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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.

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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

Ql = 60 hwe ρ q Δx                                   (1)


Ql = latent heat flow (Btu/hr)

hwe = 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 (lbh2o/lbdry_air)

Latent Heat Flow - SI-Units

The latent heat flow due to moisture in air can be expressed in SI-units (metric) as

Ql = hwe ρ q Δx / 3600                                  (2)


Ql = latent heat flow (kW)

hwe = latent heat of vaporization of water (2454 kJ/kg - in air at atmospheric pressure and 20oC)

ρ = air density at standard conditions (1.202* kg/m3)

q = air flow (m3/hr)

Δx = difference in humidity ratio (kgh2o/kgdry_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 (kgh2o/kgdry_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 (kgh2o/kgdry_air).

The latent heat flow can be calculated as

Ql = (2465.56 kJ/kg) (1.202 kg/m3) (10000 m3/h) ((0.0057 (kgh2o/kgdry_air)) - (0.0037 (kgh2o/kgdry_air))) / 3600 

    = 16.5 kW

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  • Engineering ToolBox, (2004). Latent Heat Flow. [online] Available at: [Accessed Day Mo. Year].

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