Page 119 - Advanced Gas Turbine Cycles

Chapter 6. ‘Wet’ gas turbine plants 91

and in the HRSG

But of course these two equations may be combined with Eq. (6.12) to give the steady
flow energy equation for the whole plant as

so that

(6.16)

Eq. (6.16) is essentially the same as Eq. (6.8) for the basic STIG plant which, on
reflection, is not surprising. If the states 1,2,3,4 and 5 and the steam quantity S are all the
same then expressions for the work output, the ‘heat input’ (or fuel energy supply) and the
‘heat rejected’ are all unchanged. The total amount of heat transferred from the exhaust is
also unchanged, but two separate flows, of air and of watedsteam, have been raised in
enthalpy before entry to the combustion chamber, rather than one (water/steam) in the
earlier analysis.
However in practice, for the same states 1-5 the steam raised S will be less; hence
there is no advantage in operating a STIG plant in this variation of the basic CBTX
recuperative gas turbine plant. Nonetheless, this form of analysis as developed by
Lloyd will prove to be useful in the discussion of the chemical recuperation plant in
Chapter 8.

6.3. Simple analyses of EGT type plants

63.1. A discussion of dry recuperative plants with ideal heat exchangers

Before considering the effects of water injection in an EGT type plant, it is worthwhile
to refer to the earlier studies on the performance of some dry recuperative cycles. Fig. 6.6
shows the T,s diagram of a [CBTlI[XlR cycle, with a heat exchanger effectiveness of
unity. It is implied that the surface area for heat transfer is very large, so that the outlet
temperature on the cold side is the same as the inlet temperature on the hot side. However,
due to the higher specific heat of the hot gas, its outlet temperature is higher than the inlet
temperature of the cold air.
In their original air standard cycle analysis, using constant specific heats, Hawthorne
and Davis 141 considered the dry [CBT],XR cycle. They assumed a ‘perfect’ heat
exchanger, with the specific heats of gas and air constant and identical, so that Ty becomes
equal to T2 in Fig. 6.6. From their examination of the enthalpy-entropy diagram of this