Latent Heat Flow
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/ft3 )
q = measured air flow (ft3 /min)
Δx = difference in humidity ratio (lb h2o /lb dry_air )
Latent Heat Flow - SI-Units
The latent heat flow due to moisture in air can be expressed in SI-units (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 oC)
ρ = air density at standard conditions ( 1.202* kg/m3 )
q = air flow (m3 /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 m3 /h of air through a building.
The state of the make-up (supply) air is 0 oC 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 oC 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/m3 ) (10000 m3 /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. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.
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