Face Velocity — the One Number That Sizes Your AHU Casing (and Your SFP)

Pick a casing size before you have checked face velocity and you have already decided your SFP class, your filter change frequency and half your acoustic spectrum — without meaning to. Face velocity is the quiet variable that ties every downstream number together.

What it actually is

Face velocity is airflow divided by the free cross-sectional area the air sees at a given plane in the unit:

v = q_v / A — expressed in m/s

The catch is that "the casing size" is not one number. Face velocity is evaluated per component — filter bank, coil block, heat recovery core — because each has its own effective free area once frames, tube banks and fins are subtracted from the nominal cross-section. A unit can run 2.0 m/s at the filter and 3.1 m/s through a tight coil block if the selection tool does not flag it.

Why it drives everything else

Pressure drop across almost every AHU component scales with roughly the square of face velocity. Halve the velocity and you cut component dP to roughly a quarter — which is why casing size is the single highest-leverage lever in an AHU design, ahead of picking a "better" coil or a marginally more efficient fan.

The knock-on effects:

SFP — lower velocity means lower system dP means lower absorbed fan power for the same airflow. See our SFP explained guide for how this rolls up into EN 16798 classes.
Filter life — ePM1 filters loaded at 2.5 m/s clog faster than at 1.8 m/s; replacement interval can drop by a third.
Coil performance — face velocity above ~2.5–3.0 m/s on a cooling coil risks moisture carry-over past the eliminators, a real problem for dehumidification duties, not just an efficiency footnote.
Acoustics — velocity above ~3 m/s starts generating self-noise in ductwork and at coil fins, independent of the fan's own sound power.
Casing footprint and cost — the lever runs both ways: an oversized casing to hit low velocity costs more sheet metal, more floor space and a heavier structural frame.

Target ranges by component

Practical targets used across most manufacturer selection guides (actual limits vary by coil rows, filter class and application):

Component	Typical target	Upper limit before penalties
Filter bank (ePM1/ePM2.5)	1.8–2.2 m/s	~2.5 m/s
Cooling coil (dehumidifying)	2.0–2.5 m/s	~2.8 m/s (carry-over risk)
Heating coil	2.2–3.0 m/s	~3.5 m/s
Plate heat recovery core	2.0–2.5 m/s	~3.0 m/s (leakage class impact)
Rotary wheel	2.5–3.5 m/s	~4.0 m/s

These are starting points for a design loop, not a rule to apply blindly — a unit dedicated to sensible-only cooling can tolerate higher coil velocities than one running deep dehumidification.

The sizing trap

The common failure mode is picking a casing size from a catalogue habit ("we always spec the 2000×1200 for this airflow") rather than from the actual face velocity the selection produces once real component free areas are known. Two units at the same nominal airflow can differ by 20–30% in effective free area depending on filter frame depth, coil connector boxes and heat-recovery frame design — numbers that only show up once you are selecting real hardware, not a generic box.

Where the tooling matters

In AirSelect3D, face velocity is computed per component from the actual manufacturer geometry — filter frames, coil tube-and-fin data, heat recovery core dimensions — not a flat casing assumption, and it recalculates live every time you resize the casing or swap a product. Any component crossing its practical velocity threshold flags in the results rail before it ever reaches the SFP or acoustic total, so the trade-off between footprint and running cost is visible at the moment you make the sizing decision, not after the dossier is printed.

See face velocity recalculate live as you resize the casing →