Work out heat exchanger lmtd instantly with clear inputs, formula shown and shareable results.
Because the temperature difference varies along a heat exchanger, the correct average is logarithmic: (dT1 - dT2)/ln(dT1/dT2). With 60 K at one end and 20 K at the other the LMTD is 36.4 K, not the 40 K an arithmetic mean would suggest - so using the simple mean would oversize the duty by 10%.
Log mean temperature difference
LMTD = (dT1 - dT2) / ln(dT1 / dT2)
Heat duty
Q = U A x LMTD
When the two terminal differences are within about 40% of each other, the error is under 2%. Wide ratios need the log form.
The logarithm becomes indeterminate but the limit is simply that common difference, which the calculator returns directly.