Log mean temperature difference and required heat transfer area from the four terminal temperatures — the calculation between the plant-level heat balance and checking an actual exchanger.
Standard method — stated scope limits apply
Terminal Temperatures
Required Area (optional)
F = 1.0 only for true counter-current/co-current flow. Shell-and-tube and cross-flow exchangers need F from TEMA/manufacturer charts.
Enter the four terminal temperatures.
About This Calculation
LMTD (Log Mean Temperature Difference) is the correct way to average the temperature difference driving heat transfer along a heat exchanger, because that difference changes continuously from one end to the other — a simple arithmetic average overstates the true driving force whenever the temperature profiles aren’t parallel. Required heat transfer area then follows directly from duty, the exchanger’s overall heat transfer coefficient U, and LMTD. The equation shown here assumes true counter-current (or co-current) flow; a real shell-and-tube exchanger with multiple passes needs a correction factor F (from TEMA or manufacturer charts) applied on top, since its actual flow pattern is more complex.
Frequently Asked Questions
Why use log-mean instead of a simple average of the two end temperature differences?
Because the temperature difference between hot and cold streams doesn’t fall linearly along the exchanger — it decays exponentially. The log-mean form is the mathematically correct average of that curve; a simple arithmetic mean of the two end values overstates the true average driving force.
What does the correction factor F actually correct for?
F accounts for a real exchanger not being true counter-current — multi-pass shell-and-tube units mix some co-current flow into an overall counter-current arrangement, which reduces the effective driving force below the ideal LMTD. F < 1 means the exchanger needs more area than the ideal counter-current case for the same duty.
What happens to LMTD when one side is a pure phase change (condensing steam, boiling fluid)?
That side’s temperature stays constant while it changes phase, which the standard LMTD equation still handles correctly — it’s a special case of the same formula, not a different one, as long as the constant-temperature side is entered correctly as both its own inlet and outlet value.
Why does the calculator reject some temperature combinations?
If the terminal temperatures imply the hot and cold streams’ profiles cross inside the exchanger (impossible for that flow arrangement), LMTD is mathematically undefined — that usually means the flow arrangement was entered wrong (counter-current vs co-current) or the duty isn’t physically achievable with those terminal temperatures.