Question

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)?

• A. (i) thick-walled, (ii) high
• B. (i) thin-walled, (ii) high
• C. (i) thin-walled, (ii) low
• D. (i) thick-walled, (ii) low

Hint:

When analyzing heat exchanger efficiency, consider both the physical properties of the materials involved and the geometry of the components to understand where the major resistances to heat transfer lie.

Answer 1

The Correct Option is B

The formula for the overall heat transfer coefficient \( U \) simplifies the heat transfer resistance to just the convective components on either side of the tube. This simplification is typically valid when the thermal resistance due to the tube wall itself is negligible compared to the convective resistances. This condition is generally true for:

Step 1: Tubes that are thin-walled, where the thickness of the tube does not significantly impede the heat flow through the wall material itself.

Step 2: High thermal conductivity materials, which ensure that any resistance to heat flow due to the wall is minimal. High conductivity materials effectively transport heat, reducing the relative contribution of the wall's thermal resistance to the overall process.