Figure 20-9. Effect of .ressure ratio and firing tem.erature on the .erformance of a gas turbine.
pressure ratio. Thismeans， in a practical sense， that the new compressors on these gas turbines are very susceptible to any fouling of the com-pressor， indicating that the inlet filters must be very efficient and the turbines must be performance monitored to ensure maximum operational efficiency.
The overall compressor work is calculated using the following relation-ship:
匹)1
.c=匹H2 H1)=cpavgT1 p21匹20-17)p1
the work per stage is calculated assuming the energy per stage isequal， this has been found to be a better assumption than assuming the pressure ratio per stage to be equal. It is necessary to know the work per stage if there is inter-stage bleed of the air for cooling or other reasons.
wstg =匹H2 H1)匹20-18)nstg
.12 Gas Turbine Engineering Handbook
where nstg =number of compressor stages. The computation of the com-pressor total energy requirements can now be computed.
powc =mwstgn1.匹m mb1)wstg n2.匹m mb1 mb2)wstgn3 ...匹20-19)
The work of the compressor under ideal conditions occurs at constant entropy. The actual work occurs with an increase in entropy thus the adiabatic efficiency can be written in terms of the total changes in enthalpy:
ηac = Isentropic work =匹H2T. H.T )匹20-20)Actual work 匹H2 H1T )
where H2T. =total enthalpy of the gas at isentropic exit conditions， and H2 =total enthalpy of the gas at actual exit conditions， and H1 =total enthalpy of the gas at inlet conditions for a caloricaly perfect gas the equation can be written as:
. p2 匹)1.
1
p1
ηac = ..匹20-21)
T2
1
T1
The gas turbine compressor which produces the high pressure gas at elevated temperature uses a very large part of the turbine power produced by the gas
56
55.5
55
54.5
54
53.5
0 20 40 60 80 100
Total Gas Turbine Power (%)
Figure 20-10. Gasifier .ower as a function of total gas turbine .ower.
The turbine firing temperature should be computed by knowing the gas characteristics of the combustion gas. If these characteristics are known then one can use the combustion gas equations given in the ASME performance test codes 4.4 (1991) for gas turbine HRSG. Usually the gas constituents are not known so it is not a bad assumption to use the 400% theoretical air tables in the .eenan and .aye gas tables. The following equations for
where Htit = enthalpy of the combustion gas at the firing temperature; m = mass of air; mb = bleed air; mj = mass of fuel; ηb = combustor effi-ciency (usually between 97-99%)
The calculation of the turbine firing temperature (Ttit) is based firstly on the fuel injected into the turbine and the fuel"s lower heating value (LH旦 ). The lower heating value of the gas is one in which the H2O in the products has not condensed. The lower heating value is equal to the higher heating value minus the latent heat of the condensed water vapor.
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