Heat Exchanger LMTD Calculator

The log mean temperature difference (LMTD) is the effective driving force for heat transfer in a heat exchanger. It accounts for the fact that the temperature difference between hot and cold fluids varies along the length of the exchanger. LMTD = (delta_T1 - delta_T2) / ln(delta_T1 / delta_T2).

In parallel flow, both fluids enter the exchanger at the same end and flow in the same direction. In counter flow, fluids enter from opposite ends and flow in opposite directions. Counter flow achieves a higher LMTD and can theoretically transfer more heat for the same surface area, making it more thermally efficient.

Heat duty Q = U * A * LMTD, where U is the overall heat transfer coefficient and A is the heat transfer area. Given Q, U, and LMTD, you can calculate the required area A = Q / (U * LMTD). LMTD is the key link between heat duty requirements and physical exchanger size.

For shell-and-tube exchangers with multiple passes, the effective LMTD is reduced by a correction factor F: Q = U * A * F * LMTD_countercurrent. F depends on two dimensionless parameters (P and R) and the number of tube passes. For true counter flow, F = 1. For cross-flow or multi-pass, F is obtained from charts or equations.

A small LMTD means the temperature driving force is weak. This requires a very large heat transfer area to achieve the desired duty. In practice, LMTD below about 5 degrees C requires extremely large exchangers or indicates the process temperatures should be reconsidered. Very small LMTD can make the exchanger economically unviable.