Log-mean temperature difference (LMTD) correction: In which heat exchangers is the LMTD correction factor FT applied to account for departure from true counter-current or co-current flow?

Introduction / Context:
The LMTD method assumes pure counter-current or co-current flow. Real exchangers often deviate due to multipass arrangements or crossflow geometries. The correction factor FT adjusts the ideal LMTD to reflect these departures for more accurate area sizing.

Given Data / Assumptions:

• Shell-and-tube multipass, crossflow, and other non-ideal flow arrangements are common.
• Terminal temperature differences are known.
• FT ≤ 1 and depends on configuration and temperature effectiveness ratios.

Concept / Approach:
When flows are not purely counter-current or co-current (e.g., 1–2 shell-and-tube, 2–4 passes, crossflow with baffles), temperature profiles are less efficient, reducing the effective LMTD. FT is obtained from charts or equations based on the temperature ratio and capacity rate ratio.

Step-by-Step Solution:

Verification / Alternative check:
For a true 1–1 counter-current exchanger, FT = 1; for multipass/crossflow, FT typically ranges from about 0.7 to 0.95 depending on approach temperatures.

Why Other Options Are Wrong:

• 1–1 co-current or double-pipe only: FT is generally 1 (or not needed) for ideal cases; real need is in multipass/crossflow.
• All liquid/liquid: phase has nothing to do with needing FT; geometry/arrangement does.

Common Pitfalls:
Applying FT outside its valid range; using FT too low (e.g., < 0.75) without reconsidering configuration; ignoring temperature cross limitations.

Final Answer:
All multipass configurations (and crossflow patterns)