What is calculated?
The driving temperature difference falls along the exchanger. The logarithmic mean correctly represents that variation.
Equations
ΔTm = (ΔT₁−ΔT₂)/ln(ΔT₁/ΔT₂)
Q̇ = kA·ΔTm
Fluid dynamics & thermodynamics · preliminary design
Heat-exchanger duty with LMTD
Estimate duty from UA and logarithmic mean temperature difference for a constant-temperature cold side.
IJ 5
02
Results
Enter values and run the calculation.
Application and limits
Pre-size condensers, evaporators or tank heaters.
Constant cold temperature; not a general counter/parallel-flow rating and no F correction factor.
Frequently asked questions
Why not use the arithmetic mean?
Local heat flux is proportional to local difference; integration produces the logarithmic mean.
When is the cold side constant?
Approximately during phase change or with a very large cold-side heat-capacity rate.
Technical basis
Dubbel, thermodynamics, heat-transfer section and logarithmic temperature difference (local source, 2026-09-09).
The source supports the equation structure; this calculator does not claim a complete standards verification.