KEVOS® MAKEOS · Engineering
Steam Rankine Cycle Check (Efficiency, Steam Flow, Heat Rate, Turbine Exhaust Dryness)
From steam-table values for the boiler outlet and condenser, what thermal efficiency, steam flow, heat rate and boiler and condenser duties does this steam cycle give for the power needed, and is the steam leaving the turbine dry enough?
Simple steam Rankine cycle from steam-table values: the feed pump and turbine work with their isentropic efficiencies, the heat added and rejected, the thermal efficiency, back-work ratio and heat rate, the steam flow and boiler and condenser duties for the net power you need, and the dryness of the turbine exhaust against your minimum.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Boiler outlet and turbine | ||
| Boiler (turbine inlet) pressure (absolute) | kPa | Boiler outlet (turbine inlet) pressure, absolute, from the boiler or turbine data sheet. |
| Steam enthalpy at the turbine inlet | kJ/kg | Enthalpy of the steam entering the turbine at its pressure and temperature, from the superheated steam table or IAPWS-IF97 software. |
| Enthalpy after isentropic expansion to the condenser pressure | kJ/kg | Enthalpy at condenser pressure with the same entropy as the turbine inlet (isentropic expansion), from the steam table or software. |
| Turbine isentropic efficiency | % | Turbine isentropic efficiency from the turbine maker or a test. |
| Condenser (saturation values at condenser pressure) | ||
| Condenser pressure (absolute) | kPa | Condenser pressure, absolute, from the condenser design or a gauge reading plus atmospheric pressure. |
| Saturated liquid enthalpy at condenser pressure | kJ/kg | Saturated liquid enthalpy at the condenser pressure (the condensate leaving the condenser). |
| Enthalpy of vaporisation at condenser pressure | kJ/kg | Enthalpy of vaporisation at the condenser pressure, used for the exhaust dryness. |
| Saturated liquid specific volume at condenser pressure | m³/kg | Specific volume of saturated liquid at the condenser pressure, used for the pump work. |
| Feed pump isentropic efficiency | % | Feed pump isentropic efficiency from the pump maker’s curve. |
| Duty and limit | ||
| Net power required | kW | Net electrical or shaft power the cycle must deliver. |
| Minimum dryness at the turbine exhaust | Lowest dryness fraction the turbine maker accepts at the exhaust (1 minus the moisture fraction). | |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
Boiler outlet and turbine
What this means: Boiler (turbine inlet) pressure (absolute)
- What it is
- The absolute pressure at which the boiler raises steam and delivers it to the turbine, which sets the pressure rise the feed pump must provide.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 20 kPa to 40000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3000 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher boiler pressure raises the pump work slightly; the turbine work comes from the enthalpies you enter.
- Get it right
- Use absolute pressure: a gauge reading plus atmospheric pressure.
What this means: Steam enthalpy at the turbine inlet
- What it is
- The specific enthalpy of the superheated steam entering the turbine, read from the steam table at the turbine inlet pressure and temperature.
- Units and format
- Enter a number in kJ/kg: kJ/kg. The form accepts 100 kJ/kg to 4500 kJ/kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3116.1 kJ/kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher inlet enthalpy raises the turbine work, the heat added and the exhaust dryness.
- Get it right
- Read it at the turbine stop valve conditions, not at the boiler outlet if the steam main loses pressure and heat.
What this means: Enthalpy after isentropic expansion to the cond…
- What it is
- The enthalpy the steam would have at condenser pressure after an ideal, isentropic expansion: found at the condenser pressure using the turbine inlet entropy.
- Units and format
- Enter a number in kJ/kg: kJ/kg. The form accepts 100 kJ/kg to 4500 kJ/kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2136.1 kJ/kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower value means a larger isentropic drop, more turbine work and higher efficiency.
- Get it right
- Compute it from the inlet entropy: x₄ₛ = (s₃ − s_f) ÷ s_fg at the condenser pressure, then h₄ₛ = h_f + x₄ₛ h_fg.
What this means: Turbine isentropic efficiency
- What it is
- The actual turbine work divided by the ideal isentropic work for the same inlet state and exhaust pressure, measuring friction, leakage and blade losses.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 20% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 87% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower efficiency cuts the net work, raises the steam flow and leaves a drier exhaust.
- Get it right
- Small turbines are often well below large ones; use the maker’s figure at your load.
Condenser (saturation values at condenser pressure)
What this means: Condenser pressure (absolute)
- What it is
- The absolute pressure in the condenser, which with the boiler pressure sets the pressure rise across the feed pump.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 0.7 kPa to 20000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Enters only the pump work here; the condenser saturation values carry its main effect.
- Get it right
- Condensers run well below atmospheric pressure; enter the absolute value, not the vacuum gauge reading.
What this means: Saturated liquid enthalpy at condenser pressure
- What it is
- The enthalpy of saturated liquid water at the condenser pressure, taken as the condensate entering the feed pump and the base for the exhaust dryness.
- Units and format
- Enter a number in kJ/kg: kJ/kg. The form accepts 0 kJ/kg to 2100 kJ/kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 191.81 kJ/kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher value lowers the heat to be added in the boiler and the exhaust dryness.
- Get it right
- Read it at the condenser pressure, not at the boiler pressure.
What this means: Enthalpy of vaporisation at condenser pressure
- What it is
- The enthalpy of vaporisation at the condenser pressure: the energy to turn saturated liquid into saturated vapour, used to find the exhaust dryness.
