What this calculator does
Heat exchanger sizing depends on the log-mean temperature difference, and using the arithmetic mean instead can over- or under-size a exchanger badly. This calculator computes the true counter-flow LMTD and, given a duty and an overall U-value, the heat transfer area you need.
How to use it
- Enter the four terminal temperatures: hot in, hot out, cold in and cold out.
- The calculator rejects cross-flow terminal conditions, because counter-flow LMTD does not apply there.
- Enter the heat duty in kW and the overall U-value in W/m²·K to get the required area.
- For shell-and-tube multipass exchangers apply the correction factor from TEMA before ordering.
Governing formula
ΔT_lm = (ΔT₁ − ΔT₂) / ln(ΔT₁/ΔT₂) ; A = Q/(U·F·ΔT_lm) Where:
-
ΔT₁= Hot inlet minus cold outlet [°C] -
ΔT₂= Hot outlet minus cold inlet [°C] -
Q= Heat duty [kW] -
U= Overall heat transfer coefficient [W/m²·K]
Worked example — 80 → 50 °C hot water cooling against 20 → 45 °C
The figures below were produced by running this page's own engine with the example inputs (Hot fluid inlet (°C), Hot fluid outlet (°C), Cold fluid inlet (°C), Cold fluid outlet (°C), Heat duty (kW), Overall heat transfer coefficient (U) (W/m²·K)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| LMTD | 32.44 °C |
|---|---|
| Required heat transfer area | 25.69 m² |
| Hot-side temperature drop | 30 °C |
| Cold-side temperature rise | 25 °C |
| Correction factor (F) | 1 |
| Assessment | LMTD 32.4 °C (counter-flow, F = 1.0) → heat-transfer area ≈ 25.7 m² for the 500 kW duty. |
- ΔT₁ = 80 − 45 = 35 °C and ΔT₂ = 50 − 20 = 30 °C.
- LMTD = (35 − 30) / ln(35/30) ≈ 32.5 °C.
- With U = 600 W/m²·K and a 500 kW duty the required area is about 25.6 m².
Assumptions & limitations
Engineering calculation. Counter-flow log-mean temperature difference with a correction factor of 1.0. Multipass and cross-flow arrangements need their F value from the appropriate charts.
- Counter-flow only; a cross-flow or multi-pass arrangement needs a correction factor below 1 and a different approach.
- U-value depends on the fluids, fouling films and flow arrangement — use the value for your duty, not a handbook average.
- Assumes no heat loss to the surroundings and constant flow rates.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- TEMA — Standards for heat exchangers, correction factors.
- Incropera & DeWitt, Fundamentals of Heat and Mass Transfer.
- ASME B31.3 — process piping and exchanger design.