Log Mean Temperature Difference - Thermodynamics

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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 ΔT_lm), defined in Equation below.

ΔT_{1m =} (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