Shell-and-tube heat exchangers — Why is a floating head provided in certain designs? (Choose the primary engineering reason for incorporating a floating head construction.)

Introduction:
A floating head heat exchanger is a common TEMA (Tubular Exchanger Manufacturers Association) configuration used when large temperature differences exist between shell-side and tube-side fluids. The floating head allows one end of the tube bundle to move axially, preventing excessive thermal stress. This question targets the main reason engineers choose floating-head designs.

Given Data / Assumptions:

• Shell-and-tube exchanger with significant temperature difference between fluids.
• Materials with different coefficients of thermal expansion.
• Desire to avoid tube buckling, tube-to-tubesheet leakage, or shell distortion.

Concept / Approach:
When tubes heat up more than the shell (or vice versa), they try to expand by different amounts. If both ends are rigidly fixed, large axial stresses can develop. A floating head allows one end of the bundle to slide, accommodating thermal growth without transmitting high stresses to joints and supports. Secondary benefits can include easier cleaning and bundle removal, but the primary purpose remains stress relief.

Step-by-Step Solution:
Identify thermal expansion mismatch between tubes and shell.Recognize constraint-induced stresses if both ends are fixed.Select the design feature (floating head) that permits axial movement and relieves stress.

Verification / Alternative check:
Design codes and TEMA standards recommend floating-head or U-tube designs whenever the calculated differential expansion exceeds allowable stresses for fixed-tubesheet units.

Why Other Options Are Wrong:

• (a) Cleaning is a benefit but not the primary driver.
• (b) Heat-transfer area is set by geometry; floating head does not inherently increase area.
• (d) LMTD depends on temperature profiles, not head type.
• (e) Piping expansion joints address pipe movement, not tube–shell differential expansion within the exchanger.

Common Pitfalls:
Assuming cleaning access is the sole rationale; overlooking that stress relief is the principal design reason for floating heads.

Final Answer:
To relieve thermal stresses caused by differential expansion of tubes and shell