Log Mean Temperature Difference (LMTD)
Log Mean Temperature Difference (LMTD)
A heat exchanger’s driving temperature difference changes along its length. The LMTD is the correct average to use in sizing.
The LMTD is ΔT_lm = (ΔT₁ − ΔT₂)/ln(ΔT₁/ΔT₂). where ΔT₁ and ΔT₂ are the temperature differences at the two ends of the exchanger.
Inputs
dT1 := 60 K=60 K
dT2 := 20 K=20 K
The log-mean correctly weights the changing temperature difference.
dT_lm := (dT1 - dT2)/ln(dT1/dT2)=36.41 K
Results
The LMTD always lies below the arithmetic mean. It feeds directly into Q = U·A·ΔT_lm to size the exchanger area.
Assumptions & limitations
- Constant overall U and fluid specific heats.
- Counter- or co-current single-pass flow.
- No phase change within a pass.