Recent calculations and scenario comparison
| Select | Saved | Flow | LMTD | Corrected | F | Q | Action |
|---|
Formula used
Counterflow: ΔT₁ = Th,in − Tc,out, and ΔT₂ = Th,out − Tc,in.
Parallel flow: ΔT₁ = Th,in − Tc,in, and ΔT₂ = Th,out − Tc,out.
LMTD: (ΔT₁ − ΔT₂) ÷ ln(ΔT₁ ÷ ΔT₂).
Corrected LMTD: F × LMTD.
Heat duty: Q = U × A × corrected LMTD.
Area: A = Q ÷ (U × corrected LMTD).
Overall coefficient: U = Q ÷ (A × corrected LMTD).
How to use this calculator
- Select the required calculation mode and exchanger flow arrangement.
- Choose the temperature unit and enter hot and cold stream temperatures.
- Add a correction factor, thermal coefficient, area, heat duty, or fluid data when required.
- Select shell passes, tube passes, or crossflow mixing conditions for advanced arrangements.
- Press Calculate LMTD and review warnings, detailed steps, charts, and derived engineering values.
- Save scenarios, compare alternatives, copy results, or export the calculation as CSV or PDF.
Worked example data
| Case | Hot inlet | Hot outlet | Cold inlet | Cold outlet | Flow | Typical use |
|---|---|---|---|---|---|---|
| 1 | 150°C | 90°C | 25°C | 65°C | Counterflow | Oil cooler |
| 2 | 120°C | 70°C | 20°C | 55°C | Parallel | Water heater |
| 3 | 180°C | 110°C | 30°C | 85°C | Shell-and-tube | Process exchanger |
Understanding advanced log mean temperature analysis
Log mean temperature difference represents the effective thermal driving force inside a heat exchanger. It accounts for changing stream temperatures from one end to another. This makes it more useful than a simple arithmetic average.
Counterflow exchangers usually provide a larger LMTD than parallel arrangements. The streams move in opposite directions and maintain stronger temperature separation. This often reduces the area needed for a specified heat load.
Parallel-flow exchangers place both streams in the same direction. Their temperature difference falls rapidly near the exchanger outlet. They may still be suitable for compact or controlled heating duties.
Shell-and-tube and multipass exchangers rarely behave as perfect counterflow devices. A correction factor adjusts ideal LMTD for the actual pass arrangement. Low correction factors can indicate inefficient geometry or poor temperature matching.
The R ratio compares the hot-side temperature drop with the cold-side rise. The P ratio measures the cold-side rise against the maximum inlet temperature difference. Together, these values help estimate shell-and-tube correction performance.
Heat duty follows from the product of overall coefficient, area, and corrected LMTD. Rearranging the same relationship can solve exchanger area or overall coefficient. Reliable units are essential because every factor directly affects the result.
Fluid mass flow and specific heat provide independent hot-side and cold-side duties. Close agreement supports the energy balance used in the design. Large disagreement may reveal heat loss, bad measurements, or incorrect fluid properties.
Fouling resistance lowers the effective overall heat-transfer coefficient over operating time. Deposits can form on either fluid side or across walls. Including resistance helps produce more conservative sizing and maintenance decisions.
Temperature crossover must be checked before accepting any result. LMTD cannot use zero or negative terminal differences in its logarithm. Such conditions usually signal wrong inputs or an unsuitable exchanger model.
This calculator supports quick screening, comparison, and educational analysis. Final equipment selection should also consider pressure drop, materials, vibration, phase change, and manufacturer data. Qualified engineers should verify safety-critical exchanger designs before construction.
Frequently asked questions
What does LMTD measure?
It measures the effective average temperature difference driving heat transfer through an exchanger.
Why use a logarithmic mean?
The temperature difference changes along the exchanger, so a logarithmic mean represents that variation more accurately.
When is a correction factor required?
Use a correction factor for multipass shell-and-tube, crossflow, or other arrangements that differ from ideal counterflow.
What does a low F value mean?
A low value suggests poor thermal arrangement and may require more passes, another configuration, or revised temperatures.
Can LMTD be negative?
A valid design uses positive terminal differences. Negative values indicate crossover, reversed streams, or inconsistent data.
What happens when ΔT₁ equals ΔT₂?
The LMTD equals that common temperature difference, avoiding a zero-over-zero logarithmic form.
Can this calculator determine exchanger area?
Yes. Enter heat duty and overall coefficient, then select the area calculation mode.
How is effectiveness calculated?
Effectiveness compares actual heat transfer with the maximum possible heat transfer based on the minimum heat-capacity rate.
Does the calculator support phase change?
It can estimate LMTD when a condensing or boiling stream remains nearly constant, but detailed phase-change design needs specialist data.
Are crossflow factors exact?
The included crossflow factors are estimates. Final design should use validated charts, correlations, or manufacturer software.