Physics

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LMTD Calculator.

Calculate log-mean temperature difference for a heat exchanger.

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LMTD: 39.152304 K

LMTD

39.152304K

Results update automatically as you type.

Use Cases

Heat Exchanger Sizing

Engineers use LMTD to calculate the required heat transfer area for a heat exchanger given the heat duty and overall heat transfer coefficient.

Example: For a counter-flow exchanger with ΔT1=50 K and ΔT2=30 K, LMTD ≈ 39.15 K.

Performance Evaluation

Compare actual temperature differences to theoretical LMTD to assess the effectiveness and fouling of an existing heat exchanger.

Example: If measured ΔT1=40 K and ΔT2=20 K, LMTD ≈ 28.85 K.

Frequently Asked Questions

What is the log-mean temperature difference (LMTD)?
LMTD is the logarithmic average of the temperature differences at the two ends of a heat exchanger. It is used to determine the driving force for heat transfer and is essential for sizing heat exchangers.
How do I use this LMTD calculator?
Enter the temperature difference at one end (ΔT1) and the temperature difference at the other end (ΔT2) in Kelvin. The calculator computes the LMTD using the formula: LMTD = (ΔT1 - ΔT2) / ln(ΔT1/ΔT2).
What if the two temperature differences are equal?
If ΔT1 equals ΔT2, the LMTD is equal to that value. The formula becomes indeterminate (0/0), but the limit is the common temperature difference.

Tips & Common Mistakes

Tips

  • Ensure both temperature differences are in Kelvin for consistent units.
  • For counter-current flow, the larger difference is typically at one end and the smaller at the other; enter them accordingly.
  • If the two differences are equal, the LMTD equals that value; the calculator handles this correctly.
  • Use LMTD in the equation Q = U * A * LMTD to find the heat transfer area (A).

Common Mistakes to Avoid

  • Using Celsius instead of Kelvin – always convert to Kelvin (K = °C + 273.15).
  • Entering the same value for both differences when they are actually different, which gives a misleading LMTD.
  • Forgetting that LMTD is only valid for constant heat flux and specific flow arrangements; for complex flows, use correction factors.

Last updated: August 13, 2026