Temperature Measurement · Glossary

What is Log Mean Temperature Difference?

Symbol: LMTDheat exchanger driving forceTemperature Measurement

Log mean temperature difference is the effective average temperature difference between two fluids in a heat exchanger.

Log Mean Temperature Difference Explained

The temperature difference between hot and cold fluid changes along the exchanger. LMTD gives the correct average for heat duty calculations.

For multi pass and cross flow exchangers a correction factor F is applied.

LMTD = (ΔT₁ − ΔT₂) ÷ ln(ΔT₁ ÷ ΔT₂) and Q = U A F × LMTD

ΔT₁, ΔT₂	temperature differences at the two ends
U	overall heat transfer coefficient
A	area

Example: ΔT₁ = 60 °C, ΔT₂ = 20 °C: LMTD = 40 ÷ ln 3 ≈ 36.4 °C.

What is Log Mean Temperature Difference?

Log mean temperature difference is the effective average temperature difference between two fluids in a heat exchanger.

Why not use the arithmetic mean?

It overestimates the driving force when the end differences are very different.

When is NTU better?

When outlet temperatures are unknown, the effectiveness NTU method avoids iteration.

Thermocouple and RTD installation precautions

A practical guide on instrumentationblog.in with more detail on log mean temperature difference and how it is applied on real plants.