Tropical Design Mdule 7: Basics of Heat Transfer

Basics of Heat Transfer.

Insulation is measured as the air to air rate of transmission of heat per unit area – the “U” value.

The “U” value represents the rate of heat transfer from inside air per unit area per unit difference in air temperature per unit time.

This is measured in Watts / sqm oC
U = 1 / sum of resistances

Sum of resistances include the following:
- Outside surface resistance
- Inside surface resistance
- Resistance of individual layers of the surface (Resistivity * thickness of each layer)
- Resistance of air spaces and cavities

The purpose of insulation is two-fold:

(1) to retard the flow of heat from one place to. another, and

(1) to maintain temperatures such that condensation does not occur on inside. surfaces.

... Insulation acts to reduce the rate of heat transfer by these methods

Example:
Calculate the “U” value of a brick wall with the following information:
- External surface resistance = 0.05 m2 oC / W
- Brickwall external sand rendering = 1.2 cm (resistivity 1.88 m oC / W)
- Brickwork = 20.0 cm (resistivity 0.83 m oC / W)
- Plaster (gypsum) = 1.0 cm (resistivity 2.17 m oC / W)
- internal surface resistance = 0.123 m2 oC / W

Solution:
External surface resistance: 0.050 m2 oC / W
Sand rendering resistance: 0.012 m * 1.88m oC / W = 0.025 m2 oC / W
Brickwork resistance: 0.200 m * 0.83m oC / W = 0.166 m2 oC / W
Plaster resistance: 0.010 m * 2.17m oC / W = 0.021 m2 oC / W
Internal surface resistance: 0.123 m2 oC / W

Sum of resistances: 0.385 m2 oC / W
“U” value = 1 / sum of resistances = 2.6 W / m2 oC

The rate of heat flow through a wall or roof (where the outdoor air is circulating freely through the building) is given by the formula:

Q = U * A * (t0 – t1)

Where Q = rate of heat flow (in Watts)
U = “U” value
A = superficial area (sqm)
t0-t1 = difference between inside and outside air temperature

SOL-AIR TEMPERATURE
The temperature of the outside air in contact with a shaded wall or roof which would give the same rate of heat transfer and the same temperature gradient as the combined effect of solar radiation and air temperature.
tsa = to + (X * I * ro)

Where tsa = sol-air temperature ( oC )
to = outside air temperature ( oC )
X = absorptivity of surface to solar radiation
I = intensity of radiation ( W / m2 )
ro = outside surface resistance ( m2 oC / W )

It is used to find the heating effect of the radiant heat load.

Example:
Wall surface = same as before where outside surface resistance is
0.05 m2 oC / W
Max. radiation = 600 W / m2
Absorptivity = 0.30
Air temperature = 20 oC

Then:
Sol-Air Temp = 20 oC + ( 600 W / m2 * 0.30 * 0.05 m2 oC / W )
= 20 oC + 9 oC
= 29 oC