IES
Thermal EngineeringP0

LMTD Calculator - Log Mean Temperature Difference

Calculate Log Mean Temperature Difference (LMTD) for heat exchanger design and analysis.

Inputs
Enter your values below
Result
LMTD
—
Enter values and click Calculate
Formula
(dt1 − dt2) ÷ log(dt1 ÷ dt2)
Result unit: K

LMTD = (ΔT1 − ΔT2) / ln(ΔT1/ΔT2). For counter-current: ΔT1 = Th_in − Tc_out, ΔT2 = Th_out − Tc_in. For equal ΔT at both ends, LMTD = ΔT.

Introduction

LMTD represents the effective average temperature driving force in a heat exchanger. Used in Q = U·A·LMTD. For counter-current flow (preferred), ΔT is taken at each end. For parallel flow, both streams enter at the same end.

Calculation Example

ΔT1 = 60°C, ΔT2 = 40°C → LMTD = 49.3 K (the arithmetic mean would be 50, very close when ΔT ratio < 2).

Inputs: dt1=60, dt2=40
Result: 49.3 K

Engineering Applications

  • •Heat exchanger design
  • •Process thermal rating
  • •Boiler/condenser design
  • •HVAC coil selection

Frequently Asked Questions

When is LMTD correction factor F needed?

For multi-pass shell-and-tube exchangers (1-2, 2-4 pass) or cross-flow, multiply LMTD by correction factor F (0.5-1.0) from TEMA charts based on P and R parameters.

What if ΔT1 = ΔT2?

The formula gives 0/0, but as ΔT1→ΔT2, LMTD→ΔT (the arithmetic mean). The calculator will handle this case by using ΔT directly.

Related Calculators

Heat Exchanger Calculator (LMTD Method)Heat Transfer Calculator

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Disclaimer: Calculations are for reference and educational purposes only. Always verify results independently for engineering design. See full disclaimer.