Mechanical design of exchangers: For a carbon steel shell in a shell-and-tube heat exchanger, the minimum shell thickness (including corrosion allowance) depends on shell diameter and typically falls in which range (in mm)?

Introduction / Context:
Shell thickness in heat exchangers must satisfy code rules for pressure, temperature, corrosion allowance, and fabrication. Designers consult standards to choose a minimum wall thickness that balances safety and economy. This question checks practical knowledge of the typical range for carbon steel shells.

Given Data / Assumptions:

• Material: carbon steel.
• Thickness includes corrosion allowance.
• Thickness depends on shell diameter within common industrial sizes.

Concept / Approach:
Minimum thickness increases with shell diameter and design conditions. For many moderate-pressure, moderate-diameter shells, practical minimums for carbon steel (inclusive of corrosion allowance) commonly fall in a single-digit to low double-digit millimeter range. Values significantly below ~5 mm are rarely acceptable for shells under pressure; very high minimums (≥ 12 mm) are associated with large diameters or higher pressures.

Step-by-Step Solution:
Recognize that extremely thin shells (3–5 mm) are uncommon for pressure shells.Typical practice indicates a range around 5–11 mm for many standard sizes and pressures.Therefore, select 5–11 mm as the representative range.

Verification / Alternative check:
Engineering handbooks and vendor data for common shell sizes align with minimum nominal thickness values in the single-digit millimeter range plus corrosion allowance, yielding totals around 5–11 mm for many services.

Why Other Options Are Wrong:
3–5 mm: generally insufficient for pressure shells. 12–18 mm: too high as a typical minimum; applies to larger or higher-pressure designs. 8–15 mm overlaps but exceeds common lower bounds; 5–11 mm better reflects the full typical span.

Common Pitfalls:

• Ignoring corrosion allowance or mill tolerance when quoting “minimums.”
• Assuming one minimum fits all; code calculations govern the exact value.

Final Answer:
5–11 mm