Heat Exchanger Sizing by Airflow

Estimate the core area you need from one stream's airflow, a target effectiveness, face velocity and overall heat-transfer coefficient. Works for counterflow plate cores (C_r ≈ 1).

The sizing chain

For counterflow with equal streams, NTU = ε / (1 − ε). Then NTU = UA / C_min, so core area A = NTU × C_min / U. C_min = (airflow in m³/s) × 1.2 kg/m³ × 1005 J/kgK. Face area = volume flow / face velocity, and depth ≈ A / face area. Crossflow needs a larger area for the same effectiveness — add ~10–20%.

Frequently asked questions

Why does face velocity matter?

Higher velocity shrinks the unit but raises pressure drop and fan energy, and slightly lowers effectiveness. 2–3 m/s is the usual sweet spot for plate cores.

Should I use this for rotary wheels?

The same NTU logic applies, but wheels also have carryover and moisture migration. Treat the result as a first estimate and confirm with the supplier's selection software.

Where do I go next?

Cross-check with the ε-NTU tool and our sizing & selection guide.