What is Steam?
Steam is water in its gaseous state, formed when water is heated to boiling point. Learn the physics of steam formation, phases, properties, and why steam is the most efficient heat transfer medium in industrial applications.
The Basics of Steam Formation
Steam is water in its gaseous phase. When water is heated at atmospheric pressure, its temperature rises until it reaches 100°C (212°F). At this point, any additional energy added does not increase the temperature — instead, it changes the water from liquid to vapour. This energy is called the latent heat of vaporisation, and it is what makes steam such an extraordinary heat transfer medium.
At higher pressures, water boils at higher temperatures. For example, at 10 bar gauge, water boils at approximately 184°C. The relationship between pressure and boiling temperature is defined by steam tables, which every plant engineer should be familiar with.
Three Phases of Steam
- Wet steam: A mixture of vapour and water droplets. Most industrial steam is slightly wet (95–98% dry). Wet steam has lower heat content and can cause water hammer and erosion.
- Dry saturated steam: 100% vapour at the saturation temperature for its pressure. This is the ideal steam for most process heating — it carries maximum latent heat and transfers it efficiently at a constant temperature.
- Superheated steam: Steam heated above its saturation temperature at a given pressure. Used primarily for steam turbines, as it has no moisture to erode blades. Less efficient for process heating because it must cool to saturation before releasing latent heat.
Why Steam is the Preferred Heat Transfer Medium
| Property | Value | Why it matters |
|---|---|---|
| Latent heat of vaporisation | ~2,257 kJ/kg at 0 bar | Enormous energy carried per kg of steam — 1 kg of steam at 1 bar carries the same energy as ~30 kg of water at the same temperature difference |
| Heat transfer coefficient | Very high | Steam condenses on surfaces, transferring latent heat rapidly and efficiently |
| Constant temperature release | Saturation temp at given pressure | Process heating occurs at a predictable, controllable temperature |
| Transportability | Flows through pipes easily | Steam can be distributed long distances with minimal pressure loss if pipes are sized correctly |
| Sterile and non-toxic | Pure water vapour | Safe for food, pharmaceutical, and sterilisation applications |
Key Steam Properties Every Operator Should Know
- Pressure and temperature are directly related for saturated steam — knowing one tells you the other via steam tables.
- Latent heat decreases as pressure increases — higher pressure steam carries less latent heat per kg.
- Specific volume decreases as pressure increases — higher pressure steam is denser, requiring smaller pipes.
- Condensate is formed when steam releases its latent heat — this condensate must be removed by steam traps to maintain system efficiency.
The golden rule
Steam transfers heat by condensing — when steam touches a colder surface, it condenses and releases its latent heat. The resulting condensate (water) must be drained away, or it will reduce heat transfer and cause water hammer.