Heat Exchanger LMTD Calculator
Hot water 80→60°C against cold water 27→50°C in counterflow gives an LMTD of 31.5 K — and a 100 kW duty on a plate exchanger (U = 4,000 W/m²K) needs just 0.79 m² of area.
Enter the four temperatures and pick the flow arrangement. The calculator returns the LMTD, flags impossible temperature crossings, and — given a duty and U value — the required heat transfer area. Method per D.Q. Kern, Process Heat Transfer.
Stream Temperatures
Results
| Property | Value | Basis |
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The Formulas
Log mean temperature difference (ΔT₁, ΔT₂ are the end differences for your arrangement):
Required area from duty and overall coefficient:
Method & Sources
- LMTD method — D.Q. Kern, Process Heat Transfer (1950), the standard reference. For multi-pass shell-and-tube exchangers, apply the F correction factor from TEMA / Kern charts to the counterflow LMTD — typically 0.85–0.95; this tool shows pure counterflow and parallel cases.
- Typical U values — Kern and TEMA practice ranges: plate water-water 3,000–6,000 W/m²K; shell & tube water-water 800–1,500; steam-water 1,500–4,000; tank coil 500–1,000; water-oil 100–350. The dropdown uses mid-range values; a manufacturer datasheet beats every table.
- Crossing check — in parallel flow the cold outlet can never exceed the hot outlet; the tool flags impossible inputs rather than returning a meaningless number.
- Fouling — real selections add fouling resistance (lower effective U). Size with margin or use the dirty-U from the datasheet.
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Frequently Asked Questions
What is LMTD and why not a simple average?
LMTD = (ΔT₁ − ΔT₂) ÷ ln(ΔT₁/ΔT₂) — the correct mean driving force when the temperature difference varies along the exchanger. An arithmetic average overstates the driving force whenever the end differences are unequal, which undersizes the exchanger.
How do I get the heat exchanger area?
A = Q ÷ (U × LMTD). Example: 100 kW with LMTD 31.5 K on a plate exchanger at U = 4,000 W/m²K needs 0.79 m². Add fouling margin for the real selection.
Why does counterflow beat parallel flow?
Counterflow keeps a healthy ΔT along the whole length, and the cold stream can leave hotter than the hot stream leaves — impossible in parallel. Same duty, equal or less area, every time.
What U value should I assume?
Plate water-water 3,000–6,000 W/m²K; shell & tube water-water 800–1,500; steam-water 1,500–4,000; tank coil 500–1,000; water-oil 100–350 (Kern/TEMA practice). Use the low end for fouling fluids — and the datasheet over any table.
Designing Heat Recovery?
LMTD is step one — the bigger win is usually choosing where to put the exchanger. Our engineers do pinch checks and heat pump integration as part of every industrial quote.
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