Rankine Reheat Cycle

Add two states to the basic loop

Keep states 1 and 2 at the pump inlet and outlet, and 3 at the high-pressure turbine inlet. Expansion ends at 4, reheating raises temperature to 5 at an intermediate pressure, and the low-pressure turbine exhaust is state 6.

Count both heat inputs

The boiler and reheater both draw energy from the heat source. Omitting reheater heat inflates the apparent efficiency.

Count both expansions and the pump

Add the work of both turbine stages and subtract the pump input. Use isentropic expansions only for the ideal model; real turbine efficiencies change the exit states.

Separate moisture benefits from efficiency claims

Reheating normally makes the final exhaust drier at a comparable final pressure. Its effect on efficiency depends on the chosen reheat pressure and temperatures; adding a reheater is not an unconditional efficiency guarantee. Check wet-region quality using appropriate water properties.

Worked example

Use illustrative enthalpies h₁ = 200, h₂ = 210, h₃ = 3400, h₄ = 2900, h₅ = 3500, h₆ = 2400 kJ/kg. Evaluate the energy balance without inferring steam states from these values.

1. 1.Sum boiler and reheater heat input.

   $$q_{\mathrm{in}}=(3400-210)+(3500-2900)=3790kJ/kg$$

2. 2.Sum turbine work and subtract pump work.

   $$w_{\mathrm{net}}=(3400-2900)+(3500-2400)-(210-200)=1590kJ/kg$$

3. 3.Check efficiency and heat rejection.

   $$\begin{array}{c}{\eta }_{\mathrm{th}}=1590/3790=0.4195\\ q_{\mathrm{out}}=2400-200=2200kJ/kg\end{array}$$

The illustrative balance gives 42.0% efficiency, with 3790 − 2200 = 1590 kJ/kg of net work.