Figure 2-2.. The evaporative regenerative cycle.
stream in a fine mist where it is fully mixed. The governing equations are thesame as in the previous cycle for the turbine section， but the heat added is altered because of the regenerator. The following equations govern thischange in heat addition. From the first law of thermodynamics， the mixture temperature (T4) is given by the relationship:
T4二内的αcpαT2 +内的scpw(Ts -T3)-内的shf(2-35)内的αcpα +内的scps
The enthalpy of the gas leaving the regenerator is given by the relation-ship
h5二h4 +ηreg(h7 -h4)(2-36)
Similar to the regenerative cycle， the evaporative regenerative cycle has higher efficiencies at lower pressure ratios. Figures 2-24 and 2-25 show the performance of the system at various rates of steam injection and turbineinlet temperatures. Similar to the steam injectioncycle， the steam is injected
Figure 2-24. Performance map showing the effect of pressure ratio and steam flow rate on an evaporative regenerative cycle.
Figure 2-25. Performance map showing the effect of pressure ratio and steam flow rate on afixed steam rate evaporative regenerative cycle.
at 60 psi (4 Bar) higher than the air leaving the compressor. Corrosion in theregenerator is a problem in this system. When not completelyclean， regen-erators tend to develop hot spots that can lead to fires. This problem can be overcome with proper regenerator designs. This NOx emission level is low and meets EPA standards.
The Brayton-Rankine Cycle
The combination of the gas turbine with the steam turbine is an attractiveproposal， especially for electric utilities and process industries where steam isbeing used. ln thiscycle， as shown in Figure2-26， the hot gases from the turbine exhaust are used in a supplementary fired boiler to produce super-heated steam at high temperatures for a steam turbine.
The computations of the gas turbine are the same as shown for the simple cycle. The steam turbine calculations are:
Steam generator heat
4Q1二 h1s -h4s (2-37)
Turbine work
Wts二内的s (h1s -h2s)(2-38)
Pump work
Wp二内的s(h4s -H3s)/ηp (2-39)
The combined cycle work is equal to the sum of the net gas turbine work and the steam turbine work. About one-third to one-half of the design output is available as energy in the exhaust gases. The exhaust gas fromthe turbine is used to provide heat to the recovery boiler. Thus， this heat must be credited to the overall cycle. The following equations show the overall cycle work and thermal efficiency:
Overall cycle work
Wcyc二 Wta + Wts -Wc -Wp (2-40)
Overall cycle efficiency
Wcyc
η二 内的(LH. )(2-41)
This system， as can be seen from Figure2-27， indicates that the net work is about the same as one would expect in asteam injectioncycle， but the efficiencies are much higher. The disadvantages of this system are its highinitial cost. However， just as in the steam injectioncycle， the NO x content of its exhaust remains the same and is dependent on the gas turbine used. This system is being used widely because of its high efficiency.
Summation of Cycle Analysis
Figure 2-28 and 2-29 give a good comparison of the effect of the various cycles on the output work and thermal efficiency. The curves are drawn for a
60
55
50
1800
45
2000 2200
40
2400 2600 35
2800 3000 30
25
20
Temperature 2400°F (1315°C)
300.00
250.00
200.00
150.00
100.00
50.00
0 5 10 15 20 25 30 354045
Compressor Pressure Ratio
Figure 2-28. Comparison of net work output of various cycles temperature.
turbine inlet temperature of 2400 oF (1316 oC)， which is a temperature pres-ently being used by manufacturers. The output work of the regenerativecycle is very similar to the output work of the simplecycle， and the output work of the regenerative reheat cycle is very similar to that of the reheatcycle. The most work per pound of air can be expected from the intercooling， regenerative reheat cycle.
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