Enthalpy: The One Number That Describes Heat in Water and Steam
Enthalpy is the heat content of a kilogram of water or steam, measured from an agreed starting point. It lets every heat calculation in a boiler house be done by subtraction: heat added is enthalpy out minus enthalpy in.
You'll learn
- Explain enthalpy in plain terms and read , and
- Calculate a boiler’s heat output from enthalpies
- Find the enthalpy of superheated steam
- Explain why steam leaves a pressure reducing valve superheated
Before this: Sensible heat, Latent heat
Heat content, measured from a fixed starting point
You cannot say how much heat a kilogram of steam “contains” without saying where you started counting. Steam tables fix that starting point at liquid water at 0.01 °C. Enthalpy is then the heat that has gone into a kilogram since that point: sensible heat to warm the water, latent heat to boil it, more sensible heat if the steam is superheated.
Because every value is counted from the same zero, you never need to add up the stages yourself. The heat a boiler puts into each kilogram is simply the enthalpy of the steam leaving minus the enthalpy of the water coming in. That subtraction is behind almost every boiler calculation in this curriculum.
Enthalpy in the steam tables
| Symbol | Meaning | Unit |
|---|---|---|
| Saturated water: all the sensible heat to reach boiling point | kJ/kg | |
| Latent heat to turn it into steam | kJ/kg | |
| Dry saturated steam: the total | kJ/kg |
| Symbol | Meaning | Unit |
|---|---|---|
| Heat rate into the water | kJ/h | |
| Steam (= feedwater) flow | kg/h | |
| Enthalpy of the steam produced | kJ/kg | |
| Enthalpy of the feedwater | kJ/kg |
- 2,000 kg/h of dry saturated steam at 10 kg/cm²(g), from feedwater at 80 °C.
- From a steam table: at 10 kg/cm²(g) is 2,780.1 kJ/kg; water at 80 °C is 335.8 kJ/kg.
- Heat added to each kilogram, then the heat rate:
- In other units: 1,358 kW, or 11.7 lakh kcal/h.
Superheated steam, and what a reducing valve does
Superheated steam needs two numbers to fix its state, pressure and temperature, because its temperature is no longer tied to its pressure. At 10 kg/cm²(g) and 250 °C its enthalpy is 2,940.2 kJ/kg, against 2,780.1 for dry saturated steam at the same pressure.
A pressure reducing valve does no work and, over a short distance, loses almost no heat. So enthalpy is unchanged across it: steam goes in at 2,780.1 kJ/kg and comes out at 2,780.1 kJ/kg. But at 2 kg/cm²(g), dry saturated steam only needs 2,724.5. The extra has to show up somewhere, and it shows up as superheat: the steam leaves at about 159 °C, roughly 25 °C above its saturation temperature of 133.2 °C.
Strictly, enthalpy is internal energy plus pressure times volume, . For liquid water the part is tiny, which is why feedwater enthalpy changes so little with pressure, under 1 kJ/kg across ordinary boiler pressures. For steam it is a substantial share, and it is the reason enthalpy, not internal energy, is the right quantity for anything that flows.
Key takeaways
- Enthalpy is heat content per kilogram, counted from water at 0.01 °C.
- Heat added in a boiler = steam flow × (steam enthalpy − feedwater enthalpy).
- Superheated steam needs both pressure and temperature to look up.
- Throttling keeps enthalpy constant, so a reducing valve leaves steam slightly superheated.