CONDUCTION HEAT TRANSFER
Heat Transfer
Electrical Analogy
Figure 2 Equivalent Resistance
Example:
A composite protective wall is formed of a 1 in. copper plate, a 1/8 in. layer of asbestos,
and a 2 in. layer of fiberglass. The thermal conductivities of the materials in units of
Btu/hr-ft-oF are as follows: kCu = 240, kasb = 0.048, and kfib = 0.022. The overall
temperature difference across the wall is 500°F. Calculate the thermal resistance of each
layer of the wall and the heat transfer rate per unit area (heat flux) through the composite
structure.
Solution:
RCu
DxCu
kCu
1 in
1 ft
12 in
240
Btu
hr ft °F
0.000347hr ft2°F
Btu
Rasb
Dxasb
kasb
0.125 in
1 ft
12 in
0.048
Btu
hr ft °F
0.2170hr ft2°F
Btu
Rfib
Dxfib
kfib
2 in
1 ft
12 in
0.022
Btu
hr ft °F
7.5758hr ft2°F
Btu
HT-02
Page 10
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