Why are industrial pipes most commonly circular in cross-section for fluid transport and structural economy?

Introduction / Context:
Cross-sectional shape influences both mechanical integrity and material economy in piping. A sound choice minimizes stress concentration, maximizes resistance to internal pressure, and reduces required wall area per unit flow capacity. Circular pipes dominate industrial practice for these reasons.

Given Data / Assumptions:

• Thin-walled pipe approximation for pressure analysis.
• Comparison among alternative shapes at equal enclosed flow area or volume.
• Material cost roughly proportional to surface area and wall thickness.

Concept / Approach:
For a given internal pressure, a circular cylinder develops uniform hoop stress; other shapes develop bending and stress concentrations at corners. From geometry, a circle encloses the maximum area for a given perimeter, or equivalently, provides the minimum perimeter (surface area) for a given enclosed area. Thus circular pipes achieve both better structural efficiency and material economy.

Step-by-Step Solution:

Verification / Alternative check:
Thin-cylinder stress formulas and isoperimetric inequality underpin these conclusions mathematically and are standard in mechanical design texts.

Why Other Options Are Wrong:

• (a) alone or (b) alone are incomplete—both advantages are relevant.
• “None of these” contradicts well-established design reasoning.
• “Easier to paint” is irrelevant to fundamental design criteria.

Common Pitfalls:
Ignoring fabrication/connection issues; misapplying to non-pressure structural members where other shapes may be preferred.

Final Answer:
Both (a) & (b)