Question

In a heat exchanger, floating head is provided to

• A. Facilitate cleaning of the exchanger
• B. Increase the heat transfer area
• C. Increase the log mean temperature gradient
• D. Relieve stresses caused by thermal expansion

Hint:

Heat Exchanger Mechanical Selection Guide: - Small temperature differences (\(\Delta T < 50^\circ\text{C}\)) \(\implies\) Fixed tubesheet design (low cost). - High temperature differences High Pressures \(\implies\) Floating head or U-tube design to relieve thermal expansion stresses.

Answer 1

The Correct Option is D

Concept: In industrial shell and tube heat exchangers, the fluids in the shell side and tube side often operate at vastly different temperatures. This temperature difference causes the metal tubes and the outer shell to expand or contract by different amounts, according to their thermal expansion coefficients: \[ \Delta L = L_0 \cdot \alpha \cdot \Delta T \] If both ends of the tube bundle are fixed rigidly to the outer shell (as in a fixed tubesheet design), this differential expansion is restricted, creating severe mechanical stresses (thermal stresses) within the tube sheets and shell walls. This can lead to buckled tubes, cracked joints, and catastrophic equipment failure.

Step 1: Understanding the floating head design mechanism.
To prevent these thermal stresses, designers use a floating head heat exchanger configuration. In this design, one tubesheet is clamped securely to the outer shell casing, while the opposite internal tube sheet cover sheet is left completely unattached to the shell. This unattached end is free to move or "float" axially inside the shell housing.

Step 2: Evaluating how a floating head manages thermal expansion.
When hot fluids flow through the unit, the tube bundle can expand or contract independently of the outer shell. The floating head moves axially to absorb this dimensional change: \[ \Delta L_{\text{tubes}} \neq \Delta L_{\text{shell}} \implies \text{Absorbed by the floating head motion} \] By allowing free expansion, the design completely eliminates thermal stress build-up at the joints, protecting the equipment during high-temperature operations. This directly matches Option (D).