Enthalpy of Moist Air

The enthalpy of humid air consists of sensible and latent heat

Moist air is a mixture of dry air and water vapor. In atmospheric air water vapor content varies from 0 to 3% by mass. The enthalpy of moist and humid air includes the

  • enthalpy of the dry air - the sensible heat
  • enthalpy of the evaporated water in the air - the latent heat

The total enthalpy - sensible and latent - is used when calculating cooling and heating processes.

Specific enthalpy - h - (J/kg, Btu/lb) of moist air is defined as the total enthalpy (J, Btu) of the dry air and the water vapor mixture - per unit mass (kg, lb) of dry air.

Specific Enthalpy of Moist Air

Specific enthalpy of moist air can be expressed as:

h = ha + x hw         (1)


h = specific enthalpy of moist air (kJ/kg, Btu/lb)

ha = specific enthalpy of dry air (kJ/kg, Btu/lb)

x = humidity ratio (kg/kg, lb/lb)

hw = specific enthalpy of water vapor (kJ/kg, Btu/lb)

Specific Enthalpy of Dry Air - the Sensible Heat

Assuming constant pressure conditions the specific enthalpy of dry air can be expressed as:

ha = cpa t         (2)


cpa = specific heat of air at constant pressure (kJ/kgoC, kWs/kgK, Btu/lboF)

t = air temperature (oC, oF)

For air temperature between -100oC (-150oF) and 100oC (212oF) the specific heat can be set to

cpa = 1.006 (kJ/kgoC)

    = 0.240 (Btu/lboF)

Note! - that the enthalpy is 0 kJ/kg at 0oC. This is not correct according the definition of enthalpy in the thermodynamics, but for practical purposes in air psychrometrics this assumption is good enough since our interest is the enthalpy difference.

Specific Enthalpy of Water Vapor - the Latent Heat

Assuming constant pressure conditions the specific enthalpy of water vapor can be expressed as:

hw = cpw t + hwe         (3)


cpw = specific heat of water vapor at constant pressure (kJ/kgoC, kWs/kgK)

t = water vapor temperature (oC)

hwe = evaporation heat of water at 0oC (kJ/kg)

For water vapor the specific heat can be set to

cpw = 1.84 (kJ/kgoC)

    = 0.444 (Btu/lboF)

The evaporation heat (water at 0oC) can be set to

hwe = 2501 (kJ/kg)

    = 1075 (Btu/lb)

Using (2) and (3), (1) can be modified to

h = cpa t + x [cpw t + hwe]         (1b)

(1b) in metric units

h = (1.006 kJ/kgoC) t + x [(1.84 kJ/kgoC) t + (2501 kJ/kg)]         (1c)


h = enthalpy (kJ/kg)

x = mass of water vapor (kg/kg)

t = temperature (oC)

(1b) in Imperial units

h = (0.240 Btu/lboF) t + x [(0.444 Btu/lboF) t + (1061 Btu/lb)]         (1d)


h = enthalpy (Btu/lb)

x = mass of water vapor (lb/lb)

t = temperature (oF)

Note! - that the reference points for metric and imperial enthalpies are different.

  • the "reference" enthalpy h = 0 (kJ/kg) is at t = 0 oC and x = 0 kg/kg for metric units 
  • the "reference" enthalpy h = 0 (Btu/lb) is at t = 0 oF and x = 0 lb/lb for imperial units. The evaporation heat for water at 0 oF is 1061 Btu/lb as used in eq. (1d).

You can not convert from metric to imperial enthalpy or vice versa directly.

Example - Enthalpy in Moist Air

The enthalpy of humid air at 25oC with specific moisture content x = 0.0203 kg/kg (saturation), can be calculated as:

h = (1.006 kJ/kgoC) (25oC) + (0.0203 kg/kg) [(1.84 kJ/kgoC) (25oC) + (2501 kJ/kg)]

= (25.15 kJ/kg) + [(0.93 kJ/kg) + (50.77 kJ/kg)]

= 76.9 (kJ/kg)

Note! - the latent heat due to evaporation of water is the major part of the enthalpy. The sensible heat due to heating evaporated water vapor can be almost neglected.

Enthalpy of Moist Air containing Water as Fog

If the air contains more water than limited by saturation, some of the water exists as droplets - as fog. The enthalpy of moist air with fog can be expressed as:

h = cpa t + xs [cpw t + hwe] + (x - xs) cw t         (2)


xs = humidity ratio at saturation (kg/kg)

cw = 4.19 - specific heat water (kJ/kgoC)

Enthalpy of Moist Air containing Ice or Snow

If the air contains water as ice or snow, the enthalpy of air can be expressed as:

h = cpa t + xs [cpw t + hwe] + (x - xs) ci t - (x - xs) him         (3)


ci = 2.05 - specific heat ice (kJ/kg.oC)

him= 335 - melting heat of ice (kJ/kg)

Related Topics

  • Air Psychrometrics - The study of moist and humid air - psychrometric charts, Mollier diagrams, air-condition temperatures and absolute and relative humidity and moisture content

Related Documents

  • Humidity Ratio of Air - The ratio between the mass of water vapor present in moist air - to the mass of dry air
  • Air Properties - Density, specific heat, thermal conductivity, expansion coefficient, kinematic viscosity and Prandtl's number - for air temperatures ranging from -150 oC to 400 oC
  • Heat-Recovery - Ventilation and heat-recovery calculations, sensible and latent heat -online calculators - imperial units
  • Great Sensible Heat Factor - GSHF - Great Sensible Heat Factor is the ratio between sensible and total heat in a cooling coil
  • Psychrometric Terms - Dry and Wet Bulb Temperature, Specific Volume, Relative Humidity,  Enthalpy and more ..
  • Ice - Thermal Properties - Thermal and thermodynamic properties of ice - density, thermal conductivity and specific heat at temperatures from 0 to -100 oC
  • Water - Thermal Properties - Thermal properties of water - density, freezing temperature, boiling temperature, latent heat of melting, latent heat of evaporation, critical temperature and more
  • Air Heating Systems - Using air to heat buildings - temperature rise diagram
  • Mixing Humid Air - The state of mixed moist air - enthalpy, heat, temperature and specific humidity
  • Heat Removed when Cooling Air - Heat removed when air is cooled to storage room conditions
  • Specific Heat of Dry Air - Specific heat of dry air will vary with temperature
  • Moist Air Properties - Psychrometric table with humid air properties
  • Drying Force of Air - The drying force of air can be expressed with moisture holding capacity of air and the evaporation capacity from water surface to air

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  • de: Enthalpie feuchte feuchte Luft

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