TITLE.PM5

GAS POWER CYCLES 671

dharm
\M-therm\Th13-5.pm5

T

s

5 ′

4

5

3

2 ′

2

1

6

Fig. 13.42. T-s diagram for the unit.
13.10.6. Effect of Operating Variables on Thermal Efficiency
The thermal efficiency of actual open cycle depends on the following thermodynamic
variables :
(i) Pressure ratio
(ii) Turbine inlet temperature (T 3 )
(iii) Compressor inlet temperature (T 1 )
(iv) Efficiency of the turbine (ηturbine)
(v) Efficiency of the compressor (ηcomp).
Effect of turbine inlet temperature and pressure ratio :
If the permissible turbine inlet-temperature (with the other variables being constant) of an
open cycle gas turbine power plant is increased its thermal efficiency is amply improved. A prac-
tical limitation to increasing the turbine inlet temperature, however, is the ability of the material
available for the turbine blading to withstand the high rotative and thermal stresses.
Refer Fig. 13.43. For a given turbine inlet temperature, as the pressure ratio increases, the
heat supplied as well as the heat rejected are reduced. But the ratio of change of heat supplied is
not the same as the ratio of change heat rejected. As a consequence, there exists an optimum
pressure ratio producing maximum thermal efficiency for a given turbine inlet temperature.
As the pressure ratio increases, the thermal efficiency also increases until it becomes maxi-
mum and then it drops off with a further increase in pressure ratio (Fig. 13.44). Further, as the
turbine inlet temperature increases, the peaks of the curves flatten out giving a greater range of
ratios of pressure optimum efficiency.