Difficulty: Easy
Correct Answer: Not less than one-fourth of the tube outer diameter or 3/16 inch, whichever is larger
Explanation:
Introduction / Context:
In mechanical design of shell-and-tube heat exchangers, the spacing of tube holes in the tubesheet must ensure structural integrity during drilling, rolling/welding, and service. Designers refer to the minimum free metal ligament (tube-to-tube clearance on the tubesheet) to avoid cracking between holes, maintain gasket seating, and allow manufacturability.
Given Data / Assumptions:
Concept / Approach:
The free metal ligament equals the tube pitch minus tube outer diameter at the tubesheet. If the ligament is too small, machining weakens the sheet and may cause leakage or cracking. A common shop practice is to keep the ligament no less than one-fourth of tube OD or 3/16 inch, whichever is larger, as a practical lower bound for strength and manufacturing tolerance.
Step-by-Step Solution:
Verification / Alternative check:
Fabricator standards and exchanger handbooks frequently recommend minimum ligaments around 0.25*OD with an absolute floor near 3/16 inch for common tube sizes to ensure drillability and seating during expansion/welding.
Why Other Options Are Wrong:
Common Pitfalls:
Confusing tube-to-tube clearance in bundle layout with shell-side clearance; ignoring corrosion allowance and drilling tolerances that further justify a conservative ligament.
Final Answer:
Not less than one-fourth of the tube outer diameter or 3/16 inch, whichever is larger
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