Tube layout heuristic: in most shell-and-tube exchangers, the tube pitch (center-to-center spacing) is generally what multiple of the tube outside diameter?

Introduction / Context:
Tube pitch is a key geometric parameter affecting cleanability, shell-side pressure drop, and heat transfer. Standard practice uses a pitch modestly larger than the tube outside diameter to allow for tube-to-tube clearance and manufacturing tolerances while keeping compactness.

Given Data / Assumptions:

• Standard 3/4 in or 1 in OD tubes with square or triangular pitch patterns.
• Cleanability and vibration avoidance considered.
• Focus on the general heuristic, not special fouling duties that demand wider spacing.

Concept / Approach:
Common guidelines recommend tube pitch of 1.25 to 1.50 times tube OD. This spacing provides adequate ligament for tube sheet drilling, flow passages for shell-side fluid, and space for mechanical expansion/rolling. Tighter than OD risks fabrication issues; much larger than 1.5 OD sacrifices compactness and increases shell-side bypassing.

Step-by-Step Solution:
Assess options vs. practice.Select 1.25–1.50 × OD to balance hydraulic and mechanical needs.Confirm against standard exchanger design charts.

Verification / Alternative check:
Many design standards and vendor catalogs list 1.25 OD as the minimum pitch for cleanable services on square pitch, with 1.25–1.5 OD typical on triangular pitch depending on fouling.

Why Other Options Are Wrong:
Less than OD: impossible — tubes would interfere.2.5 × OD: excessive spacing not typical unless fouling/cleaning dictates.One-fourth OD: physically meaningless in this context.

Common Pitfalls:
Ignoring service fouling, leading to too-tight spacing; neglecting flow-induced vibration limits on shell side.

Final Answer:
1.25–1.50 times the tube diameter