Question

b) A 15 MW gas turbine plant operates with a two-stage compression and a two-stage

expansion. The high-pressure turbine (HPT) drives both low-pressure compressor

(LPC) and high-pressure compressor (HPC). A low-pressure turbine (LPT) is connected

to a generator for electricity generation The overall pressure ratio is 16/1 and the

maximum cycle temperature, which is at the inlet of the HPT, is 900°C. The gases

leaving the HPT are reheated to 850°C before entering the LPT. Both compressors have

equal pressure ratios with an intercooler between the stages, and the intercooling

process is complete. Air enters the LPC at a temperature and pressure of 27°C and

100 kPa respectively. The exhaust gases leaving the LPT are passed through a heat

exchanger to heat the air leaving the HPC. Thermal ratio of the heat exchanger is 0.78.

The isentropic efficiencies for both compressors and turbines are 85%. The mass of fuel

injected is negligibly small. Assuming that the kinetic and potential energy changes are

negligibly small,

i. sketch the cycle on a temperature-entropy (T-s) diagram,

ii. determine the required air mass flow rate for the gas turbine, kg/s,

iii. determine the thermal efficiency of the power plant, %, and

iv. determine the work ratio.

For air, take cpa = 1.005 kJ/kg.K and γa = 1.40

For gas, take cpg = 1.14 kJ/kg.K and γg = 1.35

Answer 1