KEVOS® MAKEOS · Engineering
Boiling Margin Check (Clausius–Clapeyron Vapour Pressure, Keep Liquid or Make It Boil)
From one known boiling point and the heat of vaporisation, what is this liquid’s vapour pressure at my temperature, at what temperature does it boil at my pressure, and is there enough margin to keep it liquid (or to make it boil)?
Vapour pressure and boiling temperature from the integrated Clausius–Clapeyron relation, with a constant molar enthalpy of vaporisation and one known boiling point: the vapour pressure at the liquid temperature, the boiling temperature at the absolute pressure above the liquid, the temperature margin to boiling, and the pressure that gives the required margin, judged either for keeping a liquid from boiling or flashing (pump suctions, hot transfers, vacuum vessels) or for making it boil (vacuum drying, degassing, distillation).
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Known boiling temperature | °C | A temperature at which the liquid’s vapour pressure is known, such as its normal boiling point. |
| Pressure at that boiling temperature | kPa abs | The vapour pressure at that temperature (101.325 kPa for the normal boiling point). |
| Molar enthalpy of vaporisation | kJ/mol | Molar enthalpy of vaporisation, preferably a mean over the temperature range used. |
| Operating condition | ||
| Liquid temperature | °C | Temperature of the liquid. |
| Absolute pressure above the liquid | kPa abs | Absolute pressure in the vapour space above the liquid. |
| What you need | choice | Whether the liquid must stay liquid or must boil. |
| Required temperature margin | K | Temperature margin your design needs between the liquid and its boiling point. |
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.
What this means: Known boiling temperature
- What it is
- A temperature at which the vapour pressure of this liquid is known, usually its normal boiling point at one standard atmosphere.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -200 °C to 1000 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Shifts the whole vapour pressure curve along the temperature axis.
- Get it right
- Choose a known point as close as possible to the operating temperature for the best accuracy.
What this means: Pressure at that boiling temperature
- What it is
- The vapour pressure of the liquid at the known temperature: one standard atmosphere for the normal boiling point.
- Units and format
- Enter a number in kPa abs: kPa abs. The form accepts 0.001 kPa abs to 100000 kPa abs. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 101.325 kPa abs as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Every vapour pressure scales in proportion to it.
- Get it right
- Use absolute pressure; 101.325 kPa is one standard atmosphere.
What this means: Molar enthalpy of vaporisation
- What it is
- The heat needed to turn one mole of the liquid into vapour at constant temperature, which sets how steeply vapour pressure rises with temperature.
- Units and format
- Enter a number in kJ/mol: kJ/mol. The form accepts 0.1 kJ/mol to 500 kJ/mol. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40.66 kJ/mol as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger value makes vapour pressure change faster with temperature.
- Get it right
- Use a mean value over the range from the known point to the operating temperature; it falls as temperature rises.
Operating condition
What this means: Liquid temperature
- What it is
- The temperature of the liquid in the vessel, line or pump suction being checked.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -200 °C to 1000 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Vapour pressure rises steeply with it and the margin changes degree for degree.
- Get it right
- Use the hottest liquid temperature that can occur, including heat from pumps and recirculation.
What this means: Absolute pressure above the liquid
- What it is
- The absolute pressure in the space above the liquid, or at the point in the line where boiling or flashing is a concern.
- Units and format
- Enter a number in kPa abs: kPa abs. The form accepts 0.001 kPa abs to 100000 kPa abs. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 kPa abs as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher pressure raises the boiling temperature and the margin to boiling.
- Get it right
- Convert gauge readings to absolute by adding atmospheric pressure; vacuum gauges often read below atmosphere.
What this means: What you need
- What it is
- Whether the liquid must stay liquid, as in pump suctions and hot transfers, or must boil, as in vacuum drying, degassing or distillation.
- Units and format
- Choose one of 2 options from the list: Keep it liquid (no boiling or flashing); Make it boil (drying, degassing, distillation).
