What is LMTD?

The Log Mean Temperature Difference (LMTD) is the industry-standard average temperature difference used to calculate the heat transfer area of exchangers. It is more accurate than a simple arithmetic mean because the temperature difference varies non-linearly along the length of the exchanger.

[Image of counter current heat exchanger temperature profile]

Counter-Current vs. Parallel Flow

In a Counter-Current setup, fluids flow in opposite directions. This maintains a higher temperature difference across the entire length, resulting in a higher LMTD and a smaller, more efficient heat exchanger. In Parallel (Co-Current) flow, fluids travel in the same direction, which limits the thermal efficiency as the outlet temperatures converge.

Designing Heat Exchangers in Industry

While calculating LMTD is the first step, real-world design involves correction factors (Ft) for multi-pass shells and fouling factors. To learn how to design complete Shell & Tube exchangers professionally, check out our Aspen HYSYS Training Courses.