Rankine Cycle Efficiency Calculator

Example data table

Sample enthalpies (illustrative) for quick testing.

Case	h1	h2	h3	h4	ηth (expected)
Baseline	191.8	200.2	3230.0	2200.0	≈ 31–33%
Higher superheat	191.8	200.2	3330.0	2240.0	≈ 33–35%
Lower condenser pressure	170.0	178.5	3230.0	2100.0	≈ 35–38%

Formula used

All terms are per unit mass of working fluid.

• Heat added: q_in = h₃ − h₂
• Turbine work: w_t = h₃ − h₄
• Pump work: w_p = h₂ − h₁
• Net work: w_net = w_t − w_p
• Thermal efficiency: η_th = w_net / q_in

Efficiency mode relations

• Pump: η_p = (h_2s − h₁) / (h₂ − h₁) → h₂ = h₁ + (h_2s − h₁) / η_p
• Turbine: η_t = (h₃ − h₄) / (h₃ − h_4s) → h₄ = h₃ − η_t(h₃ − h_4s)

Tip: Use consistent reference states and units across all enthalpies.

How to use this calculator

1. Select your energy unit (kJ/kg or Btu/lbm).
2. Pick an input mode: direct enthalpies or efficiencies.
3. Enter h1 and h3, then fill remaining required fields.
4. Press Calculate to view results above the form.
5. Use CSV or PDF buttons to export the computed report.
6. Adjust assumptions and compare efficiency across scenarios.

If you only have pressures and temperatures, first extract enthalpies from steam tables.

Professional article

1) Why Rankine efficiency matters

The Rankine cycle converts boiler heat into shaft work by expanding steam through a turbine and rejecting heat in a condenser. Thermal efficiency, η_th, is a first-screen metric for plant performance. Many simple units fall near 30–40% net efficiency, depending on pressures, temperatures, and losses.

2) What the calculator needs

This tool works from enthalpy values at four key states: condenser outlet (h1), pump outlet (h2), boiler outlet (h3), and turbine outlet (h4). If you only have pressures and temperatures, obtain enthalpies from steam tables or trusted property software. Keep one consistent unit system.

3) Interpreting typical enthalpy data

For many water–steam cycles, h1 is a few hundred kJ/kg for saturated liquid near condenser conditions, and h3 is often above 3000 kJ/kg for superheated steam. Differences between states drive work and heat terms: q_in=h3−h2, w_t=h3−h4, and w_p=h2−h1.

4) Superheat and reheat trends

Raising turbine inlet temperature usually increases w_t and can lift η_th, provided turbine exhaust quality remains acceptable. A higher h3 can increase net work faster than it increases q_in. Reheat follows similar logic when you supply consistent state enthalpies.

5) Condenser pressure sensitivity

Lower condenser pressure reduces h4 by increasing the expansion ratio, typically boosting turbine work and η_th. The benefit can be meaningful: small drops in saturation temperature often improve efficiency. Practical limits include cooling-water temperature, heat-exchanger size, and leakage control.

6) Accounting for real component efficiencies

Ideal (isentropic) devices are rarely achieved. In efficiency mode, the calculator corrects h2 and h4 using η_p and η_t. Typical pump efficiencies are about 0.70–0.90, while turbine efficiencies are often 0.75–0.92. These losses reduce w_net.

7) Using derived indicators for decisions

Back work ratio (BWR=w_p/w_t) shows how much turbine output is consumed by pumping; it is usually small but informative. The heat-rate proxy (1/η) supports quick comparisons. Specific steam consumption (≈3600/w_net with kJ/kg) links performance to mass-flow planning.

8) Practical workflow for scenario comparison

Start with baseline enthalpies, compute results, then change one design choice at a time: turbine inlet condition, condenser condition, or component efficiency. Export each run to CSV to build a sensitivity table. This disciplined approach turns thermodynamic calculations into actionable design and operations insights.

FAQs

1) Do I need steam tables to use this tool?

Use steam tables or property software if you don’t already know enthalpies. Enter h1–h4 directly, or enter h2s, h4s with efficiencies. Consistent states and units matter most.

2) What causes negative heat input?

It usually comes from swapped states or mixed units. Verify h3 is boiler outlet and h2 is pump outlet, both in the same unit system. Then q_in=h3−h2 should be positive.

3) Why do direct and efficiency modes differ?

Efficiency mode reconstructs actual h2 and h4 from ideal endpoints and η values. If your η assumptions or isentropic enthalpies don’t match your direct data, results will not match.

4) What ηth values are typical?

Many simple Rankine cycles fall around 30–40% depending on temperatures, pressures, and losses. Extremely low or very high values often indicate incorrect enthalpies, inconsistent states, or unit mistakes.

5) How should I read back work ratio?

BWR is pump work divided by turbine work. Smaller is better. A large BWR suggests unusually high pump work or unusually low turbine work, which can occur with extreme pressure rise or input errors.

6) What does specific steam consumption mean?

It estimates mass flow required to produce 1 kWh of net work. Lower values indicate better cycle performance. Use it to compare scenarios and to approximate steam flow for a target output.

7) Can this model reheat or regeneration?

This page is a four-state, simple-cycle calculator. Reheat and feedwater heating require extra states and balances. You can still compare simplified “equivalent” cases by using representative state enthalpies.