Sizing lab exhaust duct is a chain of three linked numbers: how much air the hood needs, how fast that air must move, and the diameter that satisfies both.
Step 1 — Find the required CFM
A fume hood’s exhaust volume is governed by face velocity — the average air speed drawn through the sash opening. Most labs target 80–120 feet per minute (fpm). Multiply face velocity by the open sash area to get the required volume:
- CFM = face velocity (fpm) × sash opening area (ft²).
- Example: 100 fpm across a 6 ft² opening ≈ 600 CFM.
Step 2 — Set the transport velocity
Once you know the volume, the duct has to move it fast enough that vapors — and any entrained particulate — stay in suspension. Too slow and condensable fumes or dust settle out inside the run.
| EXHAUST TYPE | TARGET TRANSPORT VELOCITY |
|---|---|
| Gases & vapors | 1,000–2,000 fpm |
| Fumes / light smoke | 2,000–2,500 fpm |
| Fine dust / particulate | 3,000–4,000+ fpm |
Step 3 — Solve for diameter
Duct area equals volume divided by velocity: A (ft²) = CFM ÷ velocity (fpm). Convert that area to a round-duct diameter and round up to the nearest standard size. Continuing the example: 600 CFM at 1,500 fpm needs 0.4 ft², which is about an 8‑inch duct.
Round up, then re-check velocity.
Bumping to the next standard diameter lowers the actual transport velocity. Confirm the rounded-up size still keeps velocity above your minimum, especially on particulate-laden exhaust.
Step 4 — Verify static pressure
Add up the losses — straight duct friction plus every fitting. Turns matter: see 90° vs. 45° elbow pressure loss for how to keep them low. The total static pressure at the design CFM must land within the selected fan’s curve, with margin for filter loading and future changes.
- Confirm the material suits the chemistry and temperature — PVC for ambient corrosive lab exhaust, CPVC when hot.
- Minimize fittings and use gentle turns to protect your static-pressure budget.
- Balance branches with correctly sized blast gates so each hood pulls its design airflow.
For manifolded multi-hood systems, balancing and redundancy add complexity worth reviewing with a fabricator before the ducts are built.
