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.
Latent Heat Flow  English (Imperial) units
The latent heat flow due to moisture in air can be expressed in English (Imperial) units as
Q_{l } = 60 h_{we } ρ q Δx (1)
where
Q_{l } = latent heat flow (Btu/hr)
h_{we } = 1060  latent heat of vaporization of water (Btu/lb)
ρ = 0.075  air density at standard conditions (lb/ft^{3} )
q = measured air flow (ft^{3} /min)
Δx = difference in humidity ratio (lb_{h2o } /lb_{dry_air } )
Latent Heat Flow  SIUnits
The latent heat flow due to moisture in air can be expressed in SIunits (metric) as
Q_{l } = h_{we } ρ q Δx / 3600 (2)
where
Q_{l } = latent heat flow (kW)
h_{we } = latent heat of vaporization of water ( 2454 kJ/kg  in air at atmospheric pressure and 20 ^{o}C)
ρ = air density at standard conditions ( 1.202* kg/m^{3} )
q = air flow (m^{3} /hr)
Δx = difference in humidity ratio ( kg_{h2o } /kg_{dry_air } )
*Note that properties of air changes with temperature. Interpolate values if necessary.
Example  Latent Heat Flow
A ventilation system transports 10000 m^{3} /h of air through a building.
The state of the makeup (supply) air is 0 ^{o}C and 100% relative humidity. From the Mollier diagram the humidity ratio  x  can be estimated to 0.0037 ( kg_{h2o } /kg_{dry_air } ) .
The state of the room air is 20 ^{o}C and 40% relative humidity. From the Mollier diagram the humidity ratio can be estimated to 0.0057 ( kg_{h2o } /kg_{dry_air } ) .
The latent heat flow can be calculated as
Q_{l } = (2465.56 kJ/kg) (1.202 kg/m^{3} ) (10000 m^{3} /h) (( 0.0057 (kg_{h2o } /kg_{dry_air } ) )  ( 0.0037 ( kg_{h2o } /kg_{dry_air } ) )) / 3600
= 16.5 kW
Related Topics

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