Thursday, May 29, 2008

Tropical DesignModule 6: Heat Transfer


Heat transfer in buildings may take place in four (4) different ways:
• Conduction
• Convection
• Radiation
• Evaporation and condensation


The flow of heat through a material by transfer from warmer to cooler molecules in contact with each other.

Conductivity is the rate of heat transfer that occurs through a unit thickness of material for a unit area subjected to a unit difference in temperature.

Conductivity = per meter (thickness) / (area) * Temp difference (degC)

Conductivity (K value) = W/m degC
If wall thickness is 0.2m, area of wall is 12sqm and temperature difference is 3degC, then:
Conductivity = 0.2m/(12sqm * 3 degC) = 0.0056W/m degC

Resistance is the ability of a material to resist the flow of heat and is measured by the resistivity.

Resistivity = 1 / Conductivity = m degC / W
If wall thickness is 0.2m, area of wall is 12sqm and temperature difference is 3 degC, then:

Resistivity = (12sqm * 3 degC) / 0.2m = 180 m degC/W

For a given thickness of material, resistance to heat transfer is:

Resistance = Resistivity * t(mtrs) = m2 oC / W

If wall thickness is 0.20m, and resistivity is 180 m oC/W, then:

Resistance = 180 moC/W * 0.20m = 36 m2 oC/W

Convection is the transfer of heat from one place to another by the flow of molecules from one place to another.
Natural Convection is the movement of the molecules as a result of the heat energy they possess (i.e. hot air rising).
Forced Convection results from the movement of molecules by pumps, fans, or other movement caused by external forces.
The rate of flow of heat due to natural condition is usually measured by the conductance of a surface or an air movement to a building construction.
Units are Watts / m2 oC
1. Warm air rising from register (forced convection).
2. Warm air rising from all surfaces of radiator (after air in contact with radiator has been heated by conduction).
3. Warm air rising from (free convection).

Radiation is the transfer of energy through space by electromagnetic waves.
Radiation travels through air and the rate of transfer of energy is independent of the temperature of the air.
The rate of radiation transfer is measured in Watts per m2.
The rate of flow of radiant heat from the sun can be found from the radiation overlay when it is placed over the sun path diagram.
There are two terms commonly encountered while discussing radiant heat transfer:
Emittance (or emissivity), refers to the ability of a material’s surface to give off radiant energy. All materials have emissivities ranging from zero to one. The lower the emittance of a material, the lower the heat radiated from its surface. Aluminum foil has a very low emittance which explains its use in reflective insulation.
Reflectance (or reflectivity) refers to the fraction of incoming radiant energy that is reflected from the surface. Reflectivity and emissivity are related and a low emittance is indicative of a highly reflective surface. For example, aluminum with an emittance of 0.03 has a reflectance of 0.97.



Heat gain in the tropics is due mainly to solar radiation at the surface and only a smaller extent the high air temperatures.

Heat gain in hot climates are highest when there are low wind speeds and less heat is transferred to the air and more to the building surface.

Since major heat gain is from solar radiation, absorptivity of the surface to solar radiation is of primary importance.

Absorptivity and the insulation must be chosen to minimize the proportion of heat from the sun which penetrates the structure when the air temperatures inside and outside are similar.

Relative humidities in tropical regions are very high, hence air movement is the most effective way of increasing comfort by encouraging the evaporation of sweat from the skin.

Design buildings thru which wind can pass with a minimum of obstruction.

Comfort in the warm humid tropics is achieved by encouraging cross ventilation while providing protection from solar radiation.

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