LMTD Heat Exchanger Calculator
Calculate the log mean temperature difference of a heat exchanger from its inlet and outlet temperatures, and the heat duty for a given overall coefficient and area.
- ΔT1 = 65.00 K, ΔT2 = 45.00 K
- LMTD = (ΔT1 − ΔT2) ÷ ln(ΔT1 ÷ ΔT2) = 54.39 K
LMTD Formula
Counter flow: ΔT1 = Th,in − Tc,out, ΔT2 = Th,out − Tc,in
Q = U × A × LMTD
For shell and tube exchangers with multiple passes, multiply LMTD by the correction factor F from the exchanger design.
Worked example
Hot 120 → 70 °C, cold 25 → 55 °C in counter flow: ΔT1 = 65 K, ΔT2 = 45 K, LMTD = 54.4 K. With U = 800 W/m²K and 10 m², Q = 435 kW.
Frequently Asked Questions
What is LMTD?
The logarithmic mean of the temperature differences at the two ends of a heat exchanger; the effective driving force for heat transfer.
Why not use the arithmetic mean?
Temperature difference changes exponentially along the exchanger, so the arithmetic mean overestimates the driving force.
How can I estimate U?
Typical values are 800 to 1500 W/m²K for water to water and 20 to 60 W/m²K for gas to gas exchangers.
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