Fume Hood Airflow Volume (CAV/VAV) Triage
A fume hood is only as safe as its exhaust control system. Diagnostics require differentiating between older Constant Air Volume (CAV) hoods—which pull the same amount of air regardless of sash position—and modern Variable Air Volume (VAV) systems that utilize venturi air valves to instantly throttle airflow based on how high the sash window is opened.
The Physics of the Sash
Face velocity is determined by a simple equation: Velocity (FPM) = Flow (CFM) / Area (Sq.Ft.). When a researcher raises the glass sash, the physical area increases. If the exhaust fan does not compensate by pulling more CFM, the velocity drops instantly, rendering the hood unsafe.
| Hood Type | Exhaust Volume | Face Velocity Behavior |
|---|---|---|
| Standard CAV | Constant | Plummets as sash opens. Requires user to keep sash low. |
| Bypass CAV | Constant | Relatively stable. Uses a grille above the sash to divert air. |
| VAV | Variable | Maintained at exactly 100 FPM. Valve reacts to sash movement. |
Diagnostic Gateway Challenge
You are testing a VAV fume hood. The user abruptly closes the sash from 18 inches down to 2 inches. The face velocity immediately spikes to 180 FPM before settling back down to 100 FPM five seconds later. What is occurring?