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HEPA Unit — Air Speed Calculator
Ceiling-mounted HEPA — Downward discharge

Air speed decay calculator

Centreline velocity vs. distance below unit — free-jet model (ASHRAE)
Face velocity 0.45 m/s
Airflow rate 720 m³/h
Width 813 mm
Depth 552 mm
Target distance 1.8 m
Derived values
Filter area
0.449
m²
Verified face vel.
0.445
m/s
Effective diameter
0.757
m
Air speed at 1.8 m
0.090
m/s
Velocity at distance
Decay curve
Method: ASHRAE/ISO free-jet centreline decay — v(x) = v₀ × (d_eq / (d_eq + 2·k·x))² — k = 0.1 (turbulent free jet).
Assumes unobstructed downward discharge into still air. Results are indicative — actual values vary with room turbulence, obstructions, and supply air temperature.