502.7.3.1 Operation.

Mechanical ventilation shall be kept in operation at all times while spraying operations are being conducted and for a sufficient time thereafter to allow vapors from drying coated articles and finishing material residue to be exhausted. Spraying equipment shall be interlocked with the ventilation of the flammable vapor area such that spraying operations cannot be conducted unless the ventilation system is in operation.

502.7.3.2 Recirculation.

Air exhausted from spraying operations shall not be recirculated.

   Exceptions:

      1.   Air exhausted from spraying operations shall be permitted to be recirculated as makeup air for unmanned spray operations provided that:

         1.1.   The solid particulate has been removed.

         1.2.   The vapor concentration is less than 25 percent of the lower flammable limit (LFL).

         1.3.   Approved equipment is used to monitor the vapor concentration.

         1.4.   An alarm is sounded and spray operations are automatically shut down if the vapor concentration exceeds 25 percent of the LFL.

         1.5.   In the event of shutdown of the vapor concentration monitor, 100 percent of the air volume specified in Section 510 is automatically exhausted.

      2.   Air exhausted from spraying operations is allowed to be recirculated as makeup air to manned spraying operations where all of the conditions provided in Exception 1 are included in the installation and documents have been prepared to show that the installation does not pose a life safety hazard to personnel inside the spray booth, spraying space or spray room.

502.7.3.3 Air velocity.

The ventilation system shall be designed, installed and maintained so that the flammable contaminants are diluted in noncontaminated air to maintain concentrations in the exhaust air flow below 25 percent of the contaminant's lower flammable limit (LFL). In addition, the spray booth shall be provided with mechanical ventilation so that the average air velocity through openings is in accordance with Sections 502.7.3.3.1 and 502.7.3.3.2.

502.7.3.3.1 Open face or open front spray booth.

For spray application operations conducted in an open face or open front spray booth, the ventilation system shall be designed, installed and maintained so that the average air velocity into the spray booth through all openings is not less than 100 feet per minute (0.51 m/s).

   Exception: For fixed or automated electrostatic spray application equipment, the average air velocity into the spray booth through all openings shall be not less than 50 feet per minute (0.25 m/s).

502.7.3.3.2 Enclosed spray booth or spray room with openings for product conveyance.

For spray application operations conducted in an enclosed spray booth or spray room with openings for product conveyance, the ventilation system shall be designed, installed and maintained so that the average air velocity into the spray booth through openings is not less than 100 feet per minute (0.51 m/s). For occupied spray booths and occupied spray rooms, the air velocity at the point of application shall not be less than 100 feet per minute (0.51 m/s).

   Exceptions:

      1.   For fixed or automated electrostatic spray application equipment, the average air velocity into the spray booth through all openings shall be not less than 50 feet per minute (0.25 m/s).

      2.   Where methods are used to reduce cross drafts that can draw vapors and overspray through openings from the spray booth or spray room, the average air velocity into the spray booth or spray room shall be that necessary to capture and confine vapors and overspray to the spray booth or spray room.

502.7.3.4 Ventilation obstruction.

Articles being sprayed shall be positioned in a manner that does not obstruct collection of overspray.

502.7.3.5 Independent ducts.

Each spray booth and spray room shall have an independent exhaust duct system discharging to the outdoors.

   Exceptions: 

      1.   Multiple spray booths having a combined frontal area of 18 square feet (1.67 m 2 ) or less are allowed to have a common exhaust where identical spray-finishing material is used in each booth. If more than one fan serves one booth, such fans shall be interconnected so that all fans operate simultaneously.

      2.   Where treatment of exhaust is necessary for air pollution control or energy conservation, ducts shall be allowed to be manifolded if all of the following conditions are met:

         2.1.   The sprayed materials used are compatible and will not react or cause ignition of the residue in the ducts.

         2.2.   Nitrocellulose-based finishing material shall not be used.

         2.3.   A filtering system shall be provided to reduce the amount of overspray carried into the duct manifold.

         2.4.   Automatic sprinkler protection shall be provided at the junction of each booth exhaust with the manifold, in addition to the protection required by this chapter.