Page 207 - Advanced Gas Turbine Cycles

172 Advanced gas turbine cycles

The above simple analysis has to be modified for a supplementary fired CHP plant such as
that shown in Fig. 9.3c, meeting a unit electrical demand and an increased heat load Af,.
The ‘reference system’ fuel energy supplied is now
1
Af,
F’kEF = - + -.
77c 778
The CHP plant now requires a fuel energy supply of

F’ = F1 + F2 = (l/?c~) + F2, (9.9)
where F2 = Ab/+ is the Supplementary fuel energy supplied to the WHB, so that

(9.10)

The quantity $ requires discussion. The steady flow energy equation for the WHB is

M&o + Hp4 = Ab + Hps, (9.1 1)
where 4 and S are the entry and exit states, P refers to products entering (i.e. at exit from
the turbine), P‘ refers to products after the supplementary combustion and MEhm is the
enthalpy flux of the entering fuel. For a corresponding calorific value experiment at
temperature TO, again with products P entering and products P‘ leaving,
Mf2hfO + Hpo = n/r,,[CVIo + HPO. (9.12)

(9.13)

(9.14)

where (HPSt - HpO) is the new ‘heat loss’ in the stack (Q)/Nu, and this will usually be less
than (Hp4 - Hw), so that +will be greater than unity (it is not a boiler efficiency). We shall
not determine $ here but give it parametric values of 1.2 and 1.5 in the later calculations.
The fuel savings for the supplementary fired plant are given by

(9.15)

and the fuel savings ratio is

(9.16)

By way of numerical illustration of the fuel savings ratio, we consider the two plants
illustrated in Fig. 9.2. For the unfired plant of Fig. 9.2a, taking vC = 0.4 and 778 = 0.9 and