Ventilation, Smoke Control and Industrial Exhaust
Ventilation and smoke control are examined together because both concern moving air through defined paths at defined rates. Industrial ventilation asks for capture and transport; smoke control asks for tenability in escape routes and for a fan that survives 280 °C.
Industrial ventilation
- Local exhaust beats dilution. Capture velocity at the hood face depends on the contaminant: about 0.25–0.5 m/s for fumes and gases, 1–2.5 m/s for grinding dust, higher for high-velocity particles.
- Duct transport velocity must exceed the settling velocity: roughly 12–15 m/s for coarse dust and 18–23 m/s for heavy or moist dust. Too low causes silting; too high wastes fan power and erodes elbows.
- Make-up air must be at least the exhaust volume minus permitted exfiltration, otherwise the fan stalls against negative pressure and doors become hard to open.
Smoke control under GB 51251
- Stairwell pressurization about 50 Pa; lobby or refuge floor 25–30 Pa; the differential across a closed door must keep opening force within about 110 N.
- Exhaust volumes follow space type (corridor, atrium, basement, warehouse), with make-up air at not less than 50% of the exhaust rate.
- Smoke exhaust fans are rated 280 °C for 30 minutes; fire dampers in ordinary ducts close at 70 °C. Never interchange these.
Worked example
A stair door 1.0 m × 2.1 m must pass 0.7 m/s when open: flow = 2.1 × 0.7 = 1.47 m³/s = 5292 m³/h. Add closed-door leakage, Q = 0.827·A·√ΔP; with A = 0.06 m² and ΔP = 50 Pa, Q = 0.827 × 0.06 × 7.07 = 0.35 m³/s = 1263 m³/h. Total ≈ 6555 m³/h before a 1.1–1.2 safety factor, giving a fan selection near 7200–7900 m³/h.
Traps
Quoting a single pressurization value for both stair and lobby; sizing the pressurization fan on closed-door leakage only; omitting make-up air and then wondering why the exhaust rate is not achieved; using 70 °C dampers in a smoke exhaust duct; and forgetting that natural smoke exhaust needs an inlet area and a vent height, not just an outlet.