LMTD Calculator – Log Mean Temperature Difference

Written by Great Medico

Updated September 25, 2026 •2 min read

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Thermal calculations often look simple until absolute temperature, sign convention, phase, or heat-transfer mode changes the meaning. LMTD Calculator – Log Mean Temperature Difference keeps the page’s log mean temperature difference tied to the quantities and assumptions used by this specific thermodynamic relation.

What this calculator does

The LMTD Calculator – Log Mean Temperature Difference brings together Heat exchanger type, Hot inlet temperature, Hot outlet temperature, Cold inlet temperature, Cold outlet temperature around the page’s log mean temperature difference. The formula section below identifies which values actually drive that result and which fields are supporting or derived quantities, so you can check the page without assuming every visible box is an independent input.

How to use it

The main fields on this page are Heat exchanger type, Hot inlet temperature, Hot outlet temperature, Cold inlet temperature, Cold outlet temperature. Enter the quantities you actually know, keep their units consistent, and leave derived/output-style fields blank unless the formula explicitly allows solving in the opposite direction. For a clean check of log mean temperature difference, change one driving quantity at a time and confirm that the result moves in the direction predicted by the equation.

How the calculation works

The page forms the two terminal temperature differences for parallel or counterflow operation and calculates LMTD = (ΔT₁−ΔT₂)/ln(ΔT₁/ΔT₂). When the two terminal differences are equal, their common value is used directly.

Example

For example, start with the page’s populated scenario: Heat exchanger type = counter. Apply the equation above using the units shown on the page, then compare the calculated log mean temperature difference with the displayed result. As a second check, change one physical input while holding the others fixed and confirm that the direction of change makes sense for this formula.

How to interpret the result

Interpret log mean temperature difference within the stated thermodynamic model, unit system, and sign convention. Check whether temperatures are absolute where required and whether the process is idealized, because those details can matter more than the last decimal place.

Limitations and notes

Ideal thermal equations omit some combination of heat loss, contact resistance, temperature-dependent properties, radiation/convection coupling, phase behavior, and nonuniform temperature. Recheck units and process assumptions before using the result for equipment or safety decisions. For LMTD Calculator – Log Mean Temperature Difference, keep that general caution tied to the exact fields and equation shown here rather than carrying assumptions over from a different calculator.