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Publication Title | Thermodynamic Design Considerations Steam injected gas turbines

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injected gas turbine since they each affect the amount of steam that can be generated; this amount is the upper bound for the set of possible design points for the system. The needs of the end-user generally dictate the selection of a design point for which less steam is consumed.
THE BASIS FOR THIS STUDY
The engine in this study is composed of a two-spool gas generator with a free power turbine; for simplicity, it can be called a dual-spool turboshaft engine. Figure 1 provides the nomenclature and station designations on a schematic diagram for this engine.
COMPRESSORS TURBINES
are presented non-dimensionally to allow flexibility for application to other designs; Table 1 provides the specific basis for these results.
TABLE 1
APPROXIMATE DESIGN-POINT CONDITIONS
LPC - LOW-PRESSURE COMP. HPT - HIGH-PRESSURE TURB. HPC - HIGH-PRESSURE COMP. LPT - LOW-PRESSURE TURB.
Compressor pressure ratio Turbine inlet temperature (state 4) Turbine exit temperature (state 5) Exhaust duct pressure loss
Net power output
Overall efficiency
Fuel heating value (natural gas -
INLET AIR
PWT - POWER TURB. FUEL
I I EXHAUST GAS H P
LHV)
*--. ` T BUSTOR
NET WORK OUTPUT
The steam for injection is generated by transfering energy from the exhaust of the gas turbine to a stream of water via a heat exchanger. Figure 2 is a diagram showing the temperature profiles for the two fluids in a typical steam-generating heat exchanger.
I III
STATION
DESIGNATIONS 2 2.5 3 4 4.5 4.9 5
FIGURE 1
SCHEMATIC FOR DUAL-SPOOL TURBOSHAFT ENGINE
Since there are three turbines, there are three primary locations for injecting steam: prior to the high-pressure turbine, or at Station 4 ("HPT injection"); into the stators of the low-pressure turbine, or at Station 4.5 ("LPT injection"); and into the stators of the power turbine, or at Station 4.9 ("PWT injection"). Implicit in these statements is the assumption that steam is introduced into the first stage of a multiple-stage turbine. Since the temperature at Station 4 (T4) is controlled to a constant value, HPT injection can occur at any point between Stations 3 and 4 without significantly changing the effect on steam-injection on the performance of the cycle.
A computer model of this engine is used to simulate the effects of steam injection on gas-turbine performance. This model contains the following: tabulations of the performance characteristics of the individual components; tabulations of the thermodynamic properties of air, fuel, and steam; and the routines necessary to use these tables and compute engine performance at the specified conditions (Noymer, 1992). Operating conditions for the dry design point are summarized in Table 1. The results of this study
w Q
EXHAUST (STATION 5)
2
Inlet temperature (state 2) Inlet pressure (state 2) Inlet duct pressure loss Inlet air low
1520 101.3 kPa none 43 kg/sec
21:1 120550 4752C none 12.6 MW 36.50% 44.2 M,t/kg
L>TEA
SI')
W
Sr. SA•,
-
1iI
STACK (•SK•)
FEED WATER ("FW )
DISTANCE THROUGH HEAT EXCHANGER
FIGURE 2
TYPICAL TEMPERATURE VS. DISTANCE DIAGRAM FOR A STEAM-GENERATING HEAT EXCHANGER
The lower line in Figure 2, with the saturation plateau, represents the steam being generated from a water source (feedwater). The upper line represents the exhaust of the gas turbine, which eventually goes to the exhaust stack of the system. The "pinch point" occurs at the location where the temperature difference between the two streams is the smallest; this value affects the size of the heat exchanger as well as the overall performance of the cycle.
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