Wet, Dry and Superheated Steam, and the Dryness Fraction
Steam comes in three conditions. Wet steam carries droplets of water, dry saturated steam carries none, and superheated steam has been heated beyond its boiling point. Which one a plant gets, and how wet it is, changes how much heat each kilogram delivers.
You'll learn
- Tell wet, dry saturated and superheated steam apart
- Use the dryness fraction to find the enthalpy of wet steam
- Explain why heating processes want dry saturated steam, not superheated
- Understand how dryness is measured
Before this: Saturation pressure and temperature, Enthalpy
Three conditions of steam
- Wet steam: saturated steam with fine water droplets carried along in it. Most steam leaving a shell boiler is slightly wet, and it gets wetter in an uninsulated pipe as some of it condenses.
- Dry saturated steam: steam at its saturation temperature with no liquid in it. The ideal for process heating.
- Superheated steam: steam heated above its saturation temperature after all the water has boiled off. Used for turbines and some special processes.
Wet and dry saturated steam are at the same temperature for a given pressure; you cannot tell them apart with a thermometer. Superheated steam is hotter than its saturation temperature, and a thermometer shows it.
The dryness fraction
The dryness fraction, , is the share of a wet steam mixture that is actually steam. Steam with = 0.95 is 95% steam and 5% water by mass. Only the steam part has picked up latent heat, so the enthalpy is:
| Symbol | Meaning | Unit |
|---|---|---|
| Dryness fraction, 0 for water to 1 for dry steam | — | |
| Enthalpy of saturated water | kJ/kg | |
| Latent heat | kJ/kg |
- = 778.0 and = 2,002.1 kJ/kg at 10 kg/cm²(g).
- Enthalpy of the mixture:
- Latent heat it can deliver on condensing: 1,902.0 kJ/kg, against 2,002.1 for dry steam.
For superheated steam, the measure is the degree of superheat: how far above saturation it is. Steam at 10 kg/cm²(g) and 250 °C has 67 °C of superheat and an enthalpy of 2,940.2 kJ/kg.
Why process heating wants dry saturated steam
It sounds as if superheated steam should heat better: it carries more heat per kilogram. But the extra 160 kJ/kg of superheat is sensible heat in a gas, and a gas gives up heat poorly. Until superheated steam has cooled to its saturation temperature it behaves like hot air against a heat transfer surface, with a heat transfer coefficient many times lower than condensing steam. The first part of the exchanger does little work.
Superheat also breaks the link between pressure and temperature, so a process can no longer be controlled by pressure alone. That is why superheated steam from a turbine exhaust or a power boiler is usually passed through a desuperheater, which sprays in water to bring it back to saturation, before it is used for heating.
Wet steam comes from three places: priming and carryover in the boiler, condensation in poorly insulated or poorly drained mains, and droplets that a reducing valve does not dry out completely. A separator before critical users removes the water mechanically.
Key takeaways
- Wet and dry saturated steam share a temperature; superheated steam is hotter than saturation.
- Enthalpy of wet steam: . Wet steam delivers proportionally less latent heat.
- Process heating wants dry saturated steam; superheat transfers heat poorly until it is removed.
- Dryness is measured by throttling a sample and reading its temperature.