Page 149 - Advanced Gas Turbine Cycles

120 Advanced gas turbine cycles

and m, can be determined. The work output from the steam cycle can then be obtained
(allowing for the bleeding of the steam from the turbine) as

where feed pump work terms have been neglected (the feed pumping will be split for the
regenerative cycle with feed heating).
With the fuel energy input known from the calculation of the gas turbine plant
performance, F = Mf[CVl0, the combined plant efficiency is determined as

The reason for using feed heating to set the entry feed water temperature at a level Tb
above the condenser temperature T, is that Tb must exceed the dewpoint temperature Tdp of
the exhaust gases. If Tb is below Tap then condensation may occur on the outside of the
economiser tubes (the temperature of the metal on the outside of the tubes is virtually the
same as the internal water temperature because of the high heat transfer on the water side).
With Tb > Tdp possible corrosion will be avoided.
Some of Rufli’s calculations for (T~)~, for a single boiler pressure pc, are shown in
Fig. 7.7a. There are two important features here:
(a) as expected, the overall CCGT efficiency increases markedly with gas turbine
maximum temperature; and
(b) the optimum pressure ratio for maximum efficiency is low, relative to that for a
simple CBT cycle. We return to this point below in Section 7.6.
Similarly comprehensive calculations were carried out by Cerri [ 101:
(a) with and without feed heating, and
(b) with supplementary heating.
For (a), calculations showed that the presence of feed heating made little difference to
the overall efficiency. Essentially, this is because although feed heating raises the thermal
efficiency x, it leads to a higher value of TS and hence a lower value of the boiler
efficiency, 778. The overall lower cycle efficiency (qoh = 7)~- may be expected to
change little in the expression for combined cycle efficiency (vo)cp, Eq. (7.12~). However,
as pointed out before, feed heating can be used to ensure that Tb is higher than the
dewpoint temperature of the exhaust gases, Tdpr to avoid corrosion of the economiser water
tubes.
For (b), Cerri assumed that the supplementary ‘heat supplied’ was sufficient to give a
maximum temperature equal to the assumed maximum steam entry temperature T,. In
general, it was shown that for the higher values of T3 now used in CCGT plants there was
little or no benefit on overall efficiency associated with supplementary heating.
Rufli also investigated whether raising the steam at two pressure levels showed any
advantage. Typical results obtained by Rufli are also given in Fig. 7.7b. It can be seen that
there is an increase of about 2-3% on overall efficiency resulting from two stages of
heating rather than a single stage.
Results similar to the calculations of Rufli and Cerri have been obtained by many
authors [5-81.