How this calculator works
When the temperature difference changes along a heat exchanger with constant overall coefficient and heat capacity rates, integration gives a logarithmic mean. Counterflow and parallel flow pair different ends, so their terminal differences differ. Equal terminal differences use their common value without dividing zero by zero. One-shell mode applies the standard correction for an even number of tube passes; other crossflow arrangements need a factor from an applicable reference.
Formula
LMTD = (deltaT1 - deltaT2)/ln(deltaT1/deltaT2); if the differences match, LMTD = deltaT1. Corrected mean difference = F LMTD; Q = UA F LMTD.
Worked example
Terminal differences of 100 K and 50 K give LMTD = 72.134752 K. Two terminal differences of 40 K give exactly 40 K, and a supplied F = 0.9 gives a corrected value of 36 K.
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Frequently asked questions
What if one terminal difference is zero?
The limiting LMTD is zero, indicating a pinch that requires unbounded area for finite duty at finite U.
Does every exchanger use F = 1?
No. F = 1 applies to the ideal parallel and counterflow arrangements; multipass and crossflow layouts can need correction.
Can hot and cold outlet temperatures cross?
Counterflow outlet temperatures can cross each other, but each paired local terminal temperature difference must stay nonnegative.