🔧 LMTD Calculator – Log Mean Temperature Difference
Calculate the log mean temperature difference (LMTD) for a heat exchanger, a key value used to size and analyze heat transfer equipment.
What This Calculator Does
The log mean temperature difference (LMTD) is the standard way to express the average temperature driving force across a heat exchanger, used together with heat transfer area and the overall heat transfer coefficient to calculate total heat exchange (Q = U × A × LMTD).
The Formula
LMTD = (ΔT1 − ΔT2) / ln(ΔT1/ΔT2). In a counterflow arrangement, ΔT1 = Hot In − Cold Out and ΔT2 = Hot Out − Cold In. In a parallel flow arrangement, ΔT1 = Hot In − Cold In and ΔT2 = Hot Out − Cold Out.
Note: If the two temperature differences are equal, LMTD simply equals that common value (the mathematical limit of the formula). If the temperatures indicate a physically inconsistent or crossing arrangement, the calculator returns zero rather than an invalid result — double check your inlet/outlet values in that case.