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505.9 Minimum Velocities and Circulation
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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.