Saturation Temperature & Pressure Calculator — Steam Tables

Convert

Temperature (°C)

Pressure (bar abs)

Quantity	Value
Saturation pressure	–
Saturation pressure	–
Saturation temperature	–
Latent heat of vaporisation h_fg (Watson estimate)	–

The Calculation, Explained

Why saturation locks pressure to temperature

When liquid water and its vapour share a space and reach equilibrium, the pressure is fixed by the temperature alone — add heat and liquid boils rather than warming; remove the vapour space (a completely liquid-full vessel) and the rule collapses, which is why a water-solid boiler part is a pressure accident waiting for a relief valve. Every drum, deaerator, condenser and steam trap in the plant lives on this curve. A condenser at 0.10 bar abs condenses at 45.8 °C and not a degree lower; an HRSG drum at 40 bar holds its water at 250 °C whether the control room believes it or not.

The equation

The rigorous formulation is IAPWS IF97, the international steam-table standard. For engineering estimation the Antoine equation reproduces the curve closely across the whole practical range:

log₁₀ P = A − B / ( C + T ) (P in mmHg, T in °C)

with constants A = 8.07131, B = 1730.63, C = 233.426 below 99 °C and A = 8.14019, B = 1810.94, C = 244.485 above it (NIST data for water). Latent heat is estimated with the Watson correlation from its 100 °C value:

h_fg(T) = 2,257 × [ ( 647.096 − T_K ) / ( 647.096 − 373.15 ) ]^0.38 kJ/kg

Assumptions and applicability

Accuracy of the Antoine form is roughly ±0.5% on pressure across 1–374 °C — ample for operations and troubleshooting. For custody, warranty or design numbers use IAPWS IF97 steam tables.
All pressures are absolute. A gauge reading of "10 bar" in the plant is about 11 bar abs, saturating near 184 °C, not 180 °C — the commonest saturation error in the field.
The relation holds only where liquid and vapour coexist. Superheated steam and subcooled water are off the curve entirely; their temperatures are free variables.
The curve ends at the critical point, 374 °C / 221 bar. Supercritical plant has no saturation at all.

Worked example (the defaults above)

Water at 180 °C saturates at 9.97 bar abs (steam tables: 10.02 bar) — and the Watson correlation puts latent heat at ≈ 1,980 kJ/kg, about 2% under the tabulated 2,015 kJ/kg, a fair picture of estimation accuracy. Run backwards, a drum holding 10 bar abs sits at 180 °C: that is the temperature its level gauges, drain pots and the metal itself are living at, whatever the load is doing.

Practitioner's note. Saturation is the fastest instrument sanity check in the plant: compare any drum or condenser pressure against the metal/steam temperature beside it. If they are not on this curve together, one of the two instruments is lying — and it is rarely the physics.

References

IAPWS IF97 — Industrial Formulation for the Thermodynamic Properties of Water and Steam; Antoine constants per NIST Chemistry WebBook (water).

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