Log Mean Temperature Difference (LMTD) Calculator
Calculate log mean temperature difference for a heat exchanger using hot and cold inlet/outlet temperatures in consistent units.
Use measured steady-state temperatures. Transient data can distort LMTD and should be averaged over a stable interval.
Examples
- Hot in 180, hot out 120, cold in 60, cold out 90 ⇒ Log mean temperature difference: 74.00 K
- Hot in 95, hot out 70, cold in 30, cold out 55 ⇒ Log mean temperature difference: 40.00 K
FAQ
Which flow arrangement does this LMTD assume?
The formula applies to counterflow or parallel flow. For multi-pass exchangers, multiply by the appropriate correction factor.
Why is LMTD invalid when temperatures cross?
If Delta T1 or Delta T2 is zero or negative, the logarithm is undefined and indicates the assumed flow arrangement is impossible or the sensors are misassigned.
Should I use Celsius or Fahrenheit?
Either is acceptable as long as all four temperatures use the same unit. The output is reported in K to indicate a temperature difference.
Can I use this result to calculate heat duty?
Yes. Pair LMTD with the overall heat-transfer coefficient and area in Q = U × A × LMTD for steady-state sizing checks.
Additional Information
- Delta T1 is hot inlet minus cold outlet, and Delta T2 is hot outlet minus cold inlet.
- The reported K value is numerically identical to degrees Celsius for temperature differences.
- Use a correction factor if the exchanger is not true counterflow or parallel flow.