# 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/ft^{3})

q = measured air flow (ft^{3}/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^{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 make-up (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. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.

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