Heat Exchanger LMTD Calculator
Calculate the Log Mean Temperature Difference (LMTD) for parallel-flow and counter-flow heat exchangers, and determine heat duty from the LMTD method.
Results
What is it?
LMTD = (ΔT₁ − ΔT₂) / ln(ΔT₁ / ΔT₂). For counter-flow: ΔT₁ = T_h,in − T_c,out, ΔT₂ = T_h,out − T_c,in. For parallel-flow: ΔT₁ = T_h,in − T_c,in, ΔT₂ = T_h,out − T_c,out. Heat duty: Q = U × A × LMTD. Counter-flow always gives a higher LMTD (more driving force, more compact exchanger) than parallel-flow.
How to use
Enter the hot and cold fluid inlet and outlet temperatures. Select the flow arrangement. If you know U×A (product of overall heat transfer coefficient and heat transfer area), enter it to calculate the heat duty Q.
Example scenario
Counter-flow: hot 120→70°C, cold 20→60°C. ΔT₁ = 120−60 = 60 K, ΔT₂ = 70−20 = 50 K. LMTD = (60−50)/ln(60/50) = 10/0.182 = 54.9 K. With UA = 500 W/K: Q = 500 × 54.9 = 27.45 kW.
Pro tip
For multi-pass or cross-flow heat exchangers, apply an F correction factor: Q = F × U × A × LMTD where F < 1 (from standard TEMA charts). Always check that neither temperature crosses (hot outlet must be hotter than cold inlet in counter-flow). If temperatures cross, the exchanger is thermodynamically impossible for the given flow arrangement.