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252 CHAPTER 8. STEADY MICROSCOPIC BALANCES WITHOUT GEN.
Example 8.4 For the composite wall shown in Figure 8.10, related thermal wn-
ductivities are given as kA = 35W/ m. K, kg = 12 W/ m. K, kc = 23 W/ m. K,
and ko = 5 W/ m. K.
a) Determine the steady-state heat transfer rate.
b) Determine the effective thermal conductivity for the composite walls. This makes
it possible to consider the composite wall as a single material of thermal conductivity
k,-f, rather than four different materials with four different thermal conductivities.
Figure 8.10 Heat conduction through a composite wall.
Solution
a) An analogous electrical circuit for this cwe is shown below:
The equivalent resistance, R,, of the two resistances in parallel is
-1
Ro=($+&)
Thus, the electrical analog for the heat transfer process through the composite wall
can be represented as shown below:
