Figure 6-1。  Downdraft blast enclosure。

6-3。3。2 Crossdraft。 Consider the work locations of operators when positioning the replacement and exhaust air plenums。 Do not allow any operator to blast upstream of coworkers。  Use a perforated plate with 9。53 mm (3/8-in) diameter holes; see Figure

6-2, to uniformly distribute airflow over the entire cross-section of the enclosure。

Figure 6-2。  Crossdraft blast enclosure。

NOTES:

1。 For mechanically supplied replacement air, use maximum plenum take-off width of 2。44 m (8 ft)。  Plenum serves as material door。

2。 Perforated plate with 9。53 mm (3/8-in) holes。 Size open area for an airflow velocity of 10。16 m/s (2000 fpm) through holes。

3。 Size the exhaust plenum for a maximum plenum velocity of 5。08 m/s (1000 fpm)。 Size any supply plenum for a maximum plenum velocity of 2。54 m/s (500 fpm)。

4。 Lift up flap to remove material from behind plenum。

5。 Floor grating。

6。 Observation window。

7。 Personnel door。

8。 Perforated plate, from floor to ceiling and wall-to- wall, with 9。53 mm (3/8-in) holes。 Size open area for an airflow velocity of 5。080 m/s (1000 fpm) through holes。

9。 Hinged plenum equipment doors。

6-3。4 Access Doors and Observation Windows。  Provide an observation window and an access door in accordance with 29 CFR 1910。94(a)(3)(i)(d) and (e) and

ANSI Z9。4 sections 5。3 and 5。4。 Position the observation window in the blast room walls and door as necessary so workers inside the room can be seen from outside the room at all times。 Use several doors and windows in large rooms。 Provide emergency exits on opposing walls。 Make personnel and equipment doors operable from both inside and outside of the room。

6-3。5 Air Cleaning Devices。 See paragraph 2-4。4。 Design in accordance  with 29 CFR 1910。94(a)(4)(iii) and ANSI Z9。4, section 6。3。 Consider using a pulse-jet, pleated