Latent heat of vaporisation of water
| Value | 2,256,400 J/kg |
| Status | Conventional / typical value |
| Source | IAPWS-95; ASHRAE Handbook-Fundamentals |
| Categories | ThermodynamicMaterial Properties |
| joule per kilogram | 2,256,400 J/kg |
| kilojoule per kilogram | 2,256.4 kJ/kg |
| calorie per gram | 539.29254 cal/g |
| BTU per pound | 970.07739 BTU/lb |
Learning zone
Boiling a kilogram of water at 100 °C costs 2256 kJ — 6.8 times the heat of fusion, and enough to raise the same water from freezing to boiling five and a half times over. That enormous energy density is the reason steam is used to move heat around buildings and process plants: condensing one pound of steam releases about 970 BTU at constant temperature, so a steam coil delivers heat without the flow-rate and ΔT juggling a water loop demands.
The value depends strongly on pressure and temperature. It is 2501 kJ/kg at 0 °C, 2442 at 25 °C — the figure used in psychrometrics and evaporative cooling — 2256 at 100 °C, about 2015 at 10 bar, and it falls to zero at the critical point, 373.95 °C and 22.064 MPa, where liquid and vapour become indistinguishable. Evaporative cooling towers and human sweating both trade on the 25 °C value: evaporating 1 % of a water stream cools the rest by roughly 6 K.