How to use this calculator
Choose the flow arrangement and enter the four inlet and outlet temperatures. Add the overall heat transfer coefficient U and area A to get the heat duty. For shell-and-tube or crossflow units, enter the chart correction factor F.
Formulas
Worked example
Counterflow, hot 90 → 60 °C, cold 20 → 45 °C, U = 800 W/m²·K, A = 5 m² (the default)
- ΔT1 = 90 − 45 = 45 °C; ΔT2 = 60 − 20 = 40 °C.
- LMTD = (45 − 40) / ln(45/40) = 42.45 °C.
- Q = 800 × 5 × 1 × 42.45 = 169.8 kW.
In parallel flow the same temperatures would be impossible: the cold outlet (45 °C) can never pass the hot outlet in parallel flow unless the exchanger is very long.
Assumptions and limits
- Steady state, constant U along the exchanger, constant specific heats.
- No heat loss to the surroundings.
- For multipass and crossflow exchangers, F comes from charts in heat transfer textbooks.
Frequently asked questions
Why not use the plain average temperature difference?
The temperature difference changes exponentially along the exchanger, not linearly. The log mean is the exact average for that profile; the arithmetic mean slightly overestimates it.
Why is counterflow better?
For the same temperatures it gives a larger LMTD, so it needs less area, and it lets the cold outlet rise above the hot outlet.
How do I check the energy balance?
Q should equal ṁcpΔT for each fluid. If you know flow rates, the hot side’s heat loss and cold side’s gain should match Q.