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Moist Air - Enthalpy

Sensible and latent heat of moist air.

Moist air is a mixture of dry air and water vapor. In atmospheric air water vapor content varies from 0 - 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)

where

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)

where

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)

where

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= 1.86 (kJ/kgoC)

= 0.444 (Btu/lboF)

The evaporation heat (water at 0oC, 32oF) 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]                                (4)

(1b) in metric units

h = (1.006 kJ/kgoC) t + x [(1.86 kJ/kgoC) t + (2501 kJ/kg)]                           (5)

where

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 + (1075 Btu/lb)]                           (6)

where

h = enthalpy (Btu/lb)

x = mass of water vapor (lb/lb)

t = temperature (oF)

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

• for eq. (5) metric units - the "reference" point for enthalpy h = 0 (kJ/kg) is t = 0 oC(32oF) and x = 0 kg/kg
• for eq. (6) imperial units - the "reference" point for enthalpy h = 0 (Btu/lb) is t = 0 oF(-17.8oC) and x = 0 lb/lb. The evaporation heat for water at 0 oF is 1061 Btu/lb as used in eq. (6).

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) [(1.86 kJ/kgoC) (25oC) + (2501 kJ/kg)]

= (25.15 kJ/kg) + [(0.94 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) cwt                                 (7)

where

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) cit- (x - xs) him                               (8)

where

ci= 2.05 - specific heat ice (kJ/kgoC)

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

Related Topics

• Air Psychrometrics

Moist and humid air calculations. Psychrometric charts and Mollier diagrams. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.

Related Documents

• Air - Drying Force

The drying force of air depends on the air moisture holding capacity and the water surface to air evaporation capacity.
• Air - Humidifying by Adding Steam or Water

Air can be humidified by adding water or steam.
• Air - Humidifying with Steam, SI units

Using steam to humidify air.
• Air - Humidity Measurement from Dry and Wet Bulb Temperature

Relative humidity in moist air can estimated by measuring the dry and wet bulb temperature.
• Air - Humidity Ratio

The mass of water vapor present in moist air - to the mass of dry air.
• Air - Maximum Moisture Carrying Capacity

Maximum water content in humid air vs. temperature.
• Air - Specific Heat vs. Temperature at Constant Pressure

Online calculator with figures and tables showing specific heat (Cp and Cv) of dry air vs. temperature and pressure. SI and imperial units.
• Air - Thermophysical Properties

Thermal properties of air at different temperatures - density, viscosity, critical temperature and pressure, triple point, enthalpi and entropi, thermal conductivity and diffusivity and more.
• Air Cooling of Storage Roooms

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Maximum cooling tower efficiency is limited by the cooling air wet-bulb temperature.
• Dry Bulb, Wet Bulb and Dew Point Temperatures

Dry Bulb, Wet Bulb and Dew Point temperatures can be used to determine the state of humid air.
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Evaporation of water from a water surface - like a swimming pool or an open tank - depends on water temperature, air temperature, air humidity and air velocity above the water surface - online calculator.
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The Great Sensible Heat Factor is the ratio sensible to total heat in a cooling coil.
• Heat Recovery

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• Heat Recovery Efficiency

Classification of heat recovery efficiencies - temperature efficiency, moisture efficiency and enthalpy efficiency - online heat exchanger efficiency calculator.
• Humid Air - Heating

Enthalpy change and temperature rise when heating humid air without adding moisture.
• Latent Heat Flow

Latent heat is the heat when supplied to or removed from air results in a change in moisture content - the temperature of the air is not changed.
• Mixing of Humid Air

The change in state wwhen mixing moist air - enthalpy, heat, temperature and specific humidity.
• Moist Air - Degree of Saturation

Humidity ratio of moist air to humidity ratio of saturated moist air.
• Moist Air - Partial Pressure and Daltons Law

The pressure in a mixture of dry air and water vapor - humid or moist air - can be estimated by using Daltons Law of partial pressures.
• Moist Air - Properties

Psychrometric table with humid air properties like saturation pressure, specific volume, enthalpy and entropy.
• Moist Air - Psychrometric Table for Pressure 29.92 inHg

Dry and wet bulb temperatures, saturation pressure, water vapor weight, specific volume, heat and more.
• Moist Air - Psychrometric Terms

Dry and wet bulb temperature, specific volume, relative humidity,  enthalpy and more.
• Moist Air - Specific vs. Relative Humidity

Specific humidity of moist air vs. relative humidity, water vapor and air density.
• Removing Heat with Air

Calculating heat removed with air by measuring the wet bulb temperature.
• Water - Thermophysical Properties

Thermal properties of water at different temperatures like density, freezing temperature, boiling temperature, latent heat of melting, latent heat of evaporation, critical temperature and more.
• Water-Steam Mollier Diagram

Enthalpy-entropy diagram for water and steam.

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Citation

• The Engineering ToolBox (2004). Moist Air - Enthalpy . [online] Available at: https://www.engineeringtoolbox.com/enthalpy-moist-air-d_683.html [Accessed Day Month Year].

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7.8.15

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