Logarithmic mean temperature difference in heat exchangers selection

LMTD stands for Logarithmic mean temperature difference, being the logarithmic average of the temperature difference between the hot and cold feed on primary and secondary end of a heat exchanger. The value is fundamental for the calculation of the thermal exchange surface of a heat exchanger.

The LMTD is a crucial value also for the selection of the kind of heat exchanger most suitable for a certain application. Thermal transfer between two fluids presenting a short temperature gap is indeed very slow, while it will be much more faster and efficient in case of steam at 130° C to heat up cold water to a temperature of 70° C, for example. For the same reason, heat recovery is much more efficient having a small quantity of water at very high temperature instead of a lot of water at a mild temperature.

Plate heat exchangers allow to work with very narrow log mean temperature differences, thanks to possibility to work with countercurrent fluids and with turbulent flows, achieving high thermal exchange rates. Thermal transfer rates of shell and tubes exchangers are instead lower, thus requiring a higher LMTD. The Log mean TD is therefore even widened for air/water or air/steam finned pack exchangers, requiring very extended thermal transfer surfaces.

In general, the logarithmic mean temperature difference is inversely proportional to the thermal transfer surfaces. Even a half degree of difference has a strong impact on the heat transfer surface and the calculation of a heat exchanger, thus on its cost.
In Tempco we’re are willing to provide you some example of this kind of calculations!