ASHRAE’s Pocket Guide lists minimum conveying velocities that keep contaminants airborne while they travel through exhaust ductwork. Table 2.9 (2013 edition, Ch. 02, pp. 60 & 64) groups typical industrial/kitchen contaminants into velocity “bands”; designers size each run so the calculated average velocity is at or above the value for the heaviest contaminant it must carry.
| Contaminant group (per ASHRAE Pocket Guide Table 2.9) | Recommended conveying velocity, Vc (fpm) – use the higher end when dust is coarse or duct run is long |
|---|---|
| Smoke, vapours, light fumes | 1,000 – 2,000 fpm Vc |
| Grease-laden cooking vapours & light mist | 1,500 – 2,500 fpm Vc |
| Fine dry lint / light dust (e.g., cotton, flour) | 2,000 – 2,500 fpm Vc |
| Average industrial dust (sawdust, grain) | 3,000 – 3,500 fpm Vc |
| Heavy or gritty dust (sand, metal grindings) | 3,500 – 4,000 fpm Vc |
| Moist or sticky dust (coal, foundry moulding) | 3,500 – 4,000 fpm Vc |
| Chips, shavings, very heavy particles | 4,000 – 4,500 fpm Vc |
Table of Contents
Design workflow
- Determine required airflow (Q).
- Capture velocity at hood × hood area → exhaust volume.
- Apply any diversity or simultaneity factors.
- Select conveying velocity (Vc).
- Choose the highest velocity applicable to the mixture being handled (table above).
- Increase Vc 5–10% for long horizontal runs (>45 m) or where future build-up is expected.
- Size the duct.
- Area A=Q/VcA=Q/Vc.
- Convert to diameter (round) or equivalent width × height (rectangular, keep aspect ≤4:1 to avoid excess friction).
- Check pressure loss.
- Compute static‐pressure drop (friction charts or software).
- Confirm fan has adequate static pressure plus safety margin.
- Material & construction.
- Use all-welded 16-ga steel for grease or heavy-dust service; seal joints for gases.
- Provide clean-out doors at every change of direction >45° and at 12 m intervals (NFPA , SMACNA).
- Accessories.
- Balance dampers outside grease-laden airstreams.
- Spark or flame arrestors for combustible dusts.
- Automatic fire-suppression link to kitchen hoods when grease is present.
Good practice tips
- Always design from hood to fan, maintaining (or increasing) velocity at every downstream branch to prevent settling.
- Keep horizontal runs short and sloped (≥1% toward traps) for grease or moist dust so condensate/grease can drain.
- Round duct gives the lowest friction and is preferred for dust-laden airstreams; if rectangular is inevitable, internally stiffen large flat sides to limit vibration.
- Where occupant noise is a concern (e.g., laboratories), add silencers after meeting the velocity requirement—never reduce Vc below the Table 2.9 minimum just to cut noise.
By basing each segment on the appropriate conveying velocity band from ASHRAE Table 2.9 and then confirming pressure drop, structural strength and cleanability, designers ensure reliable contaminant removal while meeting fire-safety and maintainability requirements.
