Every steam system in the world is a scaled-up kettle: add heat to water until it becomes vapour, move the vapour to where the heat is wanted, and let it condense back into water on the surface that needs heating. Simple — except for one property that makes the whole industry possible, and this lesson is about exactly that property.
Evaporation and boiling are not the same thing
Evaporation happens at any temperature, from the surface only — a wet floor dries at 30 °C because the fastest molecules at the surface escape. It is slow and shallow.
Boiling is different: vapour forms in the bulk of the liquid, as bubbles, everywhere at once. It begins at one precise condition — when the water's vapour pressure equals the pressure sitting on top of it. Push on the water harder, and it must get hotter before it can boil. That single fact is the foundation of steam engineering.
The saturation lock
Because boiling depends on pressure, every pressure has exactly one boiling temperature — called the saturation temperature. At 1 atmosphere, 100 °C. Seal the vessel and raise it to 3.5 kg/cm²g, and water refuses to boil until 148 °C. At 10.5 kg/cm²g — our practice plant's boiler — it holds out to about 185 °C.
The lock works in both directions. Steam at 3.5 kg/cm²g condenses at that same 148 °C — not gradually cooling like hot oil, but giving up its entire latent heat at one constant temperature. Plot the pairs and you get the most important curve in this academy:
Two things to notice as you move the slider. First, the curve is steep at low pressure and flattens as pressure rises — the temperature reward for each extra kg/cm² keeps shrinking, which is one reason very high temperatures are better served by other carriers. Second, the latent heat readout falls as pressure rises: high-pressure steam actually carries less latent heat per kilogram. Distribute high, use low — a theme that returns in the distribution faculty.
Steam is saturated, wet or superheated
Steam sitting exactly on the curve is saturated — vapour at its boiling point, the normal working state of process steam. Real steam usually carries a fine mist of water droplets with it: wet steam, described by its dryness fraction (97% dry is typical from a well-run shell boiler). Heat steam beyond its saturation temperature — off the curve — and it becomes superheated: excellent for turbines and long transmission, surprisingly poor at transferring heat. Both states get their own lessons; for now, hold the picture: the curve is home base.
Because of the lock, the boiler's pressure gauge tells you the steam temperature without a thermometer — every operator who "knows the boiler runs at 185 °C" is reading the saturation curve from memory, whether they call it that or not.
- Any point in a saturated steam system: pressure gauge = thermometer. Use the curve.
- If measured temperature is well above saturation for the pressure — you have superheat (check the desuperheater or the PRS: reducing pressure creates superheat).
- If it is well below — suspect air in the steam space diluting it, or a gauge lying. Air is the classic silent thief; it gets a full lesson.
- Product needs a different temperature? Change the pressure set-point, not the valve position habit.
- Boiling starts when vapour pressure equals surrounding pressure — so pressure sets the boiling point.
- One pressure ↔ one saturation temperature: the curve, both ways (boiling and condensing).
- Condensation happens at constant temperature — uniform heating, no gradients.
- Higher pressure = higher temperature but less latent heat per kg.
- Saturated is on the curve; wet is below full dryness; superheated is off the curve.
The pressure cooker is older than the steam engine. In 1679 Denis Papin demonstrated his "digester" — a sealed pot with a safety valve he had to invent for it — softening bones into jelly at what we would now call about 1 kg/cm²g and 120 °C. Every pressure cooker in every Indian kitchen is a working model of this lesson: seal the pot, raise the pressure, and dinner cooks at 120 °C instead of 100.
FAQ
Does water always boil at 100 °C?
Only under 1 atmosphere. Water boils when its vapour pressure equals the pressure sitting on it, so the boiling point moves with pressure: about 92–93 °C in Shimla's thinner air, 120 °C inside a domestic pressure cooker, 185 °C in a boiler at 10.5 kg/cm²g. "100 °C" is just the sea-level entry on the saturation curve.
How can a pressure gauge tell me the steam temperature?
In a saturated steam system, pressure and temperature are locked together — one pressure, one temperature. Read the gauge, look up the saturation table, and you have the temperature without a thermometer. The trick fails only when the steam is superheated (hotter than the curve) or mixed with air (colder than the gauge promises).
What is the difference between saturated and superheated steam?
Saturated steam sits exactly at its boiling point for the pressure — the normal state of process steam, and the best for heating because it condenses at one constant temperature. Superheated steam has been heated beyond that point: good for turbines and long transmission lines, poor at giving up heat in an exchanger. Lesson A7 covers when each earns its place.
Why generate steam at high pressure but use it at low pressure?
Two curves point the same way. High pressure means smaller pipes and drier boiler steam for distribution; low pressure means a bigger latent heat per kilogram and gentler surface temperatures at the process. So plants distribute at 8–10 kg/cm²g and reduce near the point of use — the pressure-reducing station (C3) exists for exactly this reason.
What does a dryness fraction of 97% actually mean?
That every kilogram leaving the boiler is 970 g of vapour carrying 30 g of fine water mist. The mist carries no latent heat, so wet steam delivers less energy and wets heating surfaces. A well-run shell boiler gives roughly 95–98% dryness; separators and good main drainage claw back the rest. The full picture is in A9.
Check yourself
Five quick questions on this lesson. No marks, no records — the score is for you.