Page 132 - Advanced Gas Turbine Cycles

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SPECIFIC WORK
Fig. 6.17. Overall efficiency and specific work of dry and wet cycles compared.

To this figure, some of the calculations carried out by various authors for wet cycles
have been added: RWI and HAT [9]; REVAP [lo]; CHAT [ll]; TOPHAT [12].
The assumptions made by the various authors (viz. polytropic efficiencies, combustion
pressure loss and temperature ratio, etc.) are all roughly similar to those used in the
calculations of uncooled dry cycles. Some modest amounts of turbine cooling were
allowed in certain cases [9] but the effect of these on the efficiency should not be large at
T,,, = 1250°C (see later for discussion of more detailed parametric calculations by some
of these authors).
The RWI and HAT cycles may then be seen as ‘wet’ developments of the intercooled
regenerative dry cycle. These evaporative cycles show an increase in efficiency on that
of the dry CICBTX cycle-largely because of the increased turbine work (still approxi-
mately the same as the ‘heat supplied’) which is not at the expense of increased
compressor work. The HAT cycle then offers an appreciable reduction in the exergy loss
in the evaporative process compared with RWI, thus providing an added advantage in
terms of the thermal efficiency. REVAP also provides a similar advantage on efficiency.
The TOPHAT cycle has the advantage of increased turbine work together with reduced
compressor work.
The CHAT cycle may be seen as a low loss evaporative development of the dry
intercooled, reheated regenerative cycle [CICBTBTX]. It offers some thermodynamic
advantage-increase in turbine work (and ‘heat supplied’) with little or no change in the
compressor work, leading to an increased thermal efficiency and specific work output.
In summary, all these ‘wet’ cycles may be expected to deliver higher thermal
efficiencies than their original dry equivalents, at higher optimum pressure ratios. The
specific work quantities will also increase, depending on the amount of water injected.