Latent Heat of Steam: What It Is and Why It Falls as Pressure Rises
Latent heat is the heat that changes water into steam without changing its temperature, and the heat steam gives back when it condenses. It is why industry heats with steam, and it is not a fixed number: the higher the pressure, the less of it each kilogram of steam carries.
You'll learn
- Define latent heat and the enthalpy of evaporation
- Read how , and change with pressure
- Calculate the steam flow a heating load needs
- Avoid the 540 kcal/kg shortcut and the error it brings
Before this: Sensible heat
What latent heat is
Heat water at atmospheric pressure and its temperature climbs to 100 °C, then stops. The burner is still firing and heat is still going in, but the thermometer does not move until the last of the water has turned to steam. That heat has not disappeared; it has gone into pulling the water molecules apart into a vapor. Because a thermometer cannot see it, it is called latent, meaning hidden, heat.
The same heat comes back out when steam condenses, again at constant temperature. That return is the entire point of a steam system: the boiler loads heat into the steam as latent heat, the pipes carry it, and the process takes it out by condensing the steam against a cooler surface.
Why industry heats with steam
Latent heat is large. One kilogram of steam condensing at 4 kg/cm²(g) releases 2,109 kJ. To deliver the same heat from hot water cooling by 20 °C, you would need to circulate about 25 kg of water. That is why steam lines are small for the heat they carry, and why hot water and thermic fluid systems need pumps and larger pipework.
It is also released at a fixed temperature. Steam at 4 kg/cm²(g) condenses at 151.4 °C and nowhere else, so a process held at a steady pressure is held at a steady temperature. Controlling temperature with a pressure reducing valve and a gauge is a direct consequence of latent heat.
The formulas
In steam tables, latent heat is the enthalpy of evaporation, written : the difference between saturated steam, , and saturated water at the same pressure, .
| Symbol | Meaning | Unit |
|---|---|---|
| Heat released | kJ | |
| Mass of steam condensed | kg | |
| Latent heat (enthalpy of evaporation) at the steam pressure | kJ/kg |
| Symbol | Meaning | Unit |
|---|---|---|
| Enthalpy of dry saturated steam | kJ/kg | |
| Enthalpy of saturated water: the sensible heat to reach boiling point | kJ/kg | |
| Latent heat to turn that water into steam | kJ/kg |
All three are measured from water at 0.01 °C, the reference point of the steam tables, which is why they can be added and subtracted directly.
Worked example: steam flow for a heating load
- A heat exchanger needs 500 kW: 500 kJ every second.
- Dry saturated steam is available at 4 kg/cm²(g), and condensate leaves at saturation temperature, so only latent heat is used.
- Latent heat at 4 kg/cm²(g), from a steam table: 2,109.4 kJ/kg.
- Rearrange for the steam flow, converting seconds to hours:
- The same load supplied at 10 kg/cm²(g), where is 2,002.1 kJ/kg, needs 899 kg/h: more steam, because each kilogram carries less latent heat.
Two assumptions are hiding in that answer. The steam must be dry: steam that is 95% dry carries only 95% of its latent heat, so the flow rises accordingly. And the condensate must be removed as it forms, by a steam trap; condensate left in the exchanger blankets the surface and the latent heat never gets out.
Latent heat falls as pressure rises
| Pressure, kg/cm²(g) | Saturation temp., °C | , kJ/kg | , kJ/kg | , kJ/kg | , kcal/kg |
|---|---|---|---|---|---|
| 0 | 100.0 | 419.0 | 2,256.5 | 2,675.5 | 539.0 |
| 2 | 133.2 | 560.2 | 2,164.3 | 2,724.5 | 516.9 |
| 4 | 151.4 | 638.1 | 2,109.4 | 2,747.5 | 503.8 |
| 7 | 169.8 | 718.2 | 2,049.4 | 2,767.7 | 489.5 |
| 10 | 183.3 | 778.0 | 2,002.1 | 2,780.1 | 478.2 |
| 17.5 | 207.6 | 886.8 | 1,909.4 | 2,796.2 | 456.1 |
| 21 | 216.3 | 926.7 | 1,873.2 | 2,799.9 | 447.4 |
| 40 | 250.7 | 1,089.2 | 1,711.6 | 2,800.8 | 408.8 |
Computed from IAPWS-IF97 with atmospheric pressure at 1.0332 kg/cm². Gauge 0 is atmospheric.
From atmospheric pressure to 10 kg/cm²(g), the total heat in a kilogram of steam rises only from 2,676 to 2,780 kJ/kg. But the latent part falls from 2,257 to 2,002 kJ/kg, because more of the total is now sensible heat in water that boils at 183 °C instead of 100 °C. Keep raising the pressure and latent heat keeps shrinking, reaching zero at the critical point, about 224 kg/cm²(g), where water and steam stop being distinguishable. That is why a supercritical power boiler has no drum: there is no boiling to separate.
Latent heat as a rating datum: F&A 100 °C
Indian boilers are rated “from and at 100 °C”: the steam the boiler would make from water at 100 °C into steam at 100 °C. That is pure latent heat at atmospheric pressure, taken by convention as 2,256.9 kJ/kg, and it makes boilers comparable whatever pressure and feedwater temperature they will actually run at.
A boiler’s real output is lower, because at working pressure each kilogram of steam needs the latent heat plus the sensible heat to bring cold feedwater up to boiling point. The ratio between the two is the evaporation factor, which comes up again in Module 02, Boiler Fundamentals; the F&A 100 °C converter works it out for a given pressure and feedwater temperature.
Key takeaways
- Latent heat changes state at constant temperature. For steam it is , and .
- It falls as pressure rises: 2,257 kJ/kg at atmospheric pressure, 2,002 at 10 kg/cm²(g).
- Size steam supplies with at the pressure of use, never the 539 kcal/kg atmospheric figure.
- Distribute steam at high pressure, use it at the lowest pressure the process allows.