372
11 Power Gas Turbines
are approximately: diameter 5 m and length 13 m. A diesel engine with these dimensions only attains about 10 MW.
A high temperature at the combustion chamber outlet creates great technological problems for the combustor and the turbine. The melting temperature of the
materials for the combustor and the turbine blades and vanes is about 1350 °C. With
regard to sufficient strength and avoidance of creep, the highest allowable metal
temperature of turbine blades is about 900 °C. Combustor and turbine parts thus
need intensive cooling and protection against the hot gas. The technological gas
turbine design is strongly determined by cooling and heat protection aspects, which
we discuss below.
Gas turbines share similarities with steam turbines, with cycles running very
analogously for both machines. The mean difference is that the steam turbine feed
pump replaces the gas turbine compressor. With regard to efficiency potential, this
is very advantageous for the steam turbine. A pressure increase of 250 bar (typical
pressure in a steam cycle) in water requires, with 90 % efficiency, about 27.5 kJ/
kg. A compression with pressure ratio 20 (typical for a gas turbine for electricity
generation) requires about 460 kJ/kg for air at 288 K at compression start (one may
verify with the formulae of Sect. 11.2: R
J
≈ 288
,
/ kg
dW dq d v dh dU
+ =
+ +
1 2
2
, c
J kg
dW dq d v dh dU
p ≈
+ =
+ +
1000
1 2
2
/
,
, polytropic
efficiency 0.90). An expansion with the same pressure ratio starting at 1350 °C
yields about 940 kJ/kg ( R
J kg
dW dq d v dh dU
≈
+ =
+ +
288
1 2
2
/
,
, c
J kg
dW dq d v dh dU
p ≈
+ =
+ +
1250
1 2
2
/
,
). The power consumed by
the pump is almost negligible with a steam turbine. The power of the compressor
with a gas turbine is about half the power produced by the turbine part. This proportion is nearly the same for other pressure ratios. Global gas turbine efficiency is thus
very sensitive to the efficiency of the components.
11.1.3 Example of a Power Gas Turbine
Figure 11.2 shows a large gas turbine for electrical power generation. It is a
so-called heavy duty type machine, which means a very robust design meant for
long production time per year (continuous operation is possible) and long periods
between overhaul (typically 20,000 h). These machines are built for high power
(from about 50 MW to about 450 MW). They are heavy, but this is no drawback
for stationary land-based use. The compressor and turbine components are axial.
The type shown is a version of the Siemens SGT6-5000F (Siemens Energy).
The power is 208 MW at 3600 rpm (for use at 60 Hz), with compressor pressure ratio 17 realised with 16 stages (pressure ratio per stage is about 1.20). The
turbine has four stages, with three of them cooled. For other examples, we refer
to the websites of the main manufacturers (search for Siemens, Alstom, General
Electric, Mitsubishi, in combination with gas turbines or energy). E.g., the most
recent version of the SGT6-5000F reaches 232 MW with pressure ratio 18.9 in
13 stages.
The compressor is composed of discs. These are visible on Fig. 11.2. Also visible on the figure is how the compressor blades are attached to the discs by foots
11 Power Gas Turbines
are approximately: diameter 5 m and length 13 m. A diesel engine with these dimensions only attains about 10 MW.
A high temperature at the combustion chamber outlet creates great technological problems for the combustor and the turbine. The melting temperature of the
materials for the combustor and the turbine blades and vanes is about 1350 °C. With
regard to sufficient strength and avoidance of creep, the highest allowable metal
temperature of turbine blades is about 900 °C. Combustor and turbine parts thus
need intensive cooling and protection against the hot gas. The technological gas
turbine design is strongly determined by cooling and heat protection aspects, which
we discuss below.
Gas turbines share similarities with steam turbines, with cycles running very
analogously for both machines. The mean difference is that the steam turbine feed
pump replaces the gas turbine compressor. With regard to efficiency potential, this
is very advantageous for the steam turbine. A pressure increase of 250 bar (typical
pressure in a steam cycle) in water requires, with 90 % efficiency, about 27.5 kJ/
kg. A compression with pressure ratio 20 (typical for a gas turbine for electricity
generation) requires about 460 kJ/kg for air at 288 K at compression start (one may
verify with the formulae of Sect. 11.2: R
J
≈ 288
,
/ kg
dW dq d v dh dU
+ =
+ +
1 2
2
, c
J kg
dW dq d v dh dU
p ≈
+ =
+ +
1000
1 2
2
/
,
, polytropic
efficiency 0.90). An expansion with the same pressure ratio starting at 1350 °C
yields about 940 kJ/kg ( R
J kg
dW dq d v dh dU
≈
+ =
+ +
288
1 2
2
/
,
, c
J kg
dW dq d v dh dU
p ≈
+ =
+ +
1250
1 2
2
/
,
). The power consumed by
the pump is almost negligible with a steam turbine. The power of the compressor
with a gas turbine is about half the power produced by the turbine part. This proportion is nearly the same for other pressure ratios. Global gas turbine efficiency is thus
very sensitive to the efficiency of the components.
11.1.3 Example of a Power Gas Turbine
Figure 11.2 shows a large gas turbine for electrical power generation. It is a
so-called heavy duty type machine, which means a very robust design meant for
long production time per year (continuous operation is possible) and long periods
between overhaul (typically 20,000 h). These machines are built for high power
(from about 50 MW to about 450 MW). They are heavy, but this is no drawback
for stationary land-based use. The compressor and turbine components are axial.
The type shown is a version of the Siemens SGT6-5000F (Siemens Energy).
The power is 208 MW at 3600 rpm (for use at 60 Hz), with compressor pressure ratio 17 realised with 16 stages (pressure ratio per stage is about 1.20). The
turbine has four stages, with three of them cooled. For other examples, we refer
to the websites of the main manufacturers (search for Siemens, Alstom, General
Electric, Mitsubishi, in combination with gas turbines or energy). E.g., the most
recent version of the SGT6-5000F reaches 232 MW with pressure ratio 18.9 in
13 stages.
The compressor is composed of discs. These are visible on Fig. 11.2. Also visible on the figure is how the compressor blades are attached to the discs by foots