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505.1 General
A mechanical ventilation or exhaust system shall be installed to control, capture, and remove emissions generated from product use or handling where required in accordance with the building code or fire code and where such emissions result in a hazard to life or property. The design of the system shall be such that the emissions are confined to the area in which they are generated by air currents, hoods, or enclosures and shall be exhausted by a duct system to a safe location or treated by removing contaminants. Ducts conveying explosives or flammable vapors, fumes, or dusts shall extend directly to the exterior of the building without entering other spaces and shall not extend into or through ducts and plenums.
Exception: Ducts conveying vapor or fumes having flammable constituents less than 25 percent of their Lower Flammability Limit (LFL) shall be permitted to pass through other spaces.
505.1.1 Mechanical Ventilation
A mechanical ventilation system shall be interlocked to operate with the equipment used to produce vapors, fumes, or dusts that are flammable or hazardous.
505.2 Incompatible Materials
Incompatible materials shall not be conveyed in the same system. [NFPA 91:4.2.2]
505.3 Flammability Limit
Unless the circumstances stipulated in Section 505.3.1, Section 505.3.2, or Section 505.3.3 exist, in systems conveying flammable vapors, gases, or mists, the concentration shall not exceed 25 percent of the lower flammability limit (LFL). [NFPA 91:4.2.3]
505.3.1 Higher Concentrations
Higher concentrations shall be permitted where the exhaust system is designed and protected in accordance with NFPA 69 using one or more of the following techniques:
  1. Combustible concentration reduction
  2. Oxidant concentration reduction
  3. Deflagration suppression
  4. Deflagration pressure containment [NFPA 91:4.2.3.1]
Contaminated air shall not be recirculated to occupied areas unless contaminants have been removed. Air contaminated with explosive or flammable vapors, fumes, or dusts; flammable or toxic gases; or radioactive material shall not be recirculated.
505.3.2 Ovens and Furnaces
Higher concentrations shall be permitted for ovens and furnaces designed and protected in accordance with NFPA 86. [NFPA 91:4.2.3.2]
505.3.3 Deflagration
Higher concentrations shall be permitted where deflagration venting is provided in accordance with NFPA 68. [NFPA 91:4.2.3.3]
505.4 Air-Moving Devices
Air-moving devices shall be sized to establish the velocity required to capture, control, and convey materials through the exhaust system. [NFPA 91:4.2.5]
505.5 Generating Flames, Sparks, or Hot Materials
Operations generating flames, sparks, or hot material such as from grinding wheels and welding shall not be manifolded into an exhaust system that air conveys flammable or combustible materials. [NFPA 91:4.2.6]
505.6 Fire Dampers
Fire dampers shall be permitted to be installed in exhaust systems in accordance with the following:
  1. Where ducts pass through fire barriers
  2. Where a collection system installed on the end of the system is protected with an automatic extinguishing system
  3. Where the duct system is protected with an automatic extinguishing system
  4. Where ducts have been listed with interrupters
  5. Where necessary to facilitate the control of smoke pursuant to the applicable NFPA standards [NFPA 91:4.2.9]
505.6.1 Prohibited
Fire dampers shall not be installed if the material being exhausted is toxic and if a risk evaluation indicates that the toxic hazard is greater than the fire hazard. [NFPA 91:4.2.10]
505.7 Fire Detection and Alarm Systems
Unless the conditions in Section 505.7.1 or Section 505.7.2 exist, fire detection and alarm systems shall not be interlocked to shut down air-moving devices. [NFPA 91:4.2.14]
505.7.1 Automatic Extinguishing System
Where shutdown is necessary for the effective operation of an automatic extinguishing system, it shall be permitted to interlock fire detection and alarm systems to shut down air-moving devices. [NFPA 91:4.2.14.1]
505.7.2 Shut Down Permitted
Where a documented risk analysis acceptable to the Authority Having Jurisdiction shows that the risk of damage from fire and the products of combustion would be higher with air-moving devices operating, it shall be permitted to interlock fire detection and alarm systems to shut down air-moving devices. [NFPA 91:4.2.14.2]
505.8 Product-Conveying Ducts Classification
Product-conveying ducts shall be classified according to their use, as follows:
Class 1 - Ducts conveying nonabrasives, such as smoke, spray, mists, fogs, noncorrosive fumes and gases, light fine dusts, or powders.
Class 2 - Ducts conveying moderately abrasive particulate in light concentrations, such as sawdust and grain dust, and buffing and polishing dust.
Class 3 - Ducts conveying Class 2 materials in high concentrations and highly abrasive materials in low concentrations, such as manganese, steel chips, and coke.
Class 4 - Ducts conveying highly abrasive material in high concentrations.
Class 5 - Ducts conveying corrosives, such as acid vapors.
505.9 Minimum Velocities and Circulation
The velocity and circulation of air in work areas shall be such that contaminant's are captured by an airstream at the area where the emissions are generated and conveyed into a product-conveying duct system. Mixtures within work areas where contaminants are generated shall be diluted to be accordance with Section 505.3 with air that does not contain other contaminants. The velocity of air within the duct shall be not less than set forth in Table 505.9.
Systems conveying particulate matter shall be designed by employing the constant velocity method. Systems conveying explosive or radioactive materials shall be pre-balanced through duct sizing. Other systems shall be permitted to be designed with balancing devices such as dampers. Dampers provided to balance airflow shall be provided with securely fixed minimum-position blocking devices to prevent restricting flow below the required volume or velocity.
TABLE 505.9 MINIMUM DUCT DESIGN VELOCITIES*
[NFPA 91:TABLE A.4.1.5]
NATURE OF CONTAMINANTS EXAMPLES FEET PER MINUTE DESIGN VELOCITY (feet per minute)
Vapors, gases, smoke Vapors, gases, and smoke Any
Fumes Welding 2000
Fine light dusts Cotton lint, wood flour, litho powder 2500
Dry dusts and powders Fine rubber dust, molding powder dust, jute lint, cotton dust, shavings (light), soap dust, leather shavings 3000
Average industrial dusts Grinding dust, buffing lint (dry), wool jute dust (shaker waste), coffee beans, shoe dust, granite dust, silica flour, general material handling, brick cutting, clay dust, foundry (general), limestone dust, packaging and weighing asbestos dust in textile industries 3500
Heavy dusts Sawdust (heavy and wet), metal turnings, foundry tumbling barrels and shake-out, sandblast dust, wood blocks, hog waste, brass turning, cast-iron boring dust, lead dust 4000
Heavy or moist dusts Lead dust with chips, moist cement dust, asbestos chunks from transite pipe cutting machines, buffing lint (sticky), quick-lime dust 4500
For SI units: 1 foot per minute = 0.005 m/s
* Systems that are handling combustible particulate solids shall be in accordance with NFPA 654.
505.10 Makeup Air
Makeup air shall be provided to replenish air exhausted by the ventilation system. Makeup air intakes shall be located so as to avoid recirculation of contaminated air within enclosures.
505.11 Hoods and Enclosures
Hoods and enclosures shall be used where contaminants originate in a concentrated area. The design of the hood or enclosure shall be such that air currents created by the exhaust systems will capture the contaminants and transport them directly to the exhaust duct. The volume of air shall be sufficient to dilute explosive or flammable vapors, fumes, or dusts in accordance with Section 505.9. Hoods of steel shall have a base metal thickness not less than 0.027 of an inch (0.686 mm) (No. 22 gauge) for Class 1 and Class 5 metal duct systems; 0.033 of an inch (0.838 mm) (No. 20 gauge) for hoods serving a Class 2 duct system; 0.044 of an inch (1.118 mm) (No. 18 gauge) for hoods serving a Class 3 duct system; and 0.068 of an inch (1.727 mm) (No. 14 gauge) for hoods serving a Class 4 duct system.
Approved nonmetallic hoods and duct systems shall be permitted to be used for Class 5 corrosive systems where the corrosive mixture is nonflammable. Metal hoods used with Class 5 duct systems shall be protected with an approved corrosion-resistant material. Edges of hoods shall be rounded. The minimum clearance between hoods and combustible construction shall be the clearance required by the duct system.

Related Code Sections

505.0 Exhaust Systems, Product-Conveying Systems
be such that contaminant's are captured by an airstream at the area where the emissions are generated and conveyed into a product-conveying duct system. Mixtures within work ...
505.3 Exhaust Systems, Flammability Limit
Unless the circumstances stipulated in Section 505.3.1, Section 505.3.2, or Section 505.3.3 exist, in systems conveying flammable vapors, gases ...
505.4 Exhaust Systems, Air-Moving Devices
Air-moving devices shall be sized to establish the velocity required to capture, control, and convey materials through the exhaust system. [NFPA 91 ...
505.5 Exhaust Systems, Generating Flames, Sparks, or Hot Materials
Operations generating flames, sparks, or hot material such as from grinding wheels and welding shall not be manifolded into an exhaust system ...