In shell-and-tube heat exchangers, what guideline is commonly used for minimum tube-to-tube clearance on the tubesheet (i.e., the free metal ligament between adjacent tube holes)?

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:

  • Conventional shell-and-tube exchanger with drilled tubesheet.
  • Pitch patterns may be square or triangular, but the question asks a general rule-of-thumb for minimum ligament.
  • Objective: pick a conservative, widely used guideline.


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:

Recognize that “clearance” here refers to the free metal ligament on the tubesheet.Compare industry guidelines for minimum ligament vs. the options given.Select the option stating one-fourth of OD or 3/16 inch minimum, which matches common practice.


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:

  • More than one full OD or exactly one OD: unrealistic and uneconomical; would lead to very large pitch.
  • “Always greater for triangular pitch”: triangular pitch typically has closer spacing than square for a given pitch; blanket statements are misleading.
  • “No minimum clearance”: unsafe; ignores machining and strength constraints.


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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