Heat exchanger design — When using the LMTD method with correction factor FT (for multi-pass or crossflow arrangements), it is generally not desirable to design for FT below which value?

Introduction:
The log-mean temperature difference (LMTD) method uses a correction factor FT to account for deviation from true countercurrent flow. Very low FT values suggest poor thermal driving force and an oversized exchanger. Designers therefore impose a minimum acceptable FT.

Given Data / Assumptions:

• Single-phase sensible heat transfer on both sides.
• Standard shell-and-tube exchanger with pass arrangements or crossflow configuration.
• Use of LMTD correction factor FT.

Concept / Approach:
For robust designs, a common rule is FT ≥ 0.75 to 0.8. Below this, the exchanger area required becomes excessive and performance is sensitive to fouling/variability. Choosing a slightly conservative threshold (0.80) avoids marginal designs while providing reasonable compactness.

Step-by-Step Solution:
Compute true ΔT_lm,true = FT * ΔT_lm,countercurrent.If FT is too low, effective driving force shrinks, area A = Q /(U * ΔT_lm,true) grows rapidly.Adopt a practical minimum FT ≈ 0.8 for design selection.

Verification / Alternative check:
Vendor sizing tools and classic references flag FT < 0.75–0.8 as undesirable; reconfigure passes or choose a different arrangement to raise FT.

Why Other Options Are Wrong:

• 0.99 and 0.95 are unrealistically strict and would reject many sound designs.
• 0.55 is too low and typically indicates a poor configuration.

Common Pitfalls:
Ignoring condensation/boiling which require different considerations; forgetting that increased number of shell passes can improve FT but may raise pressure drop.

Final Answer:
0.80