Advanced Log Mean Temperature Calculator

Calculator Inputs

Temperature unit

Use one temperature scale consistently.

Flow arrangement

Counterflow usually gives a higher temperature driving force.

Correction factor F

Enter 1 for ideal counterflow or parallel flow.

Hot inlet temperature

Hot outlet temperature

Cold inlet temperature

Cold outlet temperature

Overall heat transfer coefficient U

Unit: W/m²·K. Optional for area and duty checks.

Heat duty Q

Unit: W. Optional for UA and area sizing.

Available surface area A

Unit: m². Optional for predicted duty and heat flux.

Temperature Profile Graph

The graph compares the hot and cold stream temperature paths along exchanger length. Counterflow plots the cold stream in the opposite direction.

Example Data Table

Case	Flow	Hot In	Hot Out	Cold In	Cold Out	F	LMTD
Oil cooler	Counterflow	180	120	40	100	1.00	80.0000
Water heater	Counterflow	200	140	60	110	0.95	84.9070
Simple heater	Parallel flow	180	130	40	90	1.00	79.8815

Formula Used

The log mean temperature difference method estimates the effective average temperature driving force between hot and cold streams in a heat exchanger.

Counterflow terminal differences
ΔT₁ = T_h,in − T_c,out
ΔT₂ = T_h,out − T_c,in

Parallel flow terminal differences
ΔT₁ = T_h,in − T_c,in
ΔT₂ = T_h,out − T_c,out

LMTD relation
LMTD = (ΔT₁ − ΔT₂) / ln(ΔT₁ / ΔT₂)

Corrected LMTD
ΔT_lm,corr = F × LMTD

Heat-transfer sizing relations
Q = U × A × ΔT_lm,corr
UA = Q / ΔT_lm,corr
A = Q / (U × ΔT_lm,corr)

If ΔT₁ and ΔT₂ become equal, the logarithmic expression approaches that same temperature difference. The calculator automatically uses that limiting value.

How to Use This Calculator

Step 1

Choose the temperature unit and exchanger arrangement. Use counterflow for opposite stream directions or parallel flow for the same direction.

Step 2

Enter hot-side inlet and outlet temperatures, then enter cold-side inlet and outlet temperatures using a consistent scale.

Step 3

Provide correction factor F. Use 1.00 for ideal arrangements, or enter a lower value for corrected multipass behavior.

Step 4

Optionally enter overall coefficient U, heat duty Q, and area A if you also want UA, required area, predicted duty, or heat flux.

Step 5

Click Calculate LMTD. The result section appears below the header and above the form, followed by the graph and supporting details.

Frequently Asked Questions

1. What does LMTD represent?

LMTD represents the effective average temperature difference between two streams across a heat exchanger. It provides the driving force used in the heat-transfer equation Q = U × A × ΔT.

2. Why is counterflow usually preferred?

Counterflow generally maintains a stronger temperature driving force over the full exchanger length. That often gives a higher LMTD and can reduce the required surface area for the same duty.

3. When should I use a correction factor?

Use a correction factor when the exchanger does not behave like a simple ideal single-pass arrangement. Shell-and-tube and multipass geometries commonly require an F value below 1.

4. Can I use Celsius or Fahrenheit?

Yes. Temperature differences are valid in Celsius, Kelvin, or Fahrenheit as long as every entered temperature uses the same scale. The calculator labels results using your chosen unit.

5. Why do I get an invalid temperature-cross message?

That message appears when one terminal temperature difference becomes zero or negative. It usually means the entered temperatures are not physically consistent with the selected exchanger arrangement.

6. What is the difference between LMTD and corrected LMTD?

LMTD comes directly from terminal temperatures. Corrected LMTD multiplies that value by factor F to account for exchanger configurations that reduce the effective temperature driving force.

7. How is required area calculated?

Required area is calculated from A = Q / (U × corrected LMTD). You must provide heat duty, overall coefficient, and temperatures for the area estimate to appear.

8. What if ΔT1 equals ΔT2?

When the terminal differences are equal, the logarithmic equation reaches a limiting value equal to that same difference. The calculator handles this case automatically without numerical instability.