How to Read a Steam Table, and What Specific Volume Tells You
A steam table lists the properties of water and steam at each pressure. Every calculation in a steam plant starts with a row from one. Reading it correctly takes three checks: gauge or absolute, kJ or kcal, and saturated or superheated.
You'll learn
- Read every column of a saturated steam table
- Avoid the gauge-versus-absolute trap
- Interpolate between rows, and know when not to
- Use specific volume to understand pipe sizes
Before this: Enthalpy, Saturation pressure and temperature
What a steam table is
A saturated steam table has one row per pressure. Along each row are the saturation temperature and the properties of the water and the steam at that point: how much heat each holds and how much space each takes up. Find your pressure, read across, and you have everything a calculation needs.
| Pressure, kg/cm²(g) | Temp., °C | , kJ/kg | , kJ/kg | , kJ/kg | , m³/kg |
|---|---|---|---|---|---|
| 5 | 158.3 | 668.1 | 2,087.4 | 2,755.5 | 0.3198 |
| 7 | 169.8 | 718.2 | 2,049.4 | 2,767.7 | 0.2439 |
| 10 | 183.3 | 778.0 | 2,002.1 | 2,780.1 | 0.1803 |
| 15 | 200.5 | 854.8 | 1,937.6 | 2,792.4 | 0.1258 |
Computed from IAPWS-IF97. GyanTosh’s full table is in the Steam Table tool.
- Temperature: the saturation temperature at that pressure.
- : heat in the water at boiling point. : latent heat. : heat in dry saturated steam.
- : specific volume of the steam, the space one kilogram occupies.
Three checks before you read a number
- Gauge or absolute? Most published tables are in absolute pressure (bar a, MPa, kg/cm² a). Plant gauges read gauge. Add atmospheric pressure, about 1.033 kg/cm², to a gauge reading before looking it up in an absolute table.
- kJ or kcal? Older Indian tables list kcal/kg. Divide kJ by 4.1868 to compare.
- Saturated or superheated? A saturated table has one row per pressure. Superheated steam needs a separate table indexed by both pressure and temperature.
- The table has rows at 7 and 10 kg/cm²(g) but not 8.5.
- 8.5 is halfway between, so take halfway between the two temperatures: 169.8 and 183.3 give 176.6 °C.
- Do the same for : 2,773.9 kJ/kg.
- The exact values at 8.5 kg/cm²(g) are 177.0 °C and 2,774.5 kJ/kg.
Specific volume: why steam pipes are the size they are
2,000 kg/h of steam at 10 kg/cm²(g) occupies 0.1803 m³/kg × 2,000 = 361 m³/h. The same mass at atmospheric pressure would be 3,347 m³/h, 9.3 times as much. Pipe sizes follow volume, not mass, so high-pressure distribution means smaller pipes.
Where tables mislead
Linear interpolation fails across wide gaps, because the saturation curve bends. Interpolating halfway between 0 and 10 kg/cm²(g) gives 141.7 °C for 5 kg/cm²(g); the true value is 158.3 °C. Use the nearest rows, or a tool that computes the property directly.
Two tables can also disagree in the last digit without either being wrong. Older tables used different formulations and sometimes a different reference state. Modern industrial tables, including GyanTosh’s, are computed from IAPWS-IF97, the international standard since 1997; if a vendor’s figures differ by more than a few tenths of a percent, ask what they are based on.
Specific volume also explains why steam is so powerful and so dangerous. At atmospheric pressure a kilogram of steam occupies about 1,604 times the volume of the water it came from. Water trapped in a hot vessel that suddenly flashes, or a pocket of condensate swept along by steam in a main, carries that violence with it.
Key takeaways
- One row per pressure: temperature, , , and specific volume.
- Check gauge vs absolute, kJ vs kcal, and saturated vs superheated before reading.
- Interpolate only between close rows; the saturation curve bends.
- Specific volume falls steeply with pressure, which is why steam is distributed at high pressure.