When should a floating-head shell-and-tube heat exchanger be selected? (Consider thermal expansion and maintenance needs.)
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AHeat transfer between highly corrosive fluids
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BLarge shell–tube temperature differences (> 50 °C) to accommodate differential expansion
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COnly co-current flow arrangements
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DOnly counter-current flow arrangements
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EApplications where tube bundle removal is unnecessary
Answer
Correct Answer: Large shell–tube temperature differences (> 50 °C) to accommodate differential expansion
Explanation
Introduction / Context:Shell-and-tube heat exchangers come in several TEMA configurations. The floating-head type allows one end of the tube bundle to move relative to the shell, accommodating differential thermal expansion and simplifying mechanical cleaning—critical in many services with large temperature differences or fouling.
Given Data / Assumptions:
- Significant temperature difference between shell-side and tube-side fluids.
- Conventional materials of construction; no exotic corrosion constraints assumed.
- Need for maintainability and cleaning may be present.
Concept / Approach:Fixed-tube-sheet exchangers are economical but susceptible to thermal stress when ΔT is large. U-tube and floating-head designs relieve thermal expansion. Floating-head units also permit easy tube-bundle removal for mechanical cleaning of both sides, making them attractive for fouling services as well as high ΔT operation.
Step-by-Step Solution:Identify the challenge: differential expansion at high ΔT causes stress in fixed configurations.Select design with axial freedom: floating head provides movement and maintenance access.Therefore, choose floating head when ΔT is large (commonly > 50 °C) and/or fouling requires bundle removal.
Verification / Alternative check:Mechanical design analyses show reduced thermal stresses with floating-head arrangements; many TEMA types (e.g., AES, AET) are specified for such duties.
Why Other Options Are Wrong:(a) Corrosion resistance is mainly a materials selection issue; floating head does not inherently solve corrosion. (c) and (d) concern flow arrangement, not head type. (e) contradicts a key floating-head advantage.
Common Pitfalls:Overlooking fouling/cleaning requirements; assuming U-tube always suffices when true counter-current temperature profiles or tube-side cleaning access is needed.
Final Answer:Large shell–tube temperature differences (> 50 °C) to accommodate differential expansion