Page 67 - Advanced Gas Turbine Cycles

Chapter 3. Basic guas turbine cycles 43

3.4. Computer calculations for open circuit gas turbines

Essentially, the analytical approach outlined above for the open circuit gas turbine
plants is that used in modem computer codes. However, gas properties, taken from tables
such as those of Keenan and Kaye [6], may be stored as data and then used directly in a
cycle calculation. Enthalpy changes are then determined directly, rather than by mean
specific heats over temperature ranges (and the estimation of n and n'), as outlined above.
A series of calculations for open circuit gas turbines, with realistic assumptions for
various parameters, have been made using a code developed by Young [7], using real gas
tables. These illustrate how the analysis developed in this chapter provides an
understanding of, and guidance to, the performance of the real practical plants. The
subscript G here indicates that the real gas effects have been included.

3.4.1. The [CBTIIG plant

Fig. 3.13 shows the overall efficiency for the [CBTIIG plant plotted against the
isentropic temperature ratio for various maximum temperatures T3 (and 6 = T3/T,, with
TI = 27°C (300 K)). The following assumptions are also made:
polytropic efficiency, qp = 0.9 for compressor and turbine;
pressure loss fraction in combustion 0.03;
fuel (methane) and air supplied at 1 bar, 27°C (300 K).
This figure may be compared with Fig. 3.3 (which showed the a/s efficiency of plant
[CHT], as a function of x only) and Fig. 3.9 (which showed the a/s efficiency of

60

50

40
z
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0
30
W
A
2
wm
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0
IO

0
1 I .5 2 2.5 3 3.5 4 4.5 5
ISENTROPIC TEMPERATURE RATD
Fig. 3.1 3. Overall efficiency of [CBTIlo cycle as a function of pressure ratio r with 7'3 (and temperature ratio e) as
a parameter.