Shell-and-tube exchangers: a practical minimum tube pitch is commonly taken as approximately how many times the outside diameter of the tube?

Introduction / Context:
Tube pitch (center-to-center spacing) in shell-and-tube heat exchangers governs mechanical clearance, tube-to-tube vibration risk, shell-side flow distribution, and cleaning access. Designers typically set a minimum pitch relative to the tube outside diameter to balance compactness with manufacturability and performance.

Given Data / Assumptions:

• Circular tubes arranged on standard layouts (triangular or square).
• Mechanical cleaning may not be required for the tightest triangular patterns.
• Normal industry rules of thumb for minimum pitch.

Concept / Approach:
A common lower bound for triangular pitch is about 1.25 times the tube OD, providing just enough ligament for drilling the tubesheet and for tube-to-tube clearance. Larger pitches are used for square layouts, mechanical cleaning lanes, or to reduce shell-side pressure drop. Tighter than 1.25 can compromise drilling integrity or increase risk of damage during expansion/rolling.

Step-by-Step Solution:

Verification / Alternative check:
TEMA and vendor practices echo ~1.25 OD as a lower limit for close pitch without mechanical cleaning lanes.

Why Other Options Are Wrong:

• 1.75, 2.5, 3.5 are not “minimums” but looser pitches used for other objectives (cleaning, lower pressure drop).
• 1.00 OD gives zero ligament—impractical.

Common Pitfalls:
Choosing minimum pitch where mechanical rodding is later required; ignoring fouling tendency that would argue for wider spacing.

Final Answer:
1.25