Log Mean Temperature Difference (LMTD)
The log mean temperature difference is the effective average temperature gap driving heat transfer between two fluids along a heat exchanger, calculated as a logarithmic mean of the end-point differences. It is the temperature term in the basic heat-exchanger sizing equation.
Because the temperature difference between hot and cold streams varies from one end of an exchanger to the other, a simple arithmetic average overstates the driving force. The LMTD weights the two end differences logarithmically to give the true mean. Multiplied by the surface area and overall heat-transfer coefficient, it gives the heat duty. For cross-flow and multi-pass arrangements a correction factor is applied to the counter-flow LMTD.
Related terms
Heat Exchanger · Approach Temperature · Fouling Factor