In shell-and-tube heat exchangers, why are smaller tube diameters often preferred for a given heat-transfer area?

Introduction / Context:
Tube size selection in shell-and-tube exchangers affects heat-transfer coefficients, pressure drop, mechanical cleaning, layout, and cost. Designers often favor smaller tubes when cost and compactness are priorities.

Given Data / Assumptions:

• Required total heat-transfer area is fixed.
• Material and tube length options are comparable.

Concept / Approach:
For a fixed area A, using smaller diameters permits more tubes within a given shell, enabling a smaller shell diameter. Shell diameter strongly influences material and fabrication costs; hence, smaller tubes can reduce total exchanger cost. However, mechanical cleaning becomes harder and fouling behavior depends on service—not inherently better for small tubes.

Step-by-Step Solution:
1) Fix A = sum(pi * d_o * L) over all tubes.2) Decrease tube outside diameter d_o → more tubes fit in the same shell pitch.3) For the same A, shell diameter can be reduced → lower shell material and bundle cost.

Verification / Alternative check:
Preliminary sizing correlations and cost charts show strong sensitivity of cost to shell diameter. Small tubes give higher velocities at the same flow, often improving inside coefficients but raising pressure drop—trade-offs considered during detailed design.

Why Other Options Are Wrong:
Easier cleaning is usually associated with larger tubes, not smaller.Fouling tendency depends on fluid chemistry, temperature, and velocity, not simply tube diameter.

Common Pitfalls:
Ignoring pressure-drop limits; overemphasizing compactness when the service demands frequent mechanical cleaning.

Final Answer:
They allow a smaller shell diameter for the same area, reducing exchanger cost.