LMTD Calculator
Enter the hot and cold stream temperatures, choose the heat exchanger flow pattern, and click calculate.
What is log mean temperature difference (LMTD)?
The log mean temperature difference is the effective average temperature driving force across a heat exchanger. Because the temperature difference between hot and cold streams changes from one end of the exchanger to the other, a simple arithmetic average is not accurate. LMTD accounts for that changing profile and gives a physically correct mean value.
Core formula used in this calculator
The calculator uses the standard equation:
LMTD = (ΔT1 − ΔT2) / ln(ΔT1/ΔT2)
- ΔT1 and ΔT2 are the terminal temperature differences at the two ends of the exchanger.
- If ΔT1 equals ΔT2, LMTD becomes that same value.
- The equation requires both terminal differences to be positive and physically meaningful.
How terminal differences are selected
Counterflow:
- ΔT1 = Th,in − Tc,out
- ΔT2 = Th,out − Tc,in
Parallel flow:
- ΔT1 = Th,in − Tc,in
- ΔT2 = Th,out − Tc,out
Why engineers use LMTD
LMTD is central to heat exchanger design and rating calculations. Combined with overall heat transfer coefficient and area, it links thermal performance to equipment size:
Q = U × A × LMTD
- Q: heat duty
- U: overall heat transfer coefficient
- A: heat transfer area
- LMTD: logarithmic mean temperature difference
A higher LMTD generally means stronger thermal driving force, which can reduce required heat transfer area for the same duty.
Step-by-step: using this LMTD calculator correctly
- Pick the correct flow arrangement (counterflow or parallel).
- Enter the four stream temperatures.
- Use consistent units across all inputs.
- Click Calculate LMTD.
- Review ΔT1, ΔT2, and final LMTD result.
Common mistakes to avoid
- Mixing units (for example °C and °F in the same calculation).
- Choosing the wrong flow pattern.
- Entering non-physical temperatures that produce negative terminal differences.
- Using arithmetic average ΔT instead of logarithmic mean ΔT for exchanger design work.
Quick practical note
For multipass shell-and-tube exchangers and crossflow exchangers, engineers often apply an LMTD correction factor (F). In that case, effective driving force is F × LMTD. This page computes the base LMTD value, which is the starting point for those cases.
FAQ
Can I use Fahrenheit values?
Yes. As long as all four temperatures use the same unit, the computed LMTD is valid in that unit.
What if ΔT1 equals ΔT2?
The logarithmic expression approaches a limit, and LMTD equals that common terminal difference.
Why did I get an error?
Errors usually indicate a sign issue in terminal differences (for example a temperature crossover or wrong stream assignment). Check your inlet/outlet labels and flow selection.