Page 115 - Advanced Gas Turbine Cycles

88 Advanced gas turbine cycles

LOW s

T13
I
HEAT TRANSFERRED
OPTIMUM S
Maximum steam exit
temperature
STEAM / WATER
Minimum pinch point
temperature
HEAT TRANSFERRED difference

Fig. 6.3. HRSG performance of STIG plant at different steam/air ratios (after Lloyd [2]). Princeton University
Library.

the superheated steam leaves at its upper temperature limit and when the pinch point
temperature difference is at its minimum, as in diagram Fig. 6.3b.
He gave an example of an industrial gas turbine with a pressure ratio of 12 and a
maximum temperature of 1100°C. For the basic CBT plant the specific work is
approximately W = WT - Wc = 650 - 350 = 300 kJkg (air) and QB = 870, so that
QA = 570 and the efficiency is 300/870 = 0.345. For a STIG plant with S = 0.1, the
turbine work output increases by about 20% to 770 giving a net work output of 420, an
increase of 40%. QB also increases somewhat, by about 23% to about 1070, and QA by
about 14% to 650. The efficiency (7) = 420/1070) therefore increases to nearly 40%,
because the work output increases substantially more than the ‘heat supplied’.
Fig. 6.4 then shows a more complete calculation of plant efficiency for varying S. The
optimum condition of maximum efficiency is reached at S = 0.208. The picture changes
for a gas turbine with a higher pressure ratio, for which the increase to maximum efficiency
is less, as is the optimum value of S [2].
A useful rule of thumb is that the turbine work in a STIG plant is increased by a factor
of about (1 + 2S), since the specific heat of the steam is about double that of the specific
heat of the ‘dry’ gas. This is in agreement with the example given above and with the
earlier detailed calculations by Fraize and Kinney [3]. (Their work was based on the
assumption that the mixture of air and steam in the turbine behaved as a semi-perfect gas,
with specific heats being determined simply by mass averaging of the values for the two
components.)
Finally, it may be noted that there is little or no point in adding steam directly to the
turbine alone-say into the first nozzle guide vane row-because its enthalpy even at best
would only be equal to the enthalpy of the steam leaving the turbine (hs6 5 h&).