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Latent Heat Flow

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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)


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)


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

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