Heat-exchanger tube layout terminology: the shortest clear distance between adjacent tubes is called the ________.

Introduction / Context:
In shell-and-tube heat exchangers, tube layout (triangular, square, rotated square) is defined by tube pitch (center-to-center spacing) and by the minimum metal ligament between holes on the tubesheet. Correct naming avoids confusion during mechanical design and thermal rating.

Given Data / Assumptions:

• Standard ASME/TEMA terminology.
• Comparison of geometric terms only (no thermal calculations involved).

Concept / Approach:
Tube pitch is the center-to-center spacing of neighboring tubes. The shortest clear distance between adjacent tube outer surfaces is commonly called tube clearance or ligament. For a given pitch, triangular layouts yield smaller minimum clearance than square layouts, improving compactness but making cleaning harder.

Step-by-Step Solution:

Verification / Alternative check:
Look at tubesheet drilling patterns: for equal center spacing, the diagonal proximity in triangular arrangements reduces the smallest ligament compared with orthogonal square arrangements.

Why Other Options Are Wrong:

• (a) Tube pitch is center-to-center, not the clear distance.
• (c) The statement is reversed; triangular layouts have smaller, not larger, minimum clearance for the same pitch.
• (d) Not applicable because a standard term exists.

Common Pitfalls:
Confusing “pitch” with “clearance,” and assuming triangular always increases all distances. In reality, triangular improves area density at the expense of cleaning access and ligament thickness.

Final Answer:
Tube clearance