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Isentropic efficiency compares the actual enthalpy drop across a turbine with the ideal drop for a reversible adiabatic expansion to the same exhaust pressure. Power output is mass flow times actual enthalpy drop. Specific steam consumption, kilograms of steam per kilowatt-hour, is the practical performance index.
Isentropic efficiency
Efficiency = (h1 - h2_actual) / (h1 - h2_isentropic)
Power
Power (MW) = mass flow (kg/s) x enthalpy drop (kJ/kg) / 1000
85-92% for a large multi-stage condensing turbine, 70-80% for small single-stage machines. Blade erosion, deposits and seal wear degrade it over time.
Lowering condenser pressure increases the available enthalpy drop substantially. A 10 mbar rise in back pressure can cost around 1% of output, which is why condenser cleanliness is closely monitored.