Log Mean Temperature Difference LMTD Calculator

The Log Mean Temperature Difference (LMTD) is the effective average temperature difference between hot and cold fluids in a heat exchanger. It is a key parameter used to calculate heat transfer and size exchangers accurately.

To make the process easier, we have created an online free LMTD Calculator.

How to Use The Log Mean Temperature Difference Calculator

1. Select heat exchanger type – Counter-flow or Parallel-flow.
2. Enter hot fluid temps – inlet and outlet.
3. Enter cold fluid temps – inlet and outlet.
4. Click “Calculate” – the tool shows ΔT₁, ΔT₂, and LMTD.
5. Check Theory tab – see step-by-step calculation.
6. Use Reset – clear inputs to start again.

What is LMTD in heat transfer?

The Log Mean Temperature Difference (LMTD) is the effective average temperature driving force for heat transfer in a heat exchanger. Unlike the arithmetic mean, it accounts for the exponential variation of temperature difference along the exchanger length, making it more accurate for both parallel-flow and counterflow configurations.

formula

Where:

• ΔT₁ = Temperature difference at one end of the exchanger
• ΔT₂ = Temperature difference at the other end of the exchanger

👉 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 Example

Example Calculation

Let’s consider a counterflow heat exchanger:

• Hot fluid inlet (Th,in): 150 °C
• Hot fluid outlet (Th,out): 80 °C
• Cold fluid inlet (Tc,in): 20 °C
• Cold fluid outlet (Tc,out): 70 °C

Step 1 – Calculate ΔT₁ and ΔT₂

• ΔT₁ = Th,in − Tc,out = 150 − 70 = 80 °C
• ΔT₂ = Th,out − Tc,in = 80 − 20 = 60 °C

So, the LMTD is 69.7 °C.

Make sure your temperatures are in °C or K before using the calculator. If you need to convert units, try our free Unit Conversion Tool

FAQ (LMTD Calculator)