- Example
- The form opens with “Keep it liquid (no boiling or flashing)” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Reverses the margin and the pass rule.
- Get it right
- Choose keep liquid for flashing or cavitation checks and make it boil for drying or solvent removal.
What this means: Required temperature margin
- What it is
- The temperature difference your design needs between the liquid and its boiling point, to allow for pressure dips, hot spots and measurement error.
- Units and format
- Enter a number in K: kelvin (a difference of 1 K equals 1 °C). The form accepts 0 K to 200 K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Moves the pass line and the pressure needed for the margin.
- Get it right
- Set it from your design rules or the equipment maker’s guidance for the duty.
Method and formulas
- Clapeyron’s relation applied to a saturated vapour, neglecting the liquid volume and treating the vapour as a perfect gas, gives d ln p ÷ dT = ΔH ÷ R T² (Guggenheim §3.37–3.39).
- With ΔH constant: ln(p ÷ p_b) = −(ΔH ÷ R)(1/T − 1/T_b), so the vapour pressure at T and the boiling temperature at p follow directly.
- Margin = boiling temperature − liquid temperature (keep liquid), or the reverse (make it boil); the pressure for the margin is the vapour pressure at the liquid temperature plus (or minus) the margin.
- Example: water from 100 °C at 101.325 kPa with 40.66 kJ/mol has 21.006 kPa of vapour pressure at 60 °C and boils at 68.29 °C under 30 kPa, a margin of 8.2895 K.
Limitations
- The enthalpy of vaporisation falls with temperature, so accuracy drops far from the known point (water at 60 °C reads about 5 % high from the 100 °C value).
- The vapour is treated as a perfect gas; near the critical point the relation fails.
- Pure liquids only; mixtures boil over a range.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check that a hot liquid will not flash in a vessel or suction line
- Find the vacuum needed to boil off a solvent or water at a set temperature
- Estimate a vapour pressure where no table is at hand
Who it is for
- Process engineers
- Mechanical engineers
- Plant operators
What you get
- Vapour pressure at the liquid temperature
- Boiling temperature at the operating pressure
- Temperature margin and pressure for the margin
Interpreting the result
- Vapour pressure rises steeply with temperature, so a modest temperature rise can make a liquid flash in a vacuum vessel or suction line.
- Where accuracy matters, take the known point close to the operating temperature.
Before relying on the result
- Check the vapour pressure against measured data for the liquid at the operating temperature.
References
- E. A. Guggenheim, Thermodynamics: An Advanced Treatment for Chemists and Physicists, 5th revised edition (North-Holland, 1967), §3.37–3.39 (Clapeyron’s relation and its application to saturated vapour). Method reference; no text, tables or examples are reproduced.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Explains the concept · KEVOS® original book
Liquefied gases, vapour pressure and the critical point
What it covers: Vapour pressure and Clausius–Clapeyron, liquid nitrogen in a microbulk tank, why warm liquid holds less, CO₂ cylinders that go liquid-full in summer, and a tank sized at the wrong density.
Method explained here · KEVOS® original book
Vapour pressure and boiling
In Solvent Vapours by the Numbers
What it covers: What vapour pressure is, how it rises steeply with temperature by the Clausius–Clapeyron relation, checking it against Antoine data, boiling, and ranking solvents by volatility.
Method explained here · KEVOS® original book
Vapour, outgassing and gas loads
What it covers: Gas load once the air is gone: vapour pressure and Clausius–Clapeyron checked against NIST, evaporation limited by heat, resin and walls, and a wet container.
Useful alongside · KEVOS® original book
NPSH and cavitation
In Centrifugal Pumps by the Numbers
What it covers: Cavitation, NPSH available and required, TP-3's 4.0 m margin and what altitude, hot water and a clogged foot valve take from it, and a pump moved up a plinth that began to cavitate.
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.