LMTD Calculator
Calculate Log Mean Temperature Difference for heat exchanger design. Supports multiple flow configurations with automatic F factor correction.
LMTD Calculator
Log Mean Temperature Difference for heat exchangers
Fluids flow in opposite directions. Most efficient arrangement.
Log Mean Temperature Difference
69.5
°C
Temperature Differences
ΔT₁
80.0
°C
ΔT₂
60.0
°C
LMTD
69.5
°C
Effectiveness (ε)
50.0%
Heat Capacity Ratio (R)
1.50
NTU (est.)
0.69
Temperature Profile
LMTD Formula
How to Use This Calculator
Select your temperature unit and flow arrangement. Counter-flow is most efficient; shell & tube requires F factor correction.
Enter the hot fluid temperatures (inlet and outlet) and cold fluid temperatures. The calculator validates your inputs to ensure a physically possible heat exchanger configuration.
For shell & tube exchangers, specify the number of shell and tube passes. The F factor is automatically calculated based on these parameters.
Flow Arrangements Explained
Counter-flow - Fluids flow in opposite directions. Highest LMTD, F = 1.0. Preferred for maximum heat recovery.
Parallel-flow - Fluids flow in same direction. Lower LMTD than counter-flow. Cold outlet cannot exceed hot outlet.
Cross-flow - Fluids flow perpendicular. Common in air-cooled exchangers. F typically 0.85-0.95.
Shell & Tube - Multiple tube passes inside a shell. Most common industrial exchanger. F depends on pass configuration (typically 0.75-0.95).
Frequently Asked Questions
What is LMTD?
Log Mean Temperature Difference (LMTD) is the driving force for heat transfer in a heat exchanger. It accounts for the varying temperature difference along the length of the exchanger and is used with Q = U × A × LMTD to size heat exchangers.
Why is counter-flow more efficient than parallel-flow?
In counter-flow, the temperature difference is more uniform along the exchanger, giving a higher LMTD. Counter-flow can also achieve closer approach temperatures - the cold fluid can exit hotter than the hot fluid outlet temperature.
What is the F correction factor?
The F factor corrects LMTD for heat exchangers that deviate from pure counter-flow, such as shell & tube or cross-flow configurations. F ranges from 0 to 1, with values below 0.75 generally indicating poor design.
When do I need to use the corrected LMTD?
Use corrected LMTD (F × LMTD) for multi-pass shell & tube exchangers, cross-flow exchangers, or any configuration that isn't pure counter-flow or parallel-flow. The correction accounts for reduced effectiveness.
What is a temperature cross?
A temperature cross occurs when the cold outlet exceeds the hot outlet temperature. This is impossible in parallel-flow but achievable in counter-flow. The calculator validates temperatures and warns of invalid configurations.
How do I use LMTD to size a heat exchanger?
Use Q = U × A × LMTD, where Q is heat duty (W), U is overall heat transfer coefficient (W/m²·K), A is heat transfer area (m²), and LMTD is from this calculator. Typical U values: water-water 850-1700, steam-water 1000-6000, gas-gas 10-30 W/m²·K.
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