Rankine cycle with reheat
Reheat expands the steam twice: once to an intermediate pressure, back to the boiler for a second pass through the superheater, then to the condenser. It raises the mean temperature of heat addition — and, more usefully, it dries the exhaust, which is what actually limits how high the boiler pressure can go in a simple cycle.
| State | h (kJ/kg) | Note |
|---|---|---|
| 1 — turbine inlet | 2777 | boiler exit |
| 2 — first stage exit | 2592 | at the reheat pressure |
| 3 — reheater exit | 3284 | back to the reheat temperature |
| 4 — second stage exit | 2503 | at the condenser pressure |
| 5 — condenser exit | 192 | saturated liquid |
| 6 — pump exit | 193.0 | feedwater |
| High-pressure work | 185 kJ/kg | h1 − h2 |
| Low-pressure work | 782 kJ/kg | h3 − h4 |
| Net work | 966 kJ/kg | turbine less pump |
| Heat input | 3276 kJ/kg | h1 − h6 plus h3 − h2 |
| Efficiency | 29.5% | reheat cycle |
| Simple cycle | 27.7% | same pressures, no reheat |
Reheat lifts efficiency by 1.8 points and the exhaust from 78.1% to 96.6% dry. The dryness is usually the more important of the two: below 88% the last stages erode, and reheat is how a high-pressure cycle stays above that line.
w₁ = h₁ − h₂ · To the reheat pressure.
q_rh = h₃ − h₂ · Back to the reheat temperature.
w₂ = h₃ − h₄ · To the condenser.
(w₁ + w₂ − w_p)/(q_b + q_rh) · Both heat inputs.
A clear path from inputs to results.
Reheat expands the steam twice: once to an intermediate pressure, back to the boiler for a second pass through the superheater, then to the condenser. It raises the mean temperature of heat addition and, more usefully, dries the exhaust.
How to use
- Set the boiler, reheat and condenser pressures.
- Choose the reheat temperature.
- Enter the turbine and pump efficiencies.
- Read the efficiency gain and the exhaust quality.
The formula
Reference: Saturated properties from IAPWS-IF97 with a constant-cp superheat approximation.
Worked example
10 bar, reheat at 3 bar to 400 °C
- Boiler 10 bar, reheat 3 bar, condenser 0.1 bar.
- Reheat to 400 °C, turbine 85%, pump 80%.
- 1
The first expansion runs from 2,777 kJ/kg to 2,592 kJ/kg at 3 bar.
- 2
Reheating to 400 °C lifts the enthalpy to 3,284 kJ/kg.
- 3
The second expansion runs to 2,503 kJ/kg at 0.1 bar.
- 4
Net work is 966 kJ/kg against a heat input of 3,276 kJ/kg.
- 5
Efficiency is 29.5% against 27.7% for the simple cycle, and the exhaust is 96.6% dry rather than 78.1%.
29.5% · +1.8 points · exhaust 96.6% dry.
Interpreting the result
The quality gain is the important one. A simple cycle from 10 bar to 0.1 bar ends at 78% dry, which would destroy the last turbine stages; reheat brings it to 96.6%. That is why reheat is universal on high-pressure plant and why the efficiency gain alone would not justify it.
Assumptions & limitations
- One reheat, back to a stated temperature.
- Constant-cp superheat approximation, cp ≈ 2.1 kJ/kg·K.
- No pressure loss in the reheater or crossover piping.
- No regeneration.
Frequently asked questions
Why does reheat improve the exhaust so much?
Because it adds enthalpy at a high temperature after the first expansion, so the second expansion starts from a much higher enthalpy and ends further inside the wet region — or above it entirely.
What is the optimum reheat pressure?
Usually a quarter to a third of the boiler pressure. Lower and the first stage does too little; higher and the reheat adds heat at a temperature too low to help.
How much does a second reheat add?
About a third of what the first one gave. Only a few very large plants use two.
Is the efficiency gain always positive?
No. Reheating only to saturation adds heat at a low temperature and can reduce efficiency. The gain requires reheating to a high temperature, which is why a real reheater sits in the furnace gas path.
Does reheat replace superheat?
No, they do different jobs. Superheat raises the turbine inlet temperature; reheat restores it part-way through the expansion.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.