Q1 - Gas Turbines with modifications (10 marks)
- In a gas turbine generating set, two stages of compression are used with an intercooler between stages. The HP turbine drives the HP compressor, and the LP turbine drives the LP compressor and the generator.
- The exhaust from the LP trubines passes through a heat exchanger which transfers heat to the air leaving the HP compressor.
- There is reheat combustion chamber between turbine stages which rasises the gas temperature to 600 °C, which is also the gas temperature at entry to the HP turbine.
- The overall pressure ratio is 10/1, each compressor having the same pressure ratio, and the air temperature at entry to the unit is 20 °C.
- The heat exchanger thermal ratio may be taken as 0.7, and intercooling is complete between compressor stages.
- Assume isentropoc efficiency of 0.8 for both compressor stages, and 0.85 for both turbine stages, and that 2% of the work of each turbine is used in overcoming friction.
- Neglecting all losses in pressure and assumning that velocity changes are negligibly small, calculate:
a) The net power output in kilowtts for a mass flow rate of 115 kg/s.
b) The overall cycle efficiency of the plant.
Given that $C_{pa}$ = 1.005 kJ/kg.K; $\gamma_a$ = 1.4; $C_{pg}$ = 1.15 kJ/kg.K; $\gamma_g$ = 1.333;
Hint: Perform the analysis of this problem following the steps illustrated in the solved class example.
Intercooler
Air inlet
HPT driving the HPC
LPT driving the LPC
and the generator
2
3
Net power
output
LP
C
HP
C
HP
LP
T
T
8
6
7
4
9
CC
Heat
(CC
10
Exhaust
Exchangers
ww
ww
Figure 1. Modified Gas Turbine Power plant
