Engineering tools · Free to use
Log Mean Temperature Difference Calculator
LMTD Calculator
Calculate the log mean temperature difference for a simple parallel-flow or counterflow heat exchanger.
Input parameters
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Result
Log mean temperature difference
°C
When the terminal differences are equal, the limit is used. Multipass and crossflow correction factors are not included.
Equation
ΔTlm = (ΔT₁ − ΔT₂) / ln(ΔT₁ / ΔT₂)
When the terminal differences are equal, the limit is used. Multipass and crossflow correction factors are not included.
Applicability
- For Q̇ ≈ UAΔTlm, assume steady single-pass parallel or counterflow, approximately constant heat-capacity rates and U, and negligible external heat loss. Computing the terminal temperature mean does not establish these conditions.
- Does not calculate multipass correction factors, phase change, or flow distribution.
- Both terminal differences must be positive; the hot stream must cool and the cold stream must warm.
- Temperatures alone cannot establish the energy balance; check flow rates and heat capacities.
Worked example
Example: counterflow with hot 150 → 100 °C and cold 20 → 70 °C has two terminal differences of 80 °C, giving 80 °C.
For engineering estimates; check the operating conditions and applicable requirements for design.