LMTD Calculator

Calculate the log-mean temperature difference for counter-flow and parallel-flow heat exchangers.

°C
°C
°C
°C

LMTD (Counter-Flow)

69.52 °C

LMTD (Parallel-Flow)55.81 °C

LMTD (Counter-Flow) vs Hot Fluid Inlet Temp

Vedi formula →

Formula

## Log-Mean Temperature Difference The LMTD is the effective average temperature difference between the hot and cold streams across a heat exchanger. It accounts for the fact that the temperature difference varies along the length. ### Formula **LMTD = (delta_T1 - delta_T2) / ln(delta_T1 / delta_T2)** For counter-flow: delta_T1 = T_hot_in - T_cold_out, delta_T2 = T_hot_out - T_cold_in. For parallel-flow: delta_T1 = T_hot_in - T_cold_in, delta_T2 = T_hot_out - T_cold_out. Counter-flow always gives a higher LMTD (and thus smaller exchanger area) than parallel-flow.

Esempio Risolto

Hot fluid: 150→90°C, Cold fluid: 30→70°C.

  1. 01Counter-flow: delta_T1 = 150 - 70 = 80°C, delta_T2 = 90 - 30 = 60°C
  2. 02LMTD = (80 - 60) / ln(80/60) = 20 / ln(1.333) = 20 / 0.2877 = 69.52°C
  3. 03Parallel-flow: delta_T1 = 150 - 30 = 120°C, delta_T2 = 90 - 70 = 20°C
  4. 04LMTD = (120 - 20) / ln(120/20) = 100 / ln(6) = 100 / 1.7918 = 55.80°C