(2) compressor efficiencies are thesame， (3) pressure ratios in both compres-
Y
sors are the same and equal to (P2/P1).
The intercooled simple cycle reduces the power consumed by the compressor. A reduction in consumed power is accomplished by cooling the inlet temperature in the second or other following stages of the com-pressor to the same as the ambient air and maintaining the same overall pressure ratio. The compressor work then can be represented by the follow-ing relationship:
Wc二(hα -h1)+(hc -h1)(2-28)
This cycle produces an increase of 30% in work output， but the overall efficiency is slightly decreased as seen in Figure 2-15. An intercooling regen-erative cycle can increase the power output and the thermal efficiency. This combination provides an increase in efficiency of about 12% and an increasein power output of about30%， as indicated in Figure 2-16. Maximumefficiency， however， occurs at lower pressure ratios， as compared with the simple or reheat cycles.
45 40 35 30 25 20 15 10 5 0
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Figure 2-15. The performance map of an intercooled gas turbine cycle.
Figure 2-16. Performance map showing the effect of pressure ratio and turbine inlet temperature on an intercooled regenerative cycle.
The Reheat Cycle
The regenerative cycles improve the efficiency of the split-shaftcycle， but do not provide any added work per pound of air flow. To achieve this lattergoal， the concept of the reheat cycle must be utili.ed. The reheatcycle， as shownin Figure2-8， consists of a two-stage turbine with a combustion chamber before each stage. The assumptions made in this chapter are that the high-pressure turbine"s only job is to drive the compressor and that the gas leaving this turbine is then reheated to the same temperature as in the first combustor before entering the low-pressure or power turbine. This reheat cycle has an efficiency which is less than that encountered in a simplecycle， but produces about 35% more shaft outputpower， as shown in Figure 2-17.
The Intercooled Regenerative Reheat Cycle
The Carnot cycle is the optimum cycle and all cycles incline toward this optimum. Maximum thermal efficiency is achieved by approaching the isothermal compressionand expansion of the Carnotcycle， or by inter-cooling in compression and reheating in the expansion process. Figure 2-18shows the intercooled regenerative reheatcycle， which approaches this opti-mum cycle in a practical fashion.
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Figure 2-17. The performance of a reheat gas turbine cycle.
This cycle achieves the maximum efficiency and work output of any of the cycles described to this point. With the insertion of an intercooler in thecompressor， the pressure ratio for maximum efficiency moves to a muchhigher ratio， as indicated in Figure 2-19.
The Steam Injection Cycle
Steam injection has been used in reciprocating engines and gas turbines for a number of years. This cycle may be an answer to the present concern
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Figure 2-19. The performance of an inter-cooled. regenerative. reheat cycle.
with pollution and higher efficiency. Corrosion problems are the major hurdle in such a system. The concept is simple and straightforward: water is injected into the compressor discharge air and increases the mass flow ratethrough theturbine， as shown in the schematic in Figure 2-20. The steam being injected downstream from the compressor does not increase the work required to drive the compressor.
The steam used in this process is generated by the turbine exhaust gas.Typically， water at 14.7 psia (1 Bar) and 80 oF (26.7 oC) enters the pump andregenerator， where it is brought up to 60 psia (4 Bar) above the compressor discharge and the same temperature as the compressor discharged air. The steam is injected after the compressor but far upstream of the burner to create a proper mixture which helps to reduce the primary .one temperature in the combustor and the NOx output. The enthalpy of State 3 (h3) is the mixture enthalpy of air and steam. The following relationship describes the flow at that point:
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