Engineering calculator reviewed for preliminary design use · Last updated: March 2026
Calculate log mean temperature difference (LMTD) for heat exchanger design. Supports both counterflow and parallel flow configurations. Also computes the F-correction factor for shell-and-tube exchangers. Use as input to Q=UA×LMTD×F for area calculation.
What this calculator is used for
LMTD is the driving force for heat transfer in a heat exchanger. Because the temperature difference varies along the exchanger, the log mean is more accurate than a simple arithmetic mean and is essential for calculating the required heat transfer area.
Typical engineering use cases
Sizing heat transfer area for shell-and-tube and plate exchangers
Comparing counter-flow versus parallel-flow arrangements
Verifying outlet temperatures and thermal effectiveness of existing units
Evaluating the F-correction factor for multi-pass configurations
Governing equation and methodology
The log mean temperature difference and required area are:
LMTD = (ΔT₁ − ΔT₂) / ln(ΔT₁/ΔT₂), A = Q / (U × LMTD × F)
For counter-flow, ΔT₁ = T₁ᵢₙ − T₂ₒᵤₜ and ΔT₂ = T₁ₒᵤₜ − T₂ᵢₙ; for parallel-flow, ΔT₁ = T₁ᵢₙ − T₂ᵢₙ and ΔT₂ = T₁ₒᵤₜ − T₂ₒᵤₜ. Q is the heat duty [W], U the overall heat transfer coefficient [W/(m²·K)], and F the multi-pass/cross-flow correction factor (typically 0.7–0.95).
Engineering assumptions and limitations
Steady state with a constant U over the exchanger
Sensible heat transfer (condensing/boiling needs zone-by-zone analysis)
Arithmetic mean is acceptable when ΔT₁ ≈ ΔT₂
Configurations giving F < 0.75 should be avoided
Practical design notes
Counter-flow achieves a higher LMTD and allows a temperature cross, enabling more compact designs. In practice, include a fouling allowance by selecting a conservative U and adding area margin. If the F-factor is low, revisit the number of tube passes or shells.
Interpretation: There is no temperature cross here, so a single counter-current pass works. Run the same temperatures in parallel flow and the cold outlet (80 °C) would approach the hot outlet (90 °C), collapsing ΔT and demanding far more area — which is exactly why counter-current is the default and why the F factor penalises multi-pass shells.
Common Mistakes & Misuse
Using the bare LMTD for a multi-pass shell-and-tube without the F correction factor (F < 1), which over-predicts the driving force.
Mixing up counterflow and parallel-flow terminal differences — parallel flow gives a smaller LMTD and cannot approach a temperature cross.
Pushing the design into a temperature cross where F collapses (often below ~0.8) and more shells in series are required.
Applying LMTD to a duty with phase change or strongly non-linear Cp, where the simple log-mean no longer represents the true profile.
Frequently Asked Questions
When should I use the F correction factor?
F correction is needed for multi-pass shell-and-tube exchangers. For pure counterflow or pure parallel flow, F = 1. For 1-2 or 2-4 configurations, F < 1 and depends on the R and P ratios.
What if LMTD is very small or zero?
A very small LMTD means the temperature driving force is weak, requiring a very large heat transfer area. If LMTD approaches zero, the exchanger design is impractical. Consider changing flow arrangement or temperatures.
For preliminary estimation and educational use only. Results may depend strongly on assumptions, input data, fluid or material properties, and the range of validity of the underlying equation. Verify critical calculations independently and follow the applicable code, specification, and formal engineering review process before using any result for design, procurement, fabrication, operation, or safety decisions.