- Units and format
- Enter a number in kJ/kg: kJ/kg. The form accepts 1 kJ/kg to 2600 kJ/kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2392.1 kJ/kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only sets the exhaust dryness; a larger value gives a lower dryness for the same exhaust enthalpy.
- Get it right
- It is h_g − h_f at the condenser pressure; do not use the value at the boiler pressure.
What this means: Saturated liquid specific volume at condenser p…
- What it is
- The specific volume of saturated liquid water at the condenser pressure, which multiplied by the pressure rise gives the ideal pump work.
- Units and format
- Enter a number in m³/kg: m³/kg. The form accepts 0.0009 m³/kg to 0.004 m³/kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.00101 m³/kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The pump work rises in proportion; the effect on efficiency is small.
- Get it right
- Water is about 0.001 m³/kg; a value a thousand times larger means the units are litres per kilogram.
What this means: Feed pump isentropic efficiency
- What it is
- The ideal pump work divided by the actual work the feed pump needs for the same pressure rise.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 20% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 85% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower efficiency raises the pump work and the back-work ratio slightly.
- Get it right
- Use the pump curve at the duty point; it is not the motor efficiency.
Duty and limit
What this means: Net power required
- What it is
- The net power the cycle must deliver: turbine output less the feed pump power, as electrical or shaft output.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.1 kW to 10000000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5000 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Steam flow and boiler and condenser duties rise in proportion; efficiency does not change.
- Get it right
- If the target is electrical output, allow for gearbox and generator losses separately.
What this means: Minimum dryness at the turbine exhaust
- What it is
- The lowest dryness fraction (mass share of vapour) allowed in the steam leaving the turbine, set to limit blade erosion by water droplets.
- Units and format
- Enter a plain number with no unit. The form accepts 0.5 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.85 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line of the exhaust dryness check.
- Get it right
- Take it from the turbine maker; it is written as a fraction, so 0.88 means 12 % moisture.
Method and formulas
- Pump: w_p = v_f (P_b − P_c) ÷ η_p (kPa × m³/kg = kJ/kg); feed water enthalpy h₂ = h_f + w_p.
- Turbine: w_t = η_t (h₃ − h₄ₛ); exhaust enthalpy h₄ = h₃ − w_t; exhaust dryness x₄ = (h₄ − h_f) ÷ h_fg.
- Net work w_net = w_t − w_p; heat added q_in = h₃ − h₂; heat rejected q_out = h₄ − h_f; efficiency η = w_net ÷ q_in; heat rate = 3,600 ÷ η kJ/kWh.
- Steam flow ṁ = net power ÷ w_net; boiler duty ṁ q_in; condenser duty ṁ q_out.
- Example: steam at 3 MPa with an inlet enthalpy of 3,116.1 kJ/kg, expanding to 10 kPa with an isentropic exit enthalpy of 2,136.1 kJ/kg, an 87 % turbine and an 85 % pump, gives 29.07 % efficiency, 5.889 kg/s of steam for 5,000 kW and an exhaust dryness of 0.8661, above a 0.85 minimum.
Limitations
- Simple cycle only: no reheat, regenerative feed heating, pressure losses in pipes and boiler, heat losses or auxiliary loads; real plant heat rates are higher.
- All steam values must come from one table or program with the same reference state; the tool cannot check them.
- A screening calculation, not a turbine selection or boiler design; pressure equipment and turbine work follow the maker’s data and qualified review.
- Example values are steam-table values chosen as an illustration. Replace them with values for your actual plant.
Typical uses
- Screen a small steam turbine or back-up generating set before asking for quotations
- Estimate steam flow, boiler duty and condenser duty for a net power output
- Check whether turbine exhaust moisture is acceptable for a chosen superheat and condenser pressure
Who it is for
- Mechanical and power engineers
- Boiler and steam plant engineers
- Engineering students and trainers
What you get
- Thermal efficiency, heat rate and back-work ratio
- Turbine, pump and net work per kilogram
- Steam flow and boiler and condenser duties for the net power
- Turbine exhaust dryness against the minimum
Interpreting the result
- A small back-work ratio is the reason steam cycles produce a lot of net work per kilogram: pumping a liquid takes little work.
- Lower condenser pressure and higher superheat both raise efficiency; superheat also keeps the exhaust drier, while lower condenser pressure makes it wetter.
Before relying on the result
- Compare the steam flow and heat rate with the turbine maker’s heat balance at the same conditions.
References
- Çengel Y. A. and Boles M. A., Thermodynamics: An Engineering Approach, 5th edition (McGraw-Hill, 2006), chapter 10 (sections 10-2 to 10-4: the ideal Rankine cycle, deviations from it, raising its efficiency). Method reference; no tables, figures or examples are reproduced.
- IAPWS Industrial Formulation 1997 for the thermodynamic properties of water and steam (IAPWS-IF97): the basis of current steam tables and software.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Method explained here · KEVOS® original book
Steam turbines: condensing and back-pressure
In On-Site Power and Heat by the Numbers
What it covers: The Rankine cycle from steam-table enthalpies: condensing plant for scale, a back-pressure turbine in place of a pressure-reducing valve, exhaust dryness, heat rate and fuel chargeable to power.
Useful alongside · KEVOS® original book
The second law and its limits
In Heat Pumps and Refrigeration by the Numbers
What it covers: Understand the second law in practical terms: heat flows downhill, entropy and irreversibility, the Carnot limit on turning heat into work, energy quality, and checking claims against the limit.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.