Steam turbines — definition of efficiency ratio The ratio of the total useful heat drop (actual work obtained from the fluid) to the total isentropic heat drop across the turbine/stage. Choose the correct name of this efficiency.

Concept/Approach

• Isentropic heat drop is the ideal enthalpy drop between inlet and exit pressures with no irreversibility.
• Useful heat drop equals the actual work extracted from the fluid in the turbine or in a single stage.
• The ratio (useful drop)/(isentropic drop) within the turbine is called internal efficiency (sometimes called turbine or stage internal efficiency).

Common pitfalls

• Stage efficiency is often used loosely, but strictly it is a product of component efficiencies. The definition asked matches internal (isentropic) efficiency.
• Rankine efficiency refers to the overall cycle thermal efficiency, not a drop ratio.

Final AnswerInternal efficiency