KEVOS® MAKEOS · Hydraulics
Vapour Pressure and Boiling Check (Clausius–Clapeyron, Boiling Point at Pressure, Margin)
At this pressure, at what temperature will the liquid boil, what is its vapour pressure at the working temperature, and is there enough margin to stop it boiling or flashing?
Vapour pressure and boiling point of a liquid from one known point on its vapour-pressure curve and its latent heat, by the integrated Clausius–Clapeyron equation (vapour treated as an ideal gas, liquid volume neglected, latent heat constant): the boiling temperature at the working pressure, the vapour pressure at the liquid temperature (the value the NPSH tools ask for), the slope of the curve, and the margin between the liquid temperature and the boiling point, against the margin you require.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Known point on the vapour-pressure curve | ||
| Reference temperature | °C | A temperature at which the vapour pressure is known, ideally close to the working temperature (the normal boiling point is a common choice). |
| Vapour pressure at the reference temperature | kPa abs | Vapour pressure at that temperature, absolute (101.325 kPa at the normal boiling point). |
| Latent heat of vaporisation | kJ/kg | Latent heat of vaporisation near the working temperature, from property tables such as the NIST Chemistry WebBook. |
| Molar mass of the liquid | g/mol | Molar mass of the liquid (water 18.015 g/mol). |
| Working condition | ||
| Working pressure on the liquid | kPa abs | Absolute pressure on the liquid surface: chamber pressure, suction pressure or the pressure after a valve. |
| Liquid temperature | °C | Temperature of the liquid. |
| Margin required below the boiling point | K | Temperature margin you require below the boiling point (from your design practice). |
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.
Known point on the vapour-pressure curve
What this means: Reference temperature
- What it is
- A temperature at which the liquid’s vapour pressure is known, anchoring the vapour-pressure curve the tool extends to the working condition.
- 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
- Moves the whole curve; the further the working temperature is from it, the larger the model error.
- Get it right
- Pick a reference point close to the working temperature; the normal boiling point is convenient but far from a cold vacuum process.
What this means: Vapour pressure at the reference temperature
- What it is
- The absolute vapour pressure of the liquid at the reference temperature, the pressure at which liquid and vapour are in balance there.
- 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
- Raising it shifts the curve to higher pressures, so the liquid boils at a lower temperature at the same working pressure.
- Get it right
- Use absolute pressure; at the normal boiling point it is one standard atmosphere, 101.325 kPa.
What this means: Latent heat of vaporisation
- What it is
- The heat needed to turn one kilogram 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/kg: kJ/kg. The form accepts 0.000001 kJ/kg to 100000 kJ/kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2257 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 larger latent heat makes the vapour pressure change faster with temperature.
- Get it right
- Take the value near the working temperature from property tables; latent heat falls as temperature rises.
What this means: Molar mass of the liquid
- What it is
- The mass of one mole of the liquid’s molecules, used to find the specific gas constant of its vapour.
- Units and format
- Enter a number in g/mol: g/mol. The form accepts 1 g/mol to 1000 g/mol. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 18.015 g/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 molar mass lowers the gas constant, steepening the vapour-pressure curve.
- Get it right
- Enter grams per mole (water 18.015); a value in kilograms per mole would be a thousand times too small.
Working condition
What this means: Working pressure on the liquid
- What it is
- The absolute pressure acting on the liquid surface in service, such as the chamber pressure in vacuum drying or the pressure at a pump inlet.
- 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 20 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 lower pressure lowers the boiling temperature and the margin.
- Get it right
- Convert vacuum gauge readings to absolute pressure: absolute = atmospheric − vacuum reading.
What this means: Liquid temperature
- What it is
- The temperature of the liquid in service, at which its vapour pressure is calculated and compared with the boiling point.
- 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 45 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher liquid temperature raises its vapour pressure and reduces the margin to boiling.
- Get it right
- Use the hottest temperature the liquid reaches, for example after heating or friction in a pump.
What this means: Margin required below the boiling point
- What it is
- The number of degrees the liquid must stay below its boiling point at the working pressure, the safety margin against boiling or flashing.
- Units and format
- Enter a number in K: kelvin (a difference of 1 K equals 1 °C). The form accepts 0 K to 500 K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 K 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; the boiling point and vapour pressure do not change.
- Get it right
- Set it from your own design practice; this tool does not recommend a value.
Method and formulas
- Specific gas constant R = 8.314462618 ÷ M kJ/(kg·K); with L constant, ln (p ÷ p₁) = −(L ÷ R)(1 ÷ T − 1 ÷ T₁) (Clausius–Clapeyron, Fermi eqs 96–98).
- Boiling temperature at p: 1 ÷ T_b = 1 ÷ T₁ − (R ÷ L) ln (p ÷ p₁); vapour pressure at the liquid temperature p_v = p₁ exp[−(L ÷ R)(1 ÷ T_w − 1 ÷ T₁)].
- Slope dp/dT = L p_v ÷ (R T_w²); margin = T_b − T_w, compared with the margin required.
- Example: at 20 kPa the liquid boils at 58.889 °C; at 45 °C its vapour pressure is 10.515 kPa.
Limitations
- Latent heat is taken as constant and the vapour as an ideal gas, so accuracy falls the further the working temperature is from the reference point; choose a reference point near the working condition.
- Pure liquids only; dissolved gases and mixtures boil differently, and dissolved air comes out of solution before the liquid boils.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check that a liquid will not boil in a vacuum degassing or drying chamber
- Find the vapour pressure to use in an NPSH check
- Find how far a hot liquid is from flashing at a pump suction or after a valve
Who it is for
- Process engineers
- Maintenance engineers
- Hydraulic and pump engineers
What you get
- Boiling temperature at the working pressure
- Vapour pressure at the liquid temperature and the slope of the curve
- Margin to boiling against the margin required
Interpreting the result
- Vapour pressure roughly doubles for every 10 to 20 K rise for many liquids near room temperature, so a small temperature rise can remove the whole margin under vacuum.
References
- Fermi E., Thermodynamics (Dover Publications, 1956; first published by Prentice-Hall, 1937), pp. 63–68. Method reference; no text, examples or tables are reproduced.
- NIST Chemistry WebBook, NIST Standard Reference Database 69 — https://webbook.nist.gov/chemistry/ (source for vapour pressures and latent heats; values are entered by the user).
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
Saturation pressure and temperature
In Process Chillers by the Numbers
What it covers: Why boiling point follows pressure, the Clapeyron equation checked against NIST R-134a data, reading gauges, superheat and subcooling, and air in a condenser.
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.
Explains the concept · 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.
Useful alongside · KEVOS® original book
Water in air: vapour pressure, humidity and dew point
In Condensation Control by the Numbers
What it covers: What vapour pressure is, why it depends only on temperature, how relative humidity, dew point and absolute humidity follow from it, and why dew point is the number to watch.
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.