Question

A steam turbine inlet is at 1200 kPa, 500°C. The exit is at 200 kPa. What is the lowest possible exit temperature? Which efficiency does that correspond to?

Answer 1

We would expect the lowest possible exit temperature when the maximum amount of work is taken out. This happens in a reversible process so if we assume it is adiabatic this becomes an isentropic process.

Exit: 200 kPa, s = sin = 7.6758 kJ/kg K ⇒ T = 241.9°C

The efficiency from measures the turbine relative to an isentropic turbine, so the efficiency will be 100%.