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GATE 2025 CH – Question 17

Heat Transfer · Heat exchangers and evaporators: types and design · 1 mark · Multiple choice

In industrial heat exchanger design, the overall heat transfer coefficient $U$ is estimated from the equation $\frac{1}{U} = \frac{1}{h_i} + \frac{1}{h_o}$ where $h_i$ and $h_o$ are the convective heat transfer coefficients on the inner and outer side of the tube, respectively. This is valid for (i) tube of (ii) thermal conductivity.

Which one of the following is the CORRECT option to fill in the gaps (i) and (ii)?

  1. (i) thick-walled, (ii) high
  2. (i) thin-walled, (ii) high
  3. (i) thin-walled, (ii) low
  4. (i) thick-walled, (ii) low

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Correct answer: (B) (i) thin-walled, (ii) high

Explanation

The equation leaves out the resistance of the tube wall, $\frac{x_w}{k}$, and treats the inner and outer areas as equal. That is justified only when the wall is thin (so that the areas are nearly equal and $x_w$ is small) and its thermal conductivity is high (so that $\frac{x_w}{k}$ is negligible).