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Steam tables / Latent heat of vaporisation

Latent heat of vaporisation of water against temperature

The heat a kilogram of steam gives up when it condenses, 2,500.9 kJ/kg at 0 °C falling to exactly zero at the critical point. Computed from IAPWS-IF97.

Latent heat is the vertical gap between saturated vapour and saturated liquid, and the whole reason to move heat with steam rather than with water: it comes out at constant temperature. The gap closes as pressure rises. At 100 °C condensing a kilogram releases 2,256.5 kJ, at 200 °C 1,939.7 kJ, at 300 °C 1,404.8 kJ, and at the critical point exactly nothing, because liquid and vapour have stopped being different substances.

Arrive here from a chart on the steam tables page and the temperature you dialled in comes with you, marked on the curve and bolded in the table that prints beside it. The sheet is then about one operating point rather than about water in general.

Table units — bar, °C, m³/kg, kJ/kg. Both columns of pressure, absolute and gauge, either way.
Latent heat of vaporisation kJ/kg
Latent heat of vaporisation of water in kilojoules per kilogram against temperature, falling from 2,501 at 0 °C to zero at the critical point020040060080010001200140016001800200022002400020406080100120140160180200220240260280300320340360Saturation temperature (°C)Latent heat of vaporisation (kJ/kg)
The rest of the state at 100 °C (default)
Saturation pressure1.0142 bar a · 0.0 psig
Specific volume, saturated vapour1.672 m³/kg
Saturated liquid enthalpy h_f419.1 kJ/kg
Saturated vapour enthalpy h_g2,675.6 kJ/kg
Latent heat of vaporisation along the whole saturation line
Temperature°CTemperature°FPressurebar aPressurepsigLatent heat of vaporisationkJ/kgLatent heat of vaporisationBTU/lb
032.00.006112-14.62,500.91,075.2
2577.00.0317-14.22,441.71,049.7
50122.00.1235-12.92,382.01,024.1
75167.00.386-9.12,320.6997.7
100212.01.0140.02,256.5970.1
125257.02.32219.02,188.0940.7
150302.04.76154.42,113.7908.7
175347.08.9241152,031.6873.4
200392.015.552111,939.7833.9
225437.025.493551,835.4789.1
250482.039.765621,715.3737.5
275527.059.468481,574.4676.9
300572.085.881,2311,404.8604.0
325617.0120.51,7331,191.1512.1
350662.0165.32,383892.7383.8
373.946705.1220.63,1850.00.0

Every row is a call into IAPWS-IF97 made when this page was built, not a transcription. Gauge pressure is absolute minus one standard atmosphere, 101.325 kPa, so it is negative everywhere below 99.97 °C, which is where water actually boils at one atmosphere, and a real barometer is never exactly that anyway.

Learning zone

Why it falls

The reason is visible in the specific volumes. Latent heat is the work of pulling molecules out of a liquid plus the work of pushing the atmosphere back to make room for the vapour, and as pressure rises the vapour is already dense, so there is less room to make. At 100 °C a kilogram of steam occupies 1.6719 m³ against the liquid’s 0.0010435, a ratio of about 1,602 to one. At 300 °C the ratio is down to about 15 to one. At the critical point vf = vg exactly, there is no room to make and no work to do, and hfg is zero.

What it means for the plant

Raising boiler pressure to get a higher steam temperature buys less heat per kilogram, so mass flow has to rise faster than the temperature gain suggests. It buys something back in pipe size, since vg collapses at the same time, but the feedwater load, the deaerator and the condensate return all follow the mass flow. This is also the number a trap sizing lives on: a coil rejecting a known duty is condensing duty divided by hfg kilograms an hour, and reading hfg off the wrong row is how a trap ends up half the size it needed to be.

IAPWS R7-97(2012), regions 1 to 5 implemented in full · saturation line from 0 °C to 373.946 °C · computed at page build, never transcribed

The other curves on this line

← Steam tables: every curve, both full tables and the saturated and superheated panels

The saturation dome, the Mollier h–s chart and the T–s diagram