How to size a local exhaust hood
Local exhaust ventilation captures a contaminant near the point of release, before it reaches the breathing zone. The airflow needed depends on the hood type, how far the hood sits from the source, and the capture velocity required to pull the contaminant in against room air currents.
The ACGIH Industrial Ventilation Manual gives the plain exterior hood equation Q = V × (10X² + A), with Q in CFM, V in fpm, X in feet and A in square feet. Adding a flange blocks useless air from behind the hood and applies a 0.75 factor. A booth or enclosure only needs the capture velocity across its face, so it reduces to Q = V × A.
| Release condition | Capture velocity |
|---|---|
| Released into quiet air with no velocity | 50-100 fpm |
| Low velocity release into moderate air | 100-200 fpm |
| Active generation, turbulent air | 200-500 fpm |
| High velocity release (grinding, blasting) | 500-2,000 fpm |
The equation holds well while the standoff distance stays within roughly 1.5 times the hood face dimension. Slot hoods and canopy hoods use different formulas, and a real installation also has to account for duct losses and fan static pressure. Have an industrial hygienist or ventilation engineer confirm the design before installation or modification.
Frequently asked questions
ACGIH ranges run about 50-100 fpm for contaminants released into still air, 100-200 fpm for moderate release such as welding, and 500-2,000 fpm for grinding or abrasive blasting.
A flange blocks air drawn in from behind the hood, which does no useful work. The same capture velocity is reached with roughly 25 percent less CFM, cutting fan size and energy cost.
Required airflow grows with the square of the distance, so doubling the standoff needs about four times the CFM. Moving the hood closer is always the cheapest improvement.