412
11 Power Gas Turbines
jected by lances and surrounded by injected air. Fuel and air mix through the vortex
motion and combustion takes places in the second annular combustion chamber.
11.4.3 Recuperator
Incorporation of a recuperator, i.e. a heat exchanger where the turbine exhaust gas
heats the air delivered by the compressor, is advantageous for efficiency with lower
pressure ratios. For efficiency gain, the outlet temperature of the compressor must
be lower than the turbine outlet temperature. With a recuperator, it is always advantageous to provide intercooling on the compressor as well. By this, compressor
work is reduced and turbine cooling is more efficient. That there is always a gain
in efficiency may be understood from the observation that lowering of the temperature at compressor outlet due to the intercooling is compensated by heating by
the recuperator, with no need for an increase of the fuel flow rate. By this we also
understand that the most efficient intercooling is reached by the minimum of the
compressor work, which means equal pressure ratios before and after the intercooling. Figure 11.20 shows the efficiency by combined recuperator and intercooling.
The air is cooled halfway the compression until the inlet temperature (minimum
compressor work). The recuperator has effectiveness 0.7. Pressure loss is set to
0.02 bar at the air side of the intercooler and the same value is used at the air side
and combustion gas side of the recuperator. The efficiency comes to about 47 % for
TIT above 1300 °C, almost independent of the pressure ratio.
Reheat, enhancing the efficiency, could be added to a cycle with recuperator.
Intermediate cooling in the compressor combined with intermediate heating in
the turbine is advantageous, in principle. The reason is that an infinite number of
intermediate coolers and intermediate heaters results in a Carnot cycle. But, reheat
causes TOT to increase and the recuperator must be able to withstand the higher
7KHUPDOHIILFLHQF\
3UHVVXUHUDWLR
Fig. 11.20 Efficiency of a cycle with intercooling and recuperator (fuel CH 4 )
11 Power Gas Turbines
jected by lances and surrounded by injected air. Fuel and air mix through the vortex
motion and combustion takes places in the second annular combustion chamber.
11.4.3 Recuperator
Incorporation of a recuperator, i.e. a heat exchanger where the turbine exhaust gas
heats the air delivered by the compressor, is advantageous for efficiency with lower
pressure ratios. For efficiency gain, the outlet temperature of the compressor must
be lower than the turbine outlet temperature. With a recuperator, it is always advantageous to provide intercooling on the compressor as well. By this, compressor
work is reduced and turbine cooling is more efficient. That there is always a gain
in efficiency may be understood from the observation that lowering of the temperature at compressor outlet due to the intercooling is compensated by heating by
the recuperator, with no need for an increase of the fuel flow rate. By this we also
understand that the most efficient intercooling is reached by the minimum of the
compressor work, which means equal pressure ratios before and after the intercooling. Figure 11.20 shows the efficiency by combined recuperator and intercooling.
The air is cooled halfway the compression until the inlet temperature (minimum
compressor work). The recuperator has effectiveness 0.7. Pressure loss is set to
0.02 bar at the air side of the intercooler and the same value is used at the air side
and combustion gas side of the recuperator. The efficiency comes to about 47 % for
TIT above 1300 °C, almost independent of the pressure ratio.
Reheat, enhancing the efficiency, could be added to a cycle with recuperator.
Intermediate cooling in the compressor combined with intermediate heating in
the turbine is advantageous, in principle. The reason is that an infinite number of
intermediate coolers and intermediate heaters results in a Carnot cycle. But, reheat
causes TOT to increase and the recuperator must be able to withstand the higher
7KHUPDOHIILFLHQF\
3UHVVXUHUDWLR
Fig. 11.20 Efficiency of a cycle with intercooling and recuperator (fuel CH 4 )