Log Mean Temperature Difference: Use the Terminal Pairs

Define the Thermal Model

For positive terminal differences ΔT₁ and ΔT₂, LMTD = (ΔT₁ − ΔT₂)/ln(ΔT₁/ΔT₂). Alfa Laval’s calculation method pairs the hot inlet with cold outlet and hot outlet with cold inlet for counter-current flow. The equal-difference limit is that common difference, rather than an undefined zero-over-zero result.

Compare a Hypothetical Case

Consider invented counter-current endpoints giving ΔT₁ = 60 K and ΔT₂ = 20 K. LMTD is 40/ln(3), approximately 36.41 K. The arithmetic mean is 40 K. For the same assumed duty and U, using 40 K would calculate only about 91% of the area obtained with LMTD. This comparison tests averaging, not a selected exchanger.

Complete the Engineering Review

Place both streams on one flow sketch and pair temperatures at the physical ends. Check that the assumed heat flow remains in the intended direction. Document any configuration correction required for multiple passes or other arrangements; a counterflow expression should not silently become a universal exchanger formula.

When discussing a manufacturing-vessel utility interface, provide the external exchanger’s actual arrangement and temperature program. The calculation does not establish the U value, pressure loss or control performance. Each input should remain traceable to its own design case.

Does the exchanger comparison use the correct terminal temperature differences?

This blank worksheet is for your own project. It contains no H M machine trial result.

Customer Questions

Related Equipment References

Review the actual offered equipment and application separately from this educational example.

• manufacturing-vessel utility interface

Primary References

These references support the technical principles discussed in this guide. The worked examples and review questions are educational.

• Alfa Laval’s calculation method
• manufacturing-vessel utility interface