Civil Engineering Reference
In-Depth Information
Table 9.3
Overall efficiency and ASR coefficient for backpressure and condensing turbines
Range ASR
Range
η
Range
η
η
T
G
Type of unit
Size (MW) AE (kJ/kg)
T
(%)
(%)
lb/kWh
kg/kWh
Backpressure
0.1-1
515.36
40 50 38
48
40.5 32.4 18.4 14.7
Single-valve/single-stage 1-5
515.36
65 75 62
71
24.9 21.6 11.3
9.8
Multivalve/multistage
5-25
515.36
75 80 71
76
21.6 20.3
9.8
9.2
Condensing
0.1-1
1,320.579
40 50 38
48
15.8 12.7
7.2
5.7
Single-valve/single-stage 3-20
1,320.579
70 76 67
72
9.0
8.3
4.1
3.8
Multivalve/multistage
2-50
1,320.579
76 80 72
76
8.3
7.9
3.8
3.6
Notes
Typical operating conditions for medium power backpressure turbine
See Case 4 in Table
9.2
with output pressure 0.34 MPa, 50 psi
Input steam 4.137 MPa, 600 psi
Typical operating conditions for condensing turbines
See Case 7 in Table
9.2
• The overall efficiency of a steam turbine diminishes with the output power rate,
so there is always a minimum useful value below which operation is not
economically worthwhile. Technical characteristics given by manufacturers
must be consulted;
• An increase in the initial steam pressure and/or steam temperature will increase
the amount of electric energy generated because of the increase in the enthalpy
inlet-outlet difference (see Table
9.2
). Since an increase of the inlet steam
temperatures generally results in an increase of the temperature of the steam
supplied to the process, a top limit exists to prevent outlet steam being too
highly superheated, as this would necessitate a de-superheating process before
delivery to the end users. In such a case, de-superheating water must be added
downstream of the turbine outlet; in consequence, the steam flow through
the turbine must generally be reduced by the quantity of water added, so that
the electric power generated is not increased by using higher inlet steam
conditions;
• An increase of the energy available for power generation is also possible if the
outlet pressure is reduced with a given set of initial steam conditions. The outlet
pressure must be the lowest value compatible with the end user needs;
• An increase in the power generated, with the same fuel consumption in the boiler
plant, is possible if feedwater is heated by using steam extracted from turbine
stages or exhausted from the process. The level of the power increase depends on
the number of heaters and on the temperatures;
• To avoid damage like the erosion of the turbine blades by liquid droplets, inlet
steam is always superheated. The outlet steam has generally a quality index of
not less than 90 % (see quality index in Sect.
6.4
).
