413
11.4 Performance of Power Gas Turbines with Enhanced Cycles
combustion gas temperature. This is a technical problem. There are no gas turbines
with recuperator and reheat. The cycle with recuperator and intercooling frequently
occurs in marine applications. Implementation of intercooling is easily realised due
to the availability of cooling water. Good efficiency is already attained with a lower
TIT and low pressure ratio. For completeness, we have to mention that many marine gas turbines are aero-derivatives and that these have a large pressure ratio. The
smaller gain in efficiency with intercooling for large pressure ratio is less attractive
and the intercooler requires space. Military ships are therefore often equipped with
simple-cycle gas turbines.
11.4.4 Combined Gas and Steam Cycles
In a combined-cycle system, the exhaust gas of a gas turbine is used for steam
generation in a recovery boiler and led to a steam turbine. For good efficiency of
the steam cycle, the inlet temperature of the steam turbine must be sufficiently high.
Figure 11.21 sketches the cooling of the combustion gas and the heating of the
steam in the T-s diagram of the gas, with rescaled entropy for steam such that gas
and steam temperatures correspond to each other, as on the surfaces of the steam
generator. The dashed line is the steam path for a single-pressure steam generator.
A so-called pinch point forms, which is the smallest temperature difference at the
beginning of the evaporation of the water. Due to the pinch point, the temperature
difference is quite high between the steam leaving the steam generator and the gas
entering it. A similar phenomenon occurs at the outlet side of the gas. These differences may be reduced by using two pressure levels in the steam generator, as shown
by the full lines. In practice, even three pressure levels with reheat between the high
pressure part and the intermediate pressure part are used. With a triple-pressure
Fig. 11.21 Single pressure
level ( dashed line) and dual
pressure level ( full line)
steam generation in a recovery boiler
11.4 Performance of Power Gas Turbines with Enhanced Cycles
combustion gas temperature. This is a technical problem. There are no gas turbines
with recuperator and reheat. The cycle with recuperator and intercooling frequently
occurs in marine applications. Implementation of intercooling is easily realised due
to the availability of cooling water. Good efficiency is already attained with a lower
TIT and low pressure ratio. For completeness, we have to mention that many marine gas turbines are aero-derivatives and that these have a large pressure ratio. The
smaller gain in efficiency with intercooling for large pressure ratio is less attractive
and the intercooler requires space. Military ships are therefore often equipped with
simple-cycle gas turbines.
11.4.4 Combined Gas and Steam Cycles
In a combined-cycle system, the exhaust gas of a gas turbine is used for steam
generation in a recovery boiler and led to a steam turbine. For good efficiency of
the steam cycle, the inlet temperature of the steam turbine must be sufficiently high.
Figure 11.21 sketches the cooling of the combustion gas and the heating of the
steam in the T-s diagram of the gas, with rescaled entropy for steam such that gas
and steam temperatures correspond to each other, as on the surfaces of the steam
generator. The dashed line is the steam path for a single-pressure steam generator.
A so-called pinch point forms, which is the smallest temperature difference at the
beginning of the evaporation of the water. Due to the pinch point, the temperature
difference is quite high between the steam leaving the steam generator and the gas
entering it. A similar phenomenon occurs at the outlet side of the gas. These differences may be reduced by using two pressure levels in the steam generator, as shown
by the full lines. In practice, even three pressure levels with reheat between the high
pressure part and the intermediate pressure part are used. With a triple-pressure
Fig. 11.21 Single pressure
level ( dashed line) and dual
pressure level ( full line)
steam generation in a recovery boiler