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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.

### 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.

## Related Documents

• ### Air - Density, Specific Weight and Thermal Expansion Coefficient vs. Temperature and Pressure

Online calculator, figures and tables showing density, specific weight and thermal expansion coefficients of air at temperatures ranging -100 to 1600 °C (-140 to 2900 °F) at atmospheric and higher pressure - Imperial and SI Units.
• ### Air - Humidity Ratio

The mass of water vapor present in moist air - to the mass of dry air.
• ### Air Heating Systems

Air heating buildings - heat supply vs. air flow and temperature.
• ### Cooling and Heating Equations

Latent and sensible cooling and heating equations - imperial units.
• ### Cooling Load - Latent and Sensible Heat

Latent and sensible cooling loads to consider when designing HVAC systems.
• ### Great Sensible Heat Factor - GSHF

The Great Sensible Heat Factor is the ratio sensible to total heat in a cooling coil.
• ### Heat Recovery Efficiency

Classification of heat recovery efficiencies - temperature efficiency, moisture efficiency and enthalpy efficiency - online heat exchanger efficiency calculator.
• ### Humid Air - Heating

Enthalpy change and temperature rise when heating humid air without adding moisture.
• ### Liquids - Latent Heat of Evaporation

Latent heat of vaporization for fluids like alcohol, ether, nitrogen, water and more.
• ### Melting and Boiling Temperatures - Evaporation and Melting Heats common Materials

Melting and boiling point temperatures, latent heat of evaporation, and melting heat of common substances like copper, gold, lead and more - SI units.
• ### Moist Air - Enthalpy

Sensible and latent heat of moist air.
• ### Propane - Latent Heat of Vaporization vs. Temperature

Latent heat with vaporized propane.
• ### Removing Heat with Air

Calculating heat removed with air by measuring the wet bulb temperature.
• ### Room Sensible Heat Factor - RSHF

Room Sensible Heat Factor - RSHF - is defined as the sensible heat load divided by the total heat load in a room
• ### Saturated Steam - Properties - Imperial Units

Steam table with sensible, latent and total heat, and specific volume at different gauge pressures and temperatures.
• ### Saturated Steam - Properties for Pressure in Bar

Saturated Steam Table with properties like boiling point, specific volume, density, specific enthalpy, specific heat and latent heat of vaporization.
• ### Sensible Heat Flow

Sensible heat is dry heat causing change in temperature but not in moisture content
• ### Sensible Heat Ratio - SHR

The ratio Sensible Heat (or Cooling) Load to the Total Heat (or Cooling) Load.
• ### Steam and Vapor Enthalpy

Vapor and steam enthalpy, specific enthalpy of saturated liquid, saturated vapor and superheated vapor.
• ### Water - Thermophysical Properties

Thermal properties of water at different temperatures like density, freezing temperature, boiling temperature, latent heat of melting, latent heat of evaporation, critical temperature and more.

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## Citation

• The Engineering ToolBox (2004). Latent Heat Flow. [online] Available at: https://www.engineeringtoolbox.com/latent-heat-flow-d_703.html [Accessed Day Month Year].

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7.8.15

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