Industrial exhaust systems carry solvent vapour, greasy smoke, combustible dust or particulate — and anything that accumulates in the duct becomes fuel. FM Global DS 7-78 covers exhaust design, in-duct suppression, material selection and the cleaning regime.
Exhaust Types
- Solvent exhaust: paint shops, chemical plants and printing — flammable vapour
- Grease exhaust: commercial kitchens, in parallel with NFPA 96 — grease deposition
- Dust exhaust: timber, metalworking and food — combustible dust explosion risk
- Hot exhaust: ovens and heat treatment — high temperature with volatiles
Duct Design Rules
- Duct velocity: a minimum of around 18 m/s to prevent dust settling, up to about 25 m/s
- Material: galvanised or stainless steel; combustible ductwork is not permitted
- Layout: in grease and dust systems, limit horizontal runs and collect vertically
- Access panels: cleaning openings at regular intervals
- Dampers: fire dampers at zone boundaries
In-Duct Suppression
- Grease exhaust: in-duct sprinklers at each elbow and at regular intervals, in parallel with NFPA 96
- Solvent exhaust: foam-water sprinklers or gaseous suppression
- Dust exhaust: inert gas rather than water, because water can turn dust into a slurry and may aggravate the event
- Activation: quick response heads at a duct temperature rating suited to the process
Hood and Filter Protection
- Kitchen hood: a listed wet chemical system with appliance and duct protection
- Solvent hood: foam-water plus gaseous suppression
- Cyclone separator: static electricity control and bonding and earthing
- Bag filter: explosion relief panels and suppression for combustible dust
Cleaning and Maintenance
- Kitchen exhaust: professional cleaning on a quarterly cycle under NFPA 96
- Solvent exhaust: annual internal inspection and cleaning
- Dust exhaust: monthly inspection with a full clean twice a year
- Filters: replaced to the manufacturer schedule, monitored by differential pressure
Quick Checklist
- Exhaust type correctly classified
- Duct velocity within limits and material non-combustible
- Correct in-duct suppression medium selected
- Hood and filter protection provided
- Cleaning schedule defined and enforced
- Explosion relief provided on dust systems
Frequently Asked Questions
Why is a minimum duct velocity specified in exhaust systems?
Below roughly 18 m/s, dust and particulate settle out and accumulate on the duct wall, creating a fuel layer that turns a small ignition into a duct fire or a dust explosion. The minimum velocity keeps material entrained until it reaches the collector, which is a fire protection requirement as much as a ventilation one.
Why is water not used on combustible dust exhaust?
Water can turn accumulated dust into a slurry that blocks the duct, and with certain metal dusts it reacts and worsens the event. Inert gas suppression is used instead, alongside explosion relief on the collector, so the system is protected without introducing a second hazard.
How often must a kitchen exhaust duct be cleaned?
Grease exhaust is cleaned professionally on a quarterly cycle under NFPA 96, more often for heavy-use operations. Grease deposition is continuous and invisible from inside the kitchen, so the interval is set by usage rather than by inspection findings alone.
What protection is required at the hood itself?
A commercial kitchen hood requires a listed wet chemical system covering the appliances and the duct collar, with interlocks that shut off fuel and make-up air on activation. Solvent hoods use foam-water or gaseous suppression, and dust collectors need bonding, earthing and explosion relief.
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Download MEP Calc on the App StoreFM Global Property Loss Prevention Data Sheet DS 7-78 (Industrial Exhaust Systems); NFPA 91 (Exhaust Systems for Air Conveying of Vapors, Gases, Mists and Particulate Solids); NFPA 96 (Commercial Cooking Operations); NFPA 654 (Combustible Dusts).