Page 124 - Advanced Gas Turbine Cycles

96 Advanced gas turbine cycles
an increase in turbine work (and heat supplied) with a constant compressor work (and heat
rejected), leads to an increase in efficiency.
A further variation of the El-Masri EGT cycle is one in which the evaporation takes
place both in an aftercooler and within the cold side of the heat exchanger (Fig. 6.8~).
Eq. (6.17) is still valid, but the efficiency is increased because more water can be injected
and the turbine work increased further.
It was shown in Ref. [5] that the arguments given above for the closed EGT cycles also
hold good for open EGT cycles, but this analysis is not repeated here. Some simple
parametric calculations were given to illustrate the increased thermal efficiency of
practical open EGT cycles, corresponding to Fig. 6.8a-c. It was assumed that water
injection was
(a) in the aftercooler (sufficient to saturate the compressor discharge gas),
(b) within the heat exchanger (cold side, to raise the effective specific heat), and
(c) in both aftercooler and heat exchanger (cold side).
Evaporative mixing at low velocity was assumed in the aftercooler, the pressure
remaining constant. Allowance was made for the real gas effects (increased specific heat of
the products of combustion at high temperature), of turbine cooling and intercooling. A
method of calculating the turbine work similar to that developed by Cerri and Arsuffi [8]
for the STIG cycle was used. It was assumed that evaporative cooling was carried out by a
small quantity of water so that the temperatures of the working gas carrying the steam
remain unchanged (except after injection for intercooling and at exit from the hot side of
the heat exchanger). The additional steam in the turbine was assumed to be superheated (at
low partial pressure) and its drop in enthalpy was obtained from steam tables knowing the
original ‘dry’ gas temperature drop.
Plots of efficiency against pressure ratio for the full injection EGT plant, for a
maximum to minimum temperature =5, are shown in Fig. 6.9, compared with lower
values of efficiency in the dry CBTX plant. There are several points to be noted: first that
an increase in efficiency is worthwhile, up to 10%; secondly that the total water injection is
up to over 10% of the air mass flow; and thirdly that the optimum pressure ratio increases
to about 8, from about 5 for that of the dry cycle.
Similar calculations (Fig. 6.10) were made for intercooled cycles, without and with
water injection, i.e. comparing the efficiency of the dry CICBTX cycle with an elementary
recuperated water injection plant, now a simple version of the so-called RWI plant (see
Section 6.4.2. I). Again there is an increase in thermal efficiency with water injection, but it
is not as great as for the simple EGT plant compared with the dry CBTX plant; the
optimum pressure ratio, about 8 for the dry intercooled plant, appears to change little with
water injection.
These smaller effects are related to the smaller amount of water that can be
injected for the intercooled cycle. Applying Eq. (6.19~) to the near optimum condition
of Fig. 6.9 (~RY = 0.5, with S = 0.1) yields (?)wET - ?DRY) = 0.08. Applying the
same equation to the near optimum condition of Fig. 6.10 = 0.53, with
S = 0.04) yields (m - %RY) = 0.035. Both these approximate estimates are very
close to the detailed calculations of the increases in thermal efficiency shown in the
two figures.