Steam, Water and Property Tables
Saturation, quality, superheat, how to read steam tables, interpolation and the boiling point against pressure.
Phases, saturation and the dome
At any pressure, water boils at a single temperature, the saturation temperature Tsat. Below it the water is a subcooled (compressed) liquid; above it the water is superheated vapor; at exactly Tsat liquid and vapor can coexist, in any proportion, as saturated mixtures. The boiling point rises with pressure, which is why a pressure cooker cooks faster and why a high-altitude kettle boils below 100 °C.
During boiling the temperature stays constant while the latent heat hfg is added: 2 257 kJ/kg at atmospheric pressure, falling as pressure rises, and zero at the critical point (22.06 MPa, 374 °C). Steam carries most of its energy as latent heat, which is why it is so effective for heating: condensing it on a surface releases hfg at a constant temperature.
Quality and wet steam
A saturated mixture is described by its quality x, the mass fraction that is vapor. The mixture’s properties are weighted averages of the liquid (subscript f) and vapor (subscript g) values:
h = hf + x hfg v = vf + x (vg − vf) s = sf + x sfg
Worked example: wet steam at 1 MPa
Steam at 1 MPa (Tsat = 179.88 °C) has a quality of 0.8.
- Enthalpy h = 762.51 + 0.8 × 2 014.6 = 2 374.2 kJ/kg.
- Specific volume v = 0.001127 + 0.8 × (0.1944 − 0.001127) = 0.1557 m³/kg: the 20 percent of liquid takes up almost no space.
- Wet steam carries less energy and erodes turbine blades and pipe bends; steam systems keep it above about 0.9 and turbines above about 0.88 at exit.
Reading the steam tables
Steam tables list properties of saturated water against pressure (or temperature). A short version, to 4 significant figures, is below; the full tables (and superheated steam) are in any thermodynamics book, the free IAPWS-IF97 formulations and the NIST WebBook.
| p (kPa) | Tsat (°C) | hf (kJ/kg) | hfg (kJ/kg) | hg (kJ/kg) | vg (m³/kg) |
|---|---|---|---|---|---|
| 10 | 45.81 | 191.81 | 2 392.1 | 2 583.9 | 14.6700 |
| 50 | 81.32 | 340.54 | 2 304.7 | 2 645.2 | 3.2400 |
| 100 | 99.61 | 417.51 | 2 257.5 | 2 675.0 | 1.6940 |
| 200 | 120.21 | 504.71 | 2 201.6 | 2 706.3 | 0.8858 |
| 500 | 151.83 | 640.09 | 2 108.0 | 2 748.1 | 0.3749 |
| 1 000 | 179.88 | 762.51 | 2 014.6 | 2 777.1 | 0.1944 |
| 2 000 | 212.38 | 908.47 | 1 889.8 | 2 798.3 | 0.0996 |
| 4 000 | 250.35 | 1 087.40 | 1 714.1 | 2 801.4 | 0.0498 |
| 5 000 | 263.94 | 1 154.50 | 1 639.7 | 2 794.2 | 0.0395 |
| 10 000 | 311.00 | 1 407.90 | 1 317.6 | 2 725.5 | 0.0180 |
Note how hfg falls and hf rises with pressure, and how the specific volume of the vapor falls by a factor of nearly 800 from 10 kPa to 10 MPa, so high-pressure steam needs far smaller pipes. The full 59-download library includes a water-property table and printable formula sheets in Downloads.
Interpolation
Real values fall between table rows, so interpolate. Over a small range a straight line is enough; over a wide range the saturation temperature is closer to a straight line against the logarithm of pressure.
Worked example: saturation at 3 MPa
- The table has 2 000 kPa (212.38 °C) and 4 000 kPa (250.35 °C). Linear in ln p: fraction = ln(3 000 ÷ 2 000) ÷ ln(4 000 ÷ 2 000) = 0.585, so T = 212.38 + 0.585 × 37.97 = 234.6 °C.
- The full table gives 233.90 °C, an error of 0.7 K: good enough for estimates. Interpolating hfg gives 1 787 kJ/kg (table 1 795.9).
- For critical work, take the row each side that is closest, or compute with a property program.
Superheated steam
Heated beyond the saturation temperature at constant pressure, dry steam becomes superheated. The degree of superheat is T − Tsat. Superheat adds sensible heat (cp about 2.1 kJ/kg·K near atmospheric, more at pressure), raises turbine efficiency and keeps the exhaust dry. Superheated tables list v, u, h and s against p and T.
Example: steam at 1 MPa and 300 °C has h = 3 051.6 kJ/kg, which is 274.5 kJ/kg more than dry saturated steam (hg = 2 777.1) for 120.1 K of superheat. Process heating usually wants saturated steam (best heat transfer), so superheated steam from a boiler is often de-superheated.
Why steam?
Worked example: heating water with steam
- Heat 1 000 kg/h of water from 20 °C to 80 °C: Q̇ = 0.278 kg/s × 4.186 × 60 = 69.8 kW.
- Using saturated steam at 200 kPa (hfg = 2 201.6 kJ/kg, 120.2 °C) which condenses completely: steam flow = 69.8 ÷ 2 201.6 = 0.0317 kg/s = 114 kg/h.
- A pressure cooker at 200 kPa absolute (about 100 kPa gauge) boils at 120.2 °C instead of 100 °C.
Related
- Previous: The Second Law, Entropy and Carnot. Next: Steam Power Cycles: Rankine.
- The Downloads library for the water-properties table.
- Psychrometric Calculator for moist air.
- Thermodynamics and Steam: the whole guide.
FAQ
What is the difference between saturated and superheated steam?
Saturated steam is at the boiling temperature for its pressure (dry or wet). Superheated steam is hotter than that, and so acts as a gas.
Why is latent heat so large?
Pulling molecules out of the liquid takes energy to break the bonds: for water, 2 257 kJ/kg, which is the energy needed to heat the same water by 540 K.
Do the tables use gauge or absolute pressure?
Absolute. Add about 101 kPa to a gauge reading.
Why does a steam burn hurt more than boiling water?
Condensing steam gives up its latent heat on the skin, many times the sensible heat of water at the same temperature.
References
- Wagner, W., Kretzschmar, H.-J., International Steam Tables (IAPWS-IF97), Springer.
- Çengel, Y. A. and Boles, M. A., Thermodynamics: An Engineering Approach, McGraw-Hill (property tables, Appendix).
- NIST Chemistry WebBook, fluid properties for water (webbook.nist.gov).
Last reviewed: October 2026. Table values are rounded to the usual four to six figures.