Heat Exchanger LMTD Calculator
Calculate log mean temperature difference for heat exchanger design.
Results
The log mean temperature difference (LMTD) is the driving force for heat transfer in heat exchangers. It accounts for the fact that the temperature difference between hot and cold fluids varies along the exchanger length. LMTD is used in the equation Q = UA × LMTD, where U is the overall heat transfer coefficient and A is the surface area.
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Frequently asked questions
ΔT1 = 90 - 50 = 40°C, ΔT2 = 60 - 20 = 40°C. Since they are equal, LMTD = 40°C. When ΔT1 = ΔT2, the LMTD equals either value.
LMTD properly weights the non-linear temperature profile. The arithmetic mean of 40°C and 40°C is 40°C, which happens to match the LMTD here, but for unequal deltas the LMTD is always lower than the arithmetic mean.
Shell-and-tube exchangers typically achieve 60-80% effectiveness. Plate heat exchangers can reach 85-95%. The example (30°C cold rise out of 70°C maximum) gives 42.9% effectiveness.
Use Q = UA × LMTD. If you need to transfer 100 kW with LMTD = 40°C and U = 500 W/m²·K, then A = 100000/(500 × 40) = 5 m² of heat transfer surface.
Counterflow (fluids flow opposite) gives higher LMTD and is more efficient. Parallel flow (same direction) has lower LMTD because the exit temperatures converge. Counterflow can achieve closer approach temperatures.