87.5
87
86.5
86
85.5
85
84.5
84
83.5
0 20 40 60 80 100
Gas Turbine Load (%)
Figure 20-11. Gas turbine com.ressor efficiency as a function of tem.erature load.
Htit =匹m mb)H2 . mj ηbLH旦匹20-22)匹m . mj mb)
.14 Gas Turbine Engineering Handbook
specific heat at constant pressure and the ratio of specific heats have been obtained based on the air tables based on a fuel with a mole weight of the combustion gas to be 28.9 3 lbm/pmole (kg/kgmole).
cp=匹 2.76 *10匹 10)T2 .1.1 28 *10匹 )T .0.237)*C1匹20-23) where C1 =1.0 in the U.S. units and C1 =4.186 in the SI units and = cp . 匹20-24)
(778.16)
cp
M.
The turbine firing temperature based the heat balance can be also computedandmustbewithinabout2-60F(on1-3 0C) of each other. The heat balance relationships as they apply to the gas turbine
Htit = powc ηmc .powg ηmt.匹m .mj )Hexit m .mj mb( ) 匹20-2 )
where powc =work of the gas turbine compressor (Btu/sec， kJ/sec);
powg =generator output; ηmc =mechanical loss in the turbine compressor drive; ηmt =mechanical loss in the turbine process compressor drive; and Hexit =enthalpy at turbine exit
Split shaft gas turbines usually have temperature measurements at the gasifier turbine exit and also at the power turbine exit. From experience andalso based on theoretical relationships， the temperature ratio of the tempera-ture at the gasifier inlet (Ttit) and the temperature of the power turbine inlet temperature (Tpit) for a given geometry remains constant even though the load and ambient conditions change. It is because of this that most manu-facturers limit the engine based on the power turbine inlet temperature.
Tr = Ttit 匹20-26)Tpit
This also enables equation (19) for the case of a split shaft turbine to be rewritten as:
powc
Htit = ηmc.匹匹mm ..mmjj m0.b6)mb)Hpit匹20-27)
where an assumption of 40% of the bleed flow was assumed to have entered the turbine through the cooling mechanisms of the first few stages of the turbine.
To ensure that the heat balance is accurate the following relationship indicates the accuracy of the computations. This heat balance ratio can be written as follows:
powc .匹m .mj )Hexit mHinlet
H.ratio = ηmtmj *LH旦匹20-28)
this ratio should be between 0.96 and 1.04.
Figure 20-12 shows the effect of the turbine firing temperature on the turbine expander efficiency. The decrease in firing temperature reduces theabsolute velocity， as also does the reduction in the massflow， both of which occur at part load conditions. Figure 20-13 shows the variation in the firing temperature and the exhaust gas temperature as a function of the load. It is interesting to note that the firing temperature of the turbine is greatly reduced while the exhaust temperature remains nearly constant accounting for the steam turbine producing more work at low part loads.
The work produced by the gasifier turbine (.gt) is equal to the gas turbine compressor work (.c):
powgt = powc 匹20-29)ηmc
100 90 80 70 60 50 40 30 20 10 0 2000 2050 2100 2150 2200 2250 2300 2350 2400 (1094 °C) (1149 C)(1204 C)(1260 C)(1288 C)
° °°°
Turbine Firing Temperature
Figure 20-12. Gas turbine efficiency as a function of firing tem.erature. .16 Gas Turbine Engineering Handbook
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