Shell-and-tube heat exchangers: identify the incorrect statement regarding baffles, spacing limits, and unsupported tube spans.

Introduction / Context:
Baffles in shell-and-tube exchangers control shell-side flow pattern, enhance heat transfer, and limit tube vibration by reducing unsupported spans. Several conventions exist for minimum and maximum baffle spacing and for typical unsupported tube spans. Knowing which statements match common practice helps avoid design misconceptions.

Given Data / Assumptions:

• Conventional TEMA-type exchanger, single or multiple shell passes.
• Standard materials such as carbon steel, stainless steel, and nonferrous alloys.
• Normal process services without extraordinary fouling or vibration issues.

Concept / Approach:
Segmental baffles (single or double) are by far the most commonly used in industry because they are simple, effective, and economical. Disk-and-doughnut configurations are used but are less common. Maximum baffle spacing is typically limited to about the shell inside diameter. Minimum baffle spacing is often the greater of roughly 50 mm or 0.2 times the shell diameter to preserve crossflow and cleaning access. Unsupported spans on the order of 75 times the tube diameter are common guide values, reduced for softer metals.

Step-by-Step Solution:

Verification / Alternative check:
Design guides and vendor standards highlight single-segmental prevalence; disk-and-doughnut is niche for pressure drop or fouling reasons.

Why Other Options Are Wrong:

• (a), (b), and (c) align with widely used rules of thumb; they are not the incorrect statements here.

Common Pitfalls:
Applying minimum spacing blindly without checking shell diameter; neglecting vibration checks for long spans; assuming nonstandard baffles are default.

Final Answer:
Disk-and-doughnut baffle is the most commonly used baffle.