reduce paint overspray, help control workers’ exposure, and protect the paint finish。 Workers must use appropriate respiratory protection irrespective of the airflow rate。 On 9 February 2000, OSHA issued an interpretation of 29 CFR 1910。94 and 1910。107, Spray Finishing Using Flammable and Combustible Materials for determining the airflow rate required for a walk-in paint booth。 In accordance with OSHA’s interpretation letter, following NFPA 33 will provide protection from fire and explosion。  The guidance listed in Subpart Z of 29 CFR 1910。94 provides protection for workers。 See Appendix B for OSHA’s interpretation。

a。 Use the Painting Operations section in the ACGIH IV manual to determine the design volumetric airflow rate。 Ensure that this design volumetric airflow rate will keep the concentration of vapors and mists in the exhaust stream of the ventilation system below the 25 percent of the LEL。 See 1910。94(c)(6)(ii) for an example of airflow rate requirement calculations。

b。 Do not re-circulate exhaust air while painting。

9-3。1。1 Exhaust Configurations。  The two main ventilation system configurations are downdraft and crossdraft。 In a downdraft booth, air enters through filters in the ceiling of the booth and leaves through filters that cover trenches under a metal grate floor。 In a crossdraft booth, air enters through filters in the front of the booth and leaves through filters in the back of the booth。 Both configurations are commercially available。

9-3。1。1。1 Downdraft Paint Spray Booths。 Downdraft booth configuration provides a cleaner paint job than the crossdraft booth configuration and controls exposures to workers better than crossdraft booth configuration。 The downdraft configuration should be the primary choice in designing or selecting of paint spray booths。 Figure 9-1 is an example of a downdraft configuration。

9-3。1。1。2 Crossdraft Paint Spray Booths。  The crossdraft paint spray booth usually requires less total volumetric airflow rate than the downdraft spray paint booth because the vertical cross-sectional area of the booth is often smaller than the booth footprint area。 Figures 9-2 and 9-3 are examples of drive-through crossdraft paint spray booth configurations。

Figure 9-1。  Walk-in downdraft paint booth。

NOTES:

1。 Size each plenum take-off for no more than 2。44 m (8 ft) of plenum width (W)。

2。 Perforated plate with 9。53-mm (3/8-in) holes。 Size open area for an airflow velocity of 5。08 m/s (1,000 fpm) through holes。

3。 Size exhaust plenum for a maximum plenum velocity of 5。08 m/s (1,000 fpm)。 Size replacement air plenum for a maximum plenum velocity of 2。54 m/s (500 fpm)。

4。 Use manufacturer’s recommendations for sizing perforated ductwork。

5。 Removable filters and floor grating。

Figure 9-2。 Drive-through cross draft paint booth with mechanical replacement air。

NOTES:

1。 Size each plenum take-off for no more than 2。44 m (8 ft) of plenum width。 Size the exhaust plenum for a maximum plenum velocity of 5。08 m/s (1,000 fpm)。 Size replacement air plenum for a maximum plenum velocity of 2。54 m/s (500 fpm)。

2。