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Log Mean Temperature Difference

Thermodynamics Directory | Heat Transfer Directory

In heat exchanger applications, the inlet and outlet temperatures are commonly specified based on the fluid in the tubes. The temperature change that takes place across the heat exchanger from the entrance to the exit is not linear. A precise temperature change between two fluids across the heat exchanger is best represented by the log mean temperature difference (LMTD or ΔTlm), defined in Equation below.

ΔT1m =  (ΔT2 - ΔT1)  / [ ln(ΔT2/ ΔT1) ]

where:

ΔT2 = the larger temperature difference between the two fluid streams at either the entrance or the exit to the heat exchanger 

ΔT1 = the smaller temperature difference between the two fluid streams at either the entrance or the exit to the heat exchanger

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