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
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