Reheating in multi-stage steam turbines: What are the principal benefits of reheating the steam between turbine stages?

Introduction / Context:Large steam power plants commonly use reheat between high-pressure (HP) and intermediate/low-pressure (IP/LP) turbine sections. Reheating influences thermodynamic efficiency, turbine output, and blade durability by controlling moisture content in the last stages.

Given Data / Assumptions:

• Expansion is split into at least two turbine sections.
• Steam is extracted from the HP exhaust, returned to the boiler reheater, and then admitted to the IP/LP section.
• Operating goal: limit wetness, increase average temperature of heat addition, and raise cycle efficiency.

Concept / Approach:Reheating raises the temperature of steam before the second expansion. This increases the mean temperature at which heat is supplied to the Rankine cycle, improving thermal efficiency. By reheating, the quality (dryness fraction) at the LP exhaust increases, reducing liquid droplets that erode and corrode last-stage blades. Also, the second expansion begins at higher enthalpy, increasing the total turbine work output compared with the non-reheat case for similar boiler and condenser limits.

Step-by-Step Solution:

Verification / Alternative check:T–s diagrams of reheat Rankine cycles clearly show increased area (work) and improved exhaust quality compared to simple Rankine cycles, validating all three benefits listed.

Why Other Options Are Wrong:

• Selecting any single benefit understates reheat; in practice it yields all listed improvements.

Common Pitfalls:Assuming reheat always increases condenser load; while mass flow may change, the dominant effect is improved efficiency and moisture control.

Final Answer:all of these