Environmental Engineering Reference
In-Depth Information
Figure 3.4
A 1500-MW steam turbine with the upper casing removed, showing the rotor stages. From right
to left: the high-pressure, intermediate-pressure, and low-pressure stages. (By permission of R. Hoche.)
For Rankine cycles using water as the working fluid, the temperature of the steam from the
boiler seldom exceeds 550
◦
C, and then only for boilers that operate at a high pressure. A high-
pressure and high-temperature steam cycle is one for which the steam pressure and temperature
exceed the values at the
critical point
of water.
16
The thermodynamic efficiency of a steam cycle is
improved if a high pressure and temperature are used. But there are additional steps that may be
taken to improve the efficiency of the basic Rankine cycle. One, called
superheating,
is illustrated
in Figure 3.3. Here the steam evolved by boiling the water is further heated in the superheater
section of the boiler to a temperature
T
5
that is higher than the boiling point
T
4
corresponding
to the boiler pressure
p
4
p
3
. Another is to reroute steam leaving the high-pressure turbine
(see Figure 3.4) back to the boiler for
reheating
to
T
5
, whereupon it returns to the lower-pressure
turbine stages, producing more turbine power than if it had not been reheated. A third improvement
involves extracting a fraction of the steam from the high-pressure turbine exhaust and using it to
heat the water leaving the feedwater pump, at temperature
T
2
. This latter is called
regenerative
feed water heating
. The net effect of any or all of these alternatives is to increase the average
temperature of the working fluid during which heat is added in the boiler. Using the Carnot cycle
as the paradigm, this increases the Rankine cycle thermodynamic efficiency. Depending upon
the circumstances, employing all these measures can add up to 10 points of efficiency to the
basic cycle.
The thermodynamic efficiency of the ideal Rankine cycle is in the range of 30-45%, depending
upon the details of the cycle complexity. But actual steam plants have lower-than-ideal efficiencies,
for several reasons. The steam turbine and feed water pumps are not 100% efficient, resulting in
=
16
The critical point is the condition for which the liquid and vapor phases are indistinguishable. For water the
critical pressure and temperature are 221.3 bar (
=
22.13 MPa
=
3210 psi) and 374.2
◦
C(
=
705
.
6
◦
F).
