NFPA Ventilation Standards
7.1 Ventilation – General.
Ventilating and exhaust systems shall be designed and installed in accordance with the applicable requirements of NFPA 91 except as amended by the requirements of this chapter.
7.2 Performance Requirements.
Each spray area shall be provided with mechanical ventilation that is capable of confining and removing vapors and mists to a safe location and is capable of confining and controlling combustible residues, dusts, and deposits. The concentration of the vapors and mists in the exhaust stream of the ventilation system shall not exceed 25 percent of the lower flammable limit. (See Annex B for additional guidance on determining the lower flammable limit.)
Exception: In confined spaces, where ventilation might not be capable of providing the necessary ventilation, a properly applied inerting procedure shall be permitted to be used. Such procedures shall meet the applicable requirements of NFPA 69 and shall be acceptable to the authority having jurisdiction.
7.2.1 Spray areas equipped with overspray collection filters shall have visible gauges, audible alarms, or an effective inspection program to ensure that the required air velocity is being maintained.
7.2.2 Powder Coating Systems. Powder coating systems also shall meet the requirements of Section 15.7.
7.2.3 Mechanical ventilation shall be kept in operation at all times while spray operations are being conducted and for a sufficient time thereafter to allow the vapors from drying coated objects or material and residues to be exhausted. Where spray operations are conducted automatically without an attendant constantly on duty, the operating controls of the spray apparatus shall be arranged so that the spray apparatus cannot function unless the exhaust fans are operating.
7.3* Make-Up Air.
An adequate supply of clean make-up air shall be provided to compensate for the air exhausted from spray operations. The intake for this make-up air shall be located so that the air exhausted from spray operations is not recirculated.
7.4 Routing of Exhaust Ducts.
Air exhausted from spray operations shall be conducted by ducts directly to the outside of the building. Exhaust ducts shall follow the most direct route to the point of discharge, but shall not penetrate a fire wall. The exhaust discharge shall be directed away from any fresh air intakes. The exhaust duct discharge point shall be at least 1830 mm (6 ft) from any exterior wall or roof. The exhaust duct shall not discharge in the direction of any combustible construction that is within 7625 mm (25 ft) of the exhaust duct discharge point nor shall it discharge in the direction of any unprotected opening in any noncombustible or limited-combustible construction that is within 7625 mm (25 ft) of the exhaust duct discharge point.
7.5 Recirculation of Exhaust.
7.5.1* Air exhausted from spray areas shall not be recirculated.
Exception: Air exhausted from a spray operation shall be permitted to be recirculated as make-up air for an unmanned spray operation or cascaded to subsequent unmanned spray operations, provided all of the following conditions have been met:
(1) Solid particulates have been removed from the recirculated air.
(2) The concentration of vapors in the exhaust airstream do not exceed 25 percent of the lower flammable limit.
(3) Listed equipment is used to monitor the concentration of vapors in all exhaust airstreams.
(4) An alarm will sound and the spray operation will automatically shut down if the concentration of any vapor in the exhaust airstream exceeds 25 percent of the lower flammable limit.
(5) Equipment installed to process and remove contaminants from the air exhausted from spray operations is approved by the authority having jurisdiction.
7.5.2* The provisions of 7.5.1 shall not disallow the use of recirculated air to occupied spaces. However, other requirements addressing the toxicity and the permissible exposure limits shall also apply.
7.6* Manifolding of Exhaust Ducts.
Individual spray booths shall be separately ducted to the building exterior.
Exception No. 1: Multiple cabinet spray booths whose combined frontal area does not exceed 1.7 m2 (18 ft2) shall be permitted to be manifolded, if the sprayed materials used will not react and cause ignition of the residue in the ducts.
Exception No. 2: Where treatment of exhaust is necessary for air pollution control or for energy conservation, ducts shall be permitted to be manifolded if all of the following conditions are met:
(1) The sprayed materials used will not react and cause ignition of the residue in the ducts.
(2) No nitrocellulose-based finishing material is used.
(3) An air-cleaning system is provided to reduce the amount of overspray carried into the duct manifold.
(4) Automatic sprinkler protection is provided at the junction of each booth exhaust with the manifold, in addition to the protection required by Chapter 9.
(5) The installation is approved by the authority having jurisdiction.
7.7 Materials of Construction.
Exhaust ducts and fasteners shall be constructed of steel.
Exception: Other materials of construction shall be permitted to be used in cases where the conveyed materials are not compatible with steel.
7.8* Support of Exhaust Ducts.
Exhaust ducts shall be supported to prevent collapse under fire conditions.
7.8.1 Duct supports shall be designed to carry the weight of the duct system itself, plus the anticipated weight of any residues. If sprinkler protection is provided inside the duct system, then the duct supports also shall be designed to carry the anticipated weight of any accumulation of sprinkler discharge.
7.8.2 Hangers and supports shall be fastened securely to the building or to the structure to avoid vibration and stress on the duct system.
7.8.3 Hangers and supports shall be designed to allow for expansion and contraction.
7.8.4 Exhaust ducts shall not use building walls, floors, ceilings, or roofs as component parts.
7.9 Exhaust Duct Access Openings.
Exhaust ducts shall be provided with doors, panels, or other means to facilitate inspection, maintenance, cleaning, and access to fire protection devices.
7.10 Exhaust Fans and Drives.
7.10.1 The rotating element of the exhaust fan shall be nonferrous or the fan shall be constructed so that a shift of the impeller or shaft will not permit two ferrous parts of the fan to rub or strike. There shall be ample clearance between the rotating element and fan casing to avoid a fire by friction, necessary allowances being made for ordinary expansion and loading and to prevent contact between moving parts and the duct or fan housing. Fan blades shall be mounted on a shaft that is sufficiently heavy to maintain alignment even when the blades of the fan are heavily loaded. All bearings shall be of the self-lubricating type or shall be lubricated from a point outside the duct and preferably shall be located outside the duct and the booth.\
7.10.2 Electric motors that drive exhaust fans shall not be placed inside any spray area unless they meet the provisions of 6.2.2.
7.10.3 Belts shall not enter any spray area unless the belt and pulley within the spray area are completely enclosed.
7.11* Drying Areas.
Freshly sprayed workpieces shall be dried only in spaces that are ventilated to prevent the concentration of vapors from exceeding 25 percent of the lower flammable limit. (See also Chapter 13.)