Log mean temperature difference

Summary

The log mean temperature difference is the logarithmic mean of the temperature differences between the two streams at the two ends of a heat exchanger. For parallel flow and counterflow with constant overall heat transfer coefficient $U$, the heat rate is exactly $q = U A \, \Delta T_{\mathrm{lm}}$. So it is the right average temperature difference for sizing an exchanger. The arithmetic mean of the end differences is larger, unless the two end differences are equal.