Log Mean Temperature Difference Calculator

Calculator Input

Hot inlet temperature

Hot outlet temperature

Cold inlet temperature

Cold outlet temperature

Flow arrangement

Correction factor F

Heat duty Q (optional)

Heat transfer area A (optional)

Overall U value (optional)

Reset

Plotly Graph

The cold-side line for counterflow is plotted opposite the hot-side travel direction to reflect reverse movement through the exchanger.

Example Data Table

Case	Flow	Hot In	Hot Out	Cold In	Cold Out	F	Approx. LMTD
1	Counterflow	150	95	30	70	1.00	72.13
2	Parallel	120	80	25	60	1.00	55.62
3	Counterflow	180	110	35	90	0.92	81.77

Formula Used

For parallel flow:
ΔT1 = Th,in − Tc,in
ΔT2 = Th,out − Tc,out

For counterflow:
ΔT1 = Th,in − Tc,out
ΔT2 = Th,out − Tc,in

LMTD = (ΔT1 − ΔT2) / ln(ΔT1 / ΔT2)

If ΔT1 = ΔT2, then LMTD = ΔT1

Corrected LMTD = F × LMTD

Heat duty relation:
Q = U × A × F × LMTD

The logarithmic mean temperature difference gives the effective average temperature driving force across a heat exchanger. It is more accurate than a simple arithmetic mean because exchanger temperature profiles are nonlinear along the flow path.

How to Use This Calculator

Enter hot-side inlet and outlet temperatures.
Enter cold-side inlet and outlet temperatures.
Select counterflow or parallel flow arrangement.
Set correction factor F if your exchanger needs it.
Optionally enter heat duty, area, or overall U value.
Click calculate to show results above the form.
Review LMTD, corrected LMTD, and any derived design values.
Download CSV or PDF reports if needed.

How to calculate log mean temperature difference: First compute the two terminal temperature differences based on exchanger arrangement. Then apply the logarithmic mean formula. If a correction factor is needed for shell-and-tube or multipass service, multiply LMTD by F before using Q = UAFΔTlm.

Frequently Asked Questions

1. What is log mean temperature difference?

It is the effective average temperature difference between hot and cold fluids in a heat exchanger. It accounts for temperature changes along the exchanger and is widely used in thermal design calculations.

2. Why is LMTD better than an arithmetic mean?

Temperature differences change nonlinearly along exchanger length. The logarithmic mean reflects that variation, so it represents the actual thermal driving force more accurately than a simple average.

3. How do I calculate effective terminal temperature differences?

For parallel flow, compare inlet-to-inlet and outlet-to-outlet temperatures. For counterflow, compare hot inlet with cold outlet, then hot outlet with cold inlet. Those two values go into the LMTD formula.

4. What happens if both terminal differences are equal?

When the two terminal temperature differences are equal, the logarithmic expression collapses to that same value. In practice, LMTD equals either terminal difference in that special case.

5. Why would I use a correction factor F?

Real exchangers like shell-and-tube or multipass units may not behave as pure parallel or pure counterflow devices. A correction factor adjusts the ideal LMTD to better represent actual thermal performance.

6. Can this calculator estimate overall heat-transfer coefficient?

Yes. If you enter heat duty, area, and temperatures, the calculator can estimate the required U value from Q = UAFΔTlm. It can also estimate heat duty if U and area are known.

7. Which flow arrangement usually gives a higher LMTD?

Counterflow often provides a larger average driving force than parallel flow for the same terminal temperatures. That usually makes counterflow more thermally efficient and can reduce required exchanger area.

8. What units should I use?

Use consistent units. Temperature differences can be in °C or K because the difference magnitude is identical. Heat duty, area, and U value should follow compatible SI or engineering unit systems.