with the amount of fuel needed to be added to this cycle as
内的二 h4 -h3 (2-31)
ηb(LH. )
The enthalpy leaving the turbine is
h5二((内的α +内的)h5α +内的sh5s)/(内的α +内的+内的s)(2-32)
Thus， the total work by the turbine is given by
Wt二(内的α +内的s +内的)(h4 -h5)ηt (2-33)
And the overall cycle efficiency is 二 Wt -Wc(2-34)
ηcyc内的(LH. )
The cycle leads to an increase in output work and an increase in overall thermal efficiency.
Figure 2-21 show the effect of 5% by weight of steam injection at a turbine inlet temperature of 2400 oF (1316 oC) on the system. With about 5% injec-tion at 2400 oF (1316 oC) and a pressure ratio of17:1， an 8.3% increase in work output is noted with an increase of about 19% in cycle efficiency over that experienced in the simple cycle. The assumption here is that steam is injected at a pressure of about 60 psi (4 Bar) above the air from the compressor discharge and that all the steam is created by heat from the turbine exhaust. Calculations indicate that there is more than enough waste heat to achieve these goals.
Figure 2-22 shows the effect of 5% steam injection at different tempera-tures and pressures. Steam injection for power augmentation has been used for many years and is a very good option for plant enhancement. This cycle"s great advantage is in the low production level of nitrogen oxides. That low level is accomplished by the steam being injected in the compressor dischargediffuserwall， well upstream from the combustor， creating a uniform mixture of steam and air throughout the region. The uniform mixture reduces theoxygen content of the fuel-to-air mixture and increases its heatcapacity， which in turn reduces the temperature of the combustion .one and the NOx formed. Field tests show that the amount of steam equivalent to the fuel flow by weight will reduce the amount of NOx emissions to acceptable levels. The major problem encountered is corrosion. The corrosion problem is being
60
50
40
Simple Cycle Gas Turbine
30
5% Steam Injection
20
10
0
Figure 2-21. Comparison between 5% steam injection and simple cycle gas turbine.
60.00
50.00
1800
40.00
2000 2200
30.00
2400
2600
2800
20.00
3000
10.00
Figure 2-22. The performance map of a steam injected gas turbine.
investigated， and progress is being made. The attractiveness of this system is that major changes are not needed to add this feature to an existing system. The location of the water injector is crucial for the proper operation of this system and cycle.
The Evaporative Regenerative Cycle
Thiscycle， as shown in Figure2-23， is a regenerative cycle with waterinjection. Theoretically， it has the advantages of both the steam injection and regenerative systems reduction of NOx emissions and higher efficiency. The work output of this system is about the same as that achieved in the steaminjectioncycle， but the thermal efficiency of the system is much higher.
A high-pressure evaporator is placed between the compressor and the regenerator to add water vapor into the air steam and in the process reduce the temperature of this mixed stream. The mixture then enters the regen-erator at a lower temperature， increasing the temperature differential across the regenerator. lncreasing the temperature differential reduces the tempera-ture of the exhaust gases considerably so that these exhaustgases， otherwiselost， are an indirect source of heat used to evaporate the water. Both the airand the evaporated water pass through the regenerator， combustion cham-ber， and turbine. The water enters at 80 oF (26.7 oC) and 14.7 psia (1 Bar)through a pump into the evaporator， where it is discharged as steam at the same temperature as the compressor discharged air and at a pressure of 60 psia (4 Bar) above the compressor discharge. lt is then injected into the air
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