Overall heat transfer coefficient (U): In which type of problems do we use the overall heat transfer coefficient concept?

Introduction / Context:
Real heat exchangers involve multiple layers and mechanisms: conduction through walls and fouling layers, plus convection on fluid sides. To simplify analysis, we combine these effects into a single parameter called the overall heat transfer coefficient U.

Given Data / Assumptions:

• Steady-state heat transfer.
• Planar or cylindrical walls separating fluids.
• Thermal resistances can be added in series.

Concept / Approach:
The overall coefficient U encapsulates all resistances between two bulk fluids. For a flat wall, 1/U = 1/h_hot + R_cond + 1/h_cold (+ fouling terms). For a tube, appropriate logarithmic mean area is used. U enables the compact expression Q = U * A * ΔT_mean for exchanger design or rating.

Step-by-Step Solution:

Verification / Alternative check:
Compare predictions with detailed CFD or layer-by-layer models; the series-resistance model provides good engineering estimates for design.

Why Other Options Are Wrong:
Using U for a single pure mechanism (conduction only or convection only) is unnecessary; radiation alone also does not require U unless combined with other modes in a composite resistance network.

Common Pitfalls:
Mixing areas when summing resistances in cylindrical systems; neglecting fouling, which significantly lowers U in service.

Final Answer:
Combined conduction and convection (resistances in series)