How to Use LMTD Calculator
- Pick flow type — Counter or parallel.
- Pick temperature unit — K, °C, or °F.
- Enter four temperatures — Hot & cold inlet/outlet.
- Read LMTD in kelvin along with the end-ΔTs.
Formula & Theory - LMTD Calculator
Counter-flow:
ΔT₁ = Th,in − Tc,out
ΔT₂ = Th,out − Tc,in
Parallel-flow:
ΔT₁ = Th,in − Tc,in
ΔT₂ = Th,out − Tc,out
LMTD = (ΔT₁ − ΔT₂) / ln(ΔT₁ / ΔT₂)
Heat duty: Q = U · A · LMTD · F (F = correction factor for multi-pass / cross flow)| Symbol | Meaning |
|---|---|
| Th,in / Th,out | Hot stream inlet/outlet temperature |
| Tc,in / Tc,out | Cold stream inlet/outlet temperature |
| ΔT₁, ΔT₂ | End-of-exchanger temperature differences |
Assumptions and Limits
- Steady-state operation, constant specific heats and overall U.
- Pure counter or parallel flow; use a correction factor F for shell-and-tube or cross-flow.
- ΔT₁ and ΔT₂ must both be positive.
Use Cases for LMTD Calculator
- Heat exchanger sizing — Once Q and U are known, solve for area A.
- Performance check — Compare measured to expected heat duty.
- Education — Visualize how flow arrangement affects driving temperature difference.
The LMTD Calculator is the bread-and-butter tool of classical heat exchanger analysis.