant with steam reheating is shown in Fig. 11.21, in
which the reheat superheater, or reheater,
is designated by RS. When reheating the steam, the turbine is a two-cylinder
unit, comprising a high-pressure turbine and a low-pressure turbine[1]
arranged on a common shaft along with a generator.
Figure 11.22 shows on a T-s diagram an internally reversible reheat cycle of
the steam power plant, practising superheating. It is clear that this cycle can be visualized as
consisting of two individual cycles, the conventional Rankine cycle (main) 5-4-6-1-2-3-5
and an additional cycle 2-7-8-9-2 (the
line 7-8 is an isobar p* = const). It can be assumed that the work done along the section 7-2 of the expansion adiabat in the main cycle is
spent to ensure adiabatic compression of the working medium on the section 2-7 of the additional cycle.
