Minimum ligament (metal between holes) for square-pitch tube layout in heat exchangers: what is the recommended minimum?

Introduction / Context:
In tube-sheet drilling for heat exchangers, the “ligament” is the narrow strip of metal between adjacent holes. Adequate ligament is crucial for mechanical strength, machining integrity, gasket sealing, and resistance to tube-sheet distortion. Designers therefore adhere to minimum ligament rules tied to pitch, tube O.D., and fabrication realities.

Given Data / Assumptions:

• Square-pitch layouts with common tube sizes (e.g., 3/4 in O.D.) and standard tube-sheet materials.
• General-purpose exchangers designed to TEMA-like practices.

Concept / Approach:
Too small a ligament weakens the tube sheet and risks drilling breakouts; too large a ligament wastes space, reduces heat-transfer area, and increases exchanger size. A widely adopted practical minimum ligament for square pitch is about 6.5 mm (roughly 1/4 in), balancing structural adequacy with compact layouts.

Step-by-Step Solution:

Verification / Alternative check:
Fabrication guides and exchanger standards commonly cite minimum ligaments on the order of 6–7 mm for carbon steel tube sheets to prevent breakthrough and ensure gasket seating.

Why Other Options Are Wrong:

• 6.5 cm: Far too large; would produce overly sparse layouts.
• Ligament equal to I.D. or O.D.: Not a recognized “minimum”; ligament depends on pitch selection, not equality to tube diameters.

Common Pitfalls:
Ignoring strength reduction from pass partition cuts; overlooking corrosion allowance that further reduces effective ligament; forgetting expanded tube deformation.

Final Answer:
6.5 mm