Understanding Reheating in the Rankine Cycle and Dryness Fraction

The Rankine cycle is a fundamental thermodynamic cycle used in steam power plants to generate electricity. It involves processes like pumping, heating (boiling), expansion in a turbine, and condensation.

One common modification to the basic Rankine cycle is reheating. Reheating involves expanding the working fluid (like steam or air) partially in the turbine, then sending it back to the boiler or a reheater to be heated again, and finally returning it to the turbine for further expansion.

Effect of Reheating on Dryness Fraction

The question specifically asks about reheating performed during the lower stages of turbine exhaust. This means the fluid has already expanded somewhat through the initial part of the turbine and its pressure and temperature have dropped. Reheating happens at an intermediate pressure during the expansion process.

• Dryness Fraction: This term refers to the mass fraction of vapor in a mixture of liquid and vapor. For steam exiting the turbine, a higher dryness fraction means less liquid water is present. A low dryness fraction (high moisture content) at the end of the turbine can cause erosion of the turbine blades.
• Reheating Process: When the partially expanded steam is reheated, heat is added to it at approximately constant pressure.
• Impact: Adding heat to the steam after it has partially expanded causes its temperature and internal energy to increase. If we look at a Temperature-Entropy (T-s) diagram or an Enthalpy-Entropy (h-s) diagram, this reheating process moves the state point towards the right, further away from the saturation line. Moving to the right on these diagrams in the two-phase region corresponds to an increase in the dryness fraction.

Therefore, reheating the steam or air during its expansion process in the turbine adds energy, which results in the steam becoming drier. This increases the dryness fraction at later stages of the turbine expansion and typically at the turbine exhaust.

Benefits of Increased Dryness Fraction

Increasing the dryness fraction at the turbine exhaust is beneficial because:

• It reduces the moisture content in the steam exiting the turbine.
• High moisture content can cause physical erosion damage to the turbine blades operating at high speeds, particularly in the later stages.
• By increasing the dryness fraction through reheating, the lifespan and reliability of the turbine are improved.

Based on this, reheating the steam during its passage through the turbine leads to an increase in its dryness fraction.