Logarithmic mean temperature difference

The LMTD is a logarithmic average of the temperature difference between the hot and cold feeds at each end of the double pipe exchanger.

The use of the LMTD arises straightforwardly from the analysis of a heat exchanger with constant flow rate and fluid thermal properties.

A correction factor is also required for other more complex geometries, such as a shell and tube exchanger with baffles.

The total exchanged energy is found by integrating the local heat transfer q from A to B: Notice that B − A is clearly the pipe length, which is distance along z, and D is the circumference.

A related quantity, the logarithmic mean pressure difference or LMPD, is often used in mass transfer for stagnant solvents with dilute solutes to simplify the bulk flow problem.