Page 146 - Advanced Gas Turbine Cycles

Chapter 7. The combined cycle gas turbine (CCGT) 1 I7

For a mass flow of air (Ma) to the compressor of the gas turbine plant, a mass flow Mf
is
of fuel (of specific enthalpy b) supplied to the two combustion chambers (Mf =
(Mf)H + The overall efficiency of the combined plant is then

(7.15)

Eq. (7.15) may be written as

(7.16)

where HPt = [Ma + (Mf)H + (Mf)L]hp~, and P‘ indicates products after supplementary
combustion.
Eq. (7.16) may be written in terms of ‘heating’ quantities as
QH = (Mf)H[cvlO and QL = (Mf)L[cvlO
and a ‘heat loss’
-
QUN = [Ma (Mf)H (M~)LI[(~P’)s (~P’IoI
Then with vL = QL/(QL + QH) and vUN = Qm/(QL + QH), it follows that
(Mfh/(Mf)H = vL/(1 - VL)r (7.17)
and
=
QUN~[(M~)H[C~IOI VUN/(~ - VL). (7.18)
so that Eq. (7.16) becomes

(7)O)CP = (7)O)H + Ih - (7)O)Hrh - %VUN - (7)0)H(l - ‘I)L)vL* (7.19)

7.5. The efficiency of an exhaust heated CCGT plant
The expression for the combined cycle efficiency

7) = (7)O)H + (7)O)L[1 - (7)O)HI (7.20)
is always valid for CCGT exhaust heated (unfired) cycles. The parametric calculation of
the efficiency of the upper open gas turbine plant (7)o)H is discussed in detail in Chapters 4
and 5. The overall efficiency of the lower steam cycle (qo)L is the product of the lower
thermal efficiency and the ‘boiler’ efficiency of the HRSG, m.