Page 131 - Advanced Gas Turbine Cycles

Chapter 6. ‘Wet’ gm turbine plants 103

6.4.3. Simpler direct water injection cycles

In the search for higher plant thermal efficiency, the simplicity of the two basic STIG
and EGT cycles, as described by Frutschi and Plancherel, has to some extent been lost in
the substantial modifications described above. But there have been other less complex
proposals for water injection into the simple unrecuperated open cycle gas turbine; one
simply involves water injection at entry to the compressor, and is usually known as inlet
fog boosting (IFB); the other involves the ‘front part’ of an RWI cycle, i.e. water injection
in an evaporative intercooler, usually in a high pressure ratio aero-derivative gas turbine
plant.
For the IFB plant the main advantage lies in the reduction of the inlet temperature,
mainly by saturating the air with a very fine spray of water droplets [13]. This, in itself,
results in an increased power output, but it is evident that the water may continue to
evaporate within the compressor, resulting in a lowering of the compressor delivery
temperature. A remarkable result observed by Utamura is an increase of some 8% in power
output for only a small water mass flow (about 1% of air mass flow). However, the
compressor performance may be adversely affected as the stages become mismatched
[ 141, even for the small water quantities injected.
In the second development, the emphasis is on taking advantage of the increased
specific work associated with evaporative intercooling and of the increased mass flow and
work output of the turbine. Any gain on the dry efficiency is likely to be marginal,
depending on the split in pressure ratio.

6.5. A discussion of the basic thermodynamics of these developments

All these cycles involve attempts to improve on the various ‘dry’ gas turbine cycles
discussed earlier in Section 6.3.
The basic STIG cycle improves on the dry CBT cycle through an element of
recuperation and by increasing the turbine work [2]. The ISTIG cycle provides a similar
improvement of the dry CICBTX cycle with the extra flow through the turbine. The
combined STIG and FAST cycles involve introducing a steam turbine giving extra
work and move the simple STIG cycle into the realms of the combined cycle plant (see
Chapter 7).
To further understand the ‘thermodynamic philosophy’ of the improvements on the
EGT cycle we recall the cycle calculations of Chapter 3 for ordinary dry gas turbine
cycles-including the simple cycle, the recuperated cycle and the intercooled and reheated
cycles.
Fig. 3.16 showed carpet plots of efficiency and specific work for several dry cycles,
including the recuperative [CBTX] cycle, the intercooled [CICBTX] cycle, the reheated
[CBTBTX] cycle and the intercooled reheated [CICBTBTX] cycle. These are replotted in
Fig. 6.17. The ratio of maximum to minimum temperature is 5: 1 (i.e. T,, = 1500 K); the
polytropic efficiencies are 0.90 (compressor), 0.88 (turbine); the recuperator effectiveness
is 0.75. The fuel assumed was methane and real gas effects were included, but no
allowance was made for turbine cooling.