Grease duct
A grease duct is a duct that is specifically designed to vent grease-laden flammable vapors from commercial cooking equipment such as stoves, deep fryers, and woks to the outside of a building or mobile food preparation trailer. Grease ducts are regulated both in terms of their construction and maintenance, forming part of the building's passive fire protection system. The cleaning schedule is typically dictated by fire code or related safety regulations, and evidence of compliance must be kept on file by the owner.

Special hazard
Vapors are created when grease is heated to and beyond its vaporization point. As the vapors cool, the grease condenses and settles on colder surfaces. It is important for occupational safety and health, as well as compliance with local fire codes, to vent such vapors outside the kitchen and outside the building where the kitchen is located.

Grease is not only slippery, but also a highly flammable hydrocarbon.  Regardless of what state it is in, vapor, liquid or solid, it ignites easily and burns very rapidly. A fire-resistance rating is intended to certify resistance to an internal grease fire as well as an external fire. Any adjacent firestop must be compatible with the grease duct system.

Design
In North America, grease ducts must be in compliance with NFPA 96 as well as the local building codes and fire codes. Grease ducts should be kept as short as possible to minimize grease build-up.

Internationally, grease ducts must be protected against weathering to prevent corrosion, and must be composed of specific materials based on the hood type. For example, commercial kitchen hoods should be of steel no thinner than .0575 inches. Duct joints must be welded flange joints, butt joints, or overlapping joints. 

A proprietary duct system that has its own inherent fire-resistance rating can be used, such as a metallic duct, either field fabricated or UL certified factory-built designs.  Field fabricated is typically made from 16 gauge carbon steel, all welded per local codes, which is then externally treated with fireproofing.  Factory-built designs are UL tested to UL 1978 and UL 2221 (for fire rated models) test standards, are made from lighter gauge stainless steel, and offered in single wall and multiple double wall insulated designs. Typical materials used for fireproofing field fabricated designs are:

 ceramic fibers
 rockwool
 calcium silicate
 vermiculite boards pressed and bonded with sodium silicate
 intumescents
 endothermic materials, sometimes on their own, sometimes in combination with ceramic fiber

Trade jurisdiction
In North American unionized construction sites, metallic ducts are typically installed by the sheet metal trade, whereas external wraps are usually installed by the insulators. Inherently fire-resistant systems are likewise installed by the sheet metal trade.

Maintenance and cleaning

Cleaning takes place typically every 3, 6, or 12 months, depending on the nature of the appliances below the exhaust hood. For instance, woks may require grease duct cleaning every 3 months, whereas normal stoves may necessitate the grease duct to be cleaned every 6 months. Compliance is proven through certificates issued by the cleaning and maintenance contractors. Purpose-designed fire suppression systems inside the hoods must also be routinely maintained. Proper cleaning must be enabled through the use of approved, fire-resistant access panels.

It is necessary to keep hoods and grease ducts clean to prevent equipment malfunction and accidents. Ducts must receive periodic inspections from the National Fire Protection Association (NFPA) and meet Occupational Safety and Health Administration safety standards. Failure to maintain this equipment can result in penalties from the NFPA. Hazards associated with improperly maintained hoods include, but are not limited to, poor ventilation, excess heat and smoke, and the potential risk of fires.

See also
Passive fire protection
Duct (flow)
Pressurisation ductwork
Smoke exhaust ductwork
Circuit integrity
Kitchen exhaust cleaning
Kitchen hood

References

External links

UL treatise on wrapping systems
NFPA 96: Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations
ASTM E2336 Standard Test Methods for Fire Resistive Grease Duct Enclosure Systems
ISO 6944-1:2008 Fire containment -- Elements of building construction -- Part 1: Ventilation ducts

Passive fire protection