411
11.4 Performance of Power Gas Turbines with Enhanced Cycles
has an LP compressor with 6 stages and a HP compressor with 14 stages. The turbine is split into three parts: two stages HP, two stages IP and five stages LP. There
are three shafts. The LP part of the turbine is the power turbine driving the load
on the hot end side of the machine. Assuming that the compressor stages all have
the same pressure ratio, the pressure ratio of the LP compressor would be about 3,
which is a value in accordance with the results of the simulation methodology for
optimum intermediate pressure.
11.4.2 Expansion with Reheat
Adding reheat to the turbine, which means expanding the combustion gas in one or
more turbine stages and reheating it to the original TIT, is principally advantageous
for efficiency, if the intermediate heating occurs at an early stage of the expansion. The reason is similar to that with compressor intercooling, as is clear from
Fig. 11.19. The exponent of the pressure-temperature relation is
/
pg
t
c
Rh ∞ , which
is about 4.80. For pressure ratios 20 and 40, the efficiency of the additional cycle
is about 0.464 and 0.536 according to formula (11.34). These values are much less
attractive than with intercooling, but are still higher than for the basic cycle. Reheat
causes, of course, a higher need for cooling flow, which degrades somewhat the
resulting efficiency. Further, there are pressure losses in the second combustion
chamber. Whether or not efficiency gain is possible in reality is a matter of debate
in the literature [9]. With the simulation methodology used here and ignoring the
pressure drop in the second combustion chamber, a small efficiency gain is obtained
for larger pressure ratios. There is no gain for compressor pressure ratios lower than
about 20 and the gain is about 2.5 % for compressor pressure ratio 35 (results not
shown in a figure). Moreover, the gain increases somewhat with lower TIT. From
these simulation results, we conclude that reheat is feasible with high pressure ratio
and preferably with lower TIT. Turbine reheat may be combined with compressor intercooling. The effects approximately add. But there are no gas turbines with
combined intercooling and reheat.
At present, there is only one machine with reheat: Alstom GT24/GT26 (see website of Alstom). GT26 is the 3000 rpm version, with pressure ratio 35 and TIT about
1250 °C. The efficiency is 40 %. According to Fig. 11.14, the efficiency would be
about 42 % without reheat. This last value is probably somewhat optimistic, but
nevertheless shows that there is no efficiency gain by the reheat. For completeness,
we have to mention that the objective of Alstom is to reach good efficiency in combined-cycle mode, which requires that the turbine outlet temperature is high enough
(see Sect. 11.4.4). With reheat, the TOT of the GT24/GT26 comes at about 605 °C,
which is much higher than would be the case without reheat (probably around
425 °C according to Fig. 11.16). Reheat forms, of course, a technical complication.
In the GT24/GT26, the second combustion chamber is placed after the first turbine
stage and the turbine has five stages in total. The combustion gas is led through 24
rectangular orifices with triangular vortex generators placed in these. The fuel is in-
11.4 Performance of Power Gas Turbines with Enhanced Cycles
has an LP compressor with 6 stages and a HP compressor with 14 stages. The turbine is split into three parts: two stages HP, two stages IP and five stages LP. There
are three shafts. The LP part of the turbine is the power turbine driving the load
on the hot end side of the machine. Assuming that the compressor stages all have
the same pressure ratio, the pressure ratio of the LP compressor would be about 3,
which is a value in accordance with the results of the simulation methodology for
optimum intermediate pressure.
11.4.2 Expansion with Reheat
Adding reheat to the turbine, which means expanding the combustion gas in one or
more turbine stages and reheating it to the original TIT, is principally advantageous
for efficiency, if the intermediate heating occurs at an early stage of the expansion. The reason is similar to that with compressor intercooling, as is clear from
Fig. 11.19. The exponent of the pressure-temperature relation is
/
pg
t
c
Rh ∞ , which
is about 4.80. For pressure ratios 20 and 40, the efficiency of the additional cycle
is about 0.464 and 0.536 according to formula (11.34). These values are much less
attractive than with intercooling, but are still higher than for the basic cycle. Reheat
causes, of course, a higher need for cooling flow, which degrades somewhat the
resulting efficiency. Further, there are pressure losses in the second combustion
chamber. Whether or not efficiency gain is possible in reality is a matter of debate
in the literature [9]. With the simulation methodology used here and ignoring the
pressure drop in the second combustion chamber, a small efficiency gain is obtained
for larger pressure ratios. There is no gain for compressor pressure ratios lower than
about 20 and the gain is about 2.5 % for compressor pressure ratio 35 (results not
shown in a figure). Moreover, the gain increases somewhat with lower TIT. From
these simulation results, we conclude that reheat is feasible with high pressure ratio
and preferably with lower TIT. Turbine reheat may be combined with compressor intercooling. The effects approximately add. But there are no gas turbines with
combined intercooling and reheat.
At present, there is only one machine with reheat: Alstom GT24/GT26 (see website of Alstom). GT26 is the 3000 rpm version, with pressure ratio 35 and TIT about
1250 °C. The efficiency is 40 %. According to Fig. 11.14, the efficiency would be
about 42 % without reheat. This last value is probably somewhat optimistic, but
nevertheless shows that there is no efficiency gain by the reheat. For completeness,
we have to mention that the objective of Alstom is to reach good efficiency in combined-cycle mode, which requires that the turbine outlet temperature is high enough
(see Sect. 11.4.4). With reheat, the TOT of the GT24/GT26 comes at about 605 °C,
which is much higher than would be the case without reheat (probably around
425 °C according to Fig. 11.16). Reheat forms, of course, a technical complication.
In the GT24/GT26, the second combustion chamber is placed after the first turbine
stage and the turbine has five stages in total. The combustion gas is led through 24
rectangular orifices with triangular vortex generators placed in these. The fuel is in-