Reheat Rankine Cycle

Reheating improves turbine durability and efficiency by reducing moisture content in later turbine stages.

Reheating

The process of heating the steam output of one turbine, and feeding it to another. Steam is reheated at constant pressure.

Allows multiple turbines. Increases thermal efficiency, turbine work output. Reduces wetness at turbine exit.

Steps:

• Steam generated in boiler and superheated.
• Expanded through high-pressure turbine to an intermediate pressure.
• Returned to boiler/reheater and superheated again.
• Expanded through next turbine stages (IP/LP).
• Condensed to saturated liquid.
• Pumped back to boiler.

T–S Diagram Features

The T–S diagram for reheat cycles shows repeated constant-pressure heating segments.

If 2 turbines are installed:

• First expansion from superheated region to intermediate pressure.
• Reheat back to high temperature at constant pressure.
• Second expansion down to condenser pressure.

Performance

Efficiency is evaluated through turbine work, pump work, and heat addition in two stages.

Suppose 2 turbines and 1 pump are in the setup.

Net Work

Two turbines. One pump.

$$w_\text{net} = (h_3 - h_4) + (h_5 - h_6) - w_\text{pump}$$

Heat Input

Two heating processes: boiler + reheat.

$$q_\text{in} = (h_3 - h_2) + (h_5 - h_4)$$

Mass Flow Rate

For power output $P$:

$$\dot{m} = \frac{P}{w_\text{net}}$$

Summary

Reheat increases numerator (work) more than denominator (heat input), raising efficiency.