Question

In a Rankine cycle, what is the primary thermodynamic effect on cycle efficiency when increasing superheating temperature, assuming turbine inlet pressure remains constant?

Accepted Answer

Increasing the superheating temperature in a Rankine cycle, while keeping the turbine inlet pressure constant, primarily increases the cycle efficiency due to a higher average temperature during heat addition. Superheating refers to heating the steam above its saturation temperature (the boiling point at a given pressure) at a constant pressure. When steam is superheated, it enters the turbine at a higher temperature. This higher temperature translates to a greater enthalpy (total heat content) difference between the turbine inlet and outlet. Since the turbine extracts energy from the steam based on this enthalpy difference, more work is produced per unit mass of steam. Consequently, the amount of heat rejected in the condenser remains relatively unchanged or increases slightly, while the work output significantly increases. This greater work output, relative to the heat input, leads to a higher thermal efficiency, which is defined as the ratio of net work output to heat input. Furthermore, superheating reduces the moisture content at the turbine exit. High moisture content can lead to erosion of turbine blades. Thus, superheating not only increases efficiency but also enhances the reliability and lifespan of the turbine.

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In a Rankine cycle, what is the primary thermodynamic effect on cycle efficiency when increasing superheating temperature, assuming turbine inlet pressure remains constant?