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Question 11.24: Determine the overall heat-transfer coefficient for the comp...

Determine the overall heat-transfer coefficient for the composite wall of Illustrative Problem 11.4 if h on the hot side is 0.9 Btu/(h·ft .^{2}·°F) and h on the cold side is 1.5 Btu/(h·ft .^{2}·°F). The temperatures given are to be the respective air temperatures (see Figure 11.7).

 

11,7
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For a plane wall, the areas are all the same, and if we use 1 ft.2 of the wall surface as the reference area,

\begin{array}{r}\text { brick resistance }=\frac{\Delta x}{k A}=\frac{6 / 12}{0.4 \times 1}=1.25 \\\text { concrete resistance }=\frac{\Delta x}{k A}=\frac{\frac{1}{2} / 12}{0.8 \times 1}=0.052 \\\text { plaster resistance }=\frac{\Delta x}{k A}=\frac{\frac{1}{2} / 12}{0.3 \times 1}=0.139 \\\text { "hot film" resistance }=\frac{1}{h A}=\frac{1}{0.9 \times 1}=1.11 \\\text { "cold film" resistance }=\frac{1}{h A}=\frac{1}{1.5 \times 1}=0.67 \\ \\ \hline \\ \text { total resistance }= 3.22\end{array}

The overall conductance (or overall heat-transfer coefficient) U = 1/(overall resistance) = 1/3.22 = 0.31 Btu/h·ft .^{2}. In Illustrative Problem 11.24, the solution is straightforward, because the heat-transfer area is constant for all series resistances.

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