Enthalpy of Moist and Humid Air

The enthalpy of moist and humid air consist of sensible heat and latent heat. The enthalpy is important for calculating cooling and heating processes

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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

Specific enthalpy - h - (kJ/kg) of moist air is defined as the total enthalpy of the dry air and the water vapor mixture per kg of moist air.

Specific Enthalpy of Moist Air

Specific enthalpy of moist air can be expressed as:

h = ha + x hw (1)

where

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

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

x = humidity ratio (kg/kg)

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

Specific Enthalpy of Dry Air - Sensible Heat

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

ha = cpa t (2)

where

cpa = specific heat capacity of air at constant pressure (kJ/kg.oC, kWs/kg.K)

t = air temperature (oC)

For air temperatures between -100oC and 100oC the specific heat capacity can be set to

cpa = 1.006 (kJ/kgoC)

Note! that the enthalpy is 0 at 0oC. This is of course not correct according the enthalpy definition in the thermodynamics, but for practical purposes in air psychrometrics it do because our interest is the enthalpy difference.

Specific Enthalpy of Water Vapor - Latent Heat

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

hw = cpw t + hwe (3)

where

cpw = specific heat capacity of water vapor at constant pressure (kJ/kg.oC, kWs/kg.K)

t = water vapor temperature (oC)

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

For water vapor the specific heat capacity can be set to

cpw = 1.84 (kJ/kg.oC)

The evaporation heat can be set to

hwe = 2,502 (kJ/kg)

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

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

or

h = 1.006 (kJ/kg.oC) t + x [1.84 (kJ/kg.oC) t + 2,502 (kJ/kg)] (1c)

Example - Enthalpy in Moist Air

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

h = 1.006 (kJ/kg.oC) 25oC + 0.0203 (kg/kg) [1.84 (kJ/kg.oC) 25oC + 2,502 (kJ/kg)]

= 25.15 (kJ/kg) + 0.93 (kJ/kg) + 50.79 (kJ/kg)

= 76.87 (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 almost be neglected.

Enthalpy of Moist Air containing Water - 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)

where

xs = humidity ratio at saturation (kg/kg)

cw = 4.19 - specific heat capacity of water (kJ/kg.oC)

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)

where

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

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

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Related Topics

  • Air Psychrometrics Air psychrometrics are the study of moist and humid air and the change of air condition - psychrometric charts, Mollier diagrams, air temperature and absolute and relative humidity, moisture content ..

Related Documents

  • Air Properties Air properties - temperature, density, specific heat, thermal conductivity, expansion coefficient, kinematic viscosity and Prandtl number for temperatures between -150oC and 400oC.
  • Water - Thermal Properties Thermal properties of water - density, freezing temperature, boiling temperature, latent heat of melting, latent heat of evaporation, critical temperature and more
  • Common Psychrometric Terms Common psychometric terms as Dry Bulb and Wet Bulb Temperature, Specific Volume, Relative Humidity and Enthalpy
  • Heat-Recovery Ventilation and heat-recovery calculations, sensible and latent heat - online calculators - Imperial units
  • Ice - Thermal Properties Thermal and thermodynamic properties of ice - density, thermal conductivity and specific heat at temperatures from zero to -100oC
  • Humidity Ratio of Air Humidity ratio is the ratio between actual mass of water vapor present in moist air - to the mass of dry air
  • Mixing Humid Air Calculating enthalpy, heat, temperature and specific humidity when mixing humid air
  • Great Sensible Heat Factor - GSHF The Great Sensible Heat Factor is the ratio between the sensible and total heat for a cooling coil
  • Specific Heat Capacity of Dry Air The Specific Heat Capacity of dry air varies with temperature.
  • Air Heating Systems Using air to heat buildings - temperature rise diagram
  • Moist Air Properties Psychrometric Table of Moist Humid Air Properties

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