Question 11.29: A heat exchanger has one shell pass and two tube passes simi...

Thermodynamics and Heat Power

A heat exchanger has one shell pass and two tube passes similar to Figure 11.27c. Oil flows in the tube side and is cooled from 280°F to 140°F. On the shell side, water is heated from 85°F to 115°F. Determine the true mean temperature difference.

11.27c

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In order to use Figure 11.32, which is applicable to this problem, we need to calculate P and R. From the figure,

\begin{aligned}&P=\frac{t_{2}-t_{1}}{T_{1}-t_{1}}=\frac{140-280}{85-280}=0.72 \\ \\ &R=\frac{T_{1}-T_{2}}{t_{2}-t_{1}}=\frac{85-115}{140-280}=0.21\end{aligned}

From Figure 11.32, F = 0.91. Thus, true mean temperature difference = 0.91 × LMTD counterflow

\begin{aligned}&=0.91 \frac{(280-115)-(140-85)}{\ln \frac{(280-115)}{(140-85)}} \\&=91^{\circ} F\end{aligned}

11.32

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