Page 148 - Advanced Gas Turbine Cycles

Chapter 7. The combined cycle gas turbine (CCGT) 119

of the heat balance equation for the whole HRSG,
(h4 - hS) = p(he - hb), (7.22)

yields the enthalpy and temperature at the stack, (hs, Ts).
Even for this simplest CCGT plant, iterations on such a calculation are required, with
various values of pc, in order to meet the requirements set on Te, the steam turbine entry
temperature, and Ts (the calculated value of Ts has to be such that the dewpoint
temperature of the gas (Tdp) is below the economiser water entry temperature (Tb) and that
may not be achievable). But with the ratio p satisfactorily determined, the work output
from the lower cycle WL can be estimated and the combined plant efficiency obtained from
770 = (WH + wL)/Mf[cvlO, (7.23)

as the fuel energy input to the higher cycle and its work output is already known.
This is essentially the approach adopted by Rufli [9] in a comprehensive set of
calculations, but he assumed that the economiser entry water temperature Tb is raised
above the condenser temperature by feed heating, which was specified for all his
calculations. The T,s diagram is shown in Fig. 7.6; the feed pump work terms are
neglected so that ha = hb' and hat = hb.
Knowing the turbine efficiency, an approximate condition line for the expansion
through the steam turbine can be drawn (to state f' at pressure pb') and an estimate made of
the steam enthalpy hp. If a fraction of the steam flow in, is bled at this point then the heat
balance for a direct heater raising the water from near the condenser temperature T, to Tb is

(7.24)

=g v P%
Fig. 7.6. CCGT plant with feed water heating by bled steam (after Ref. [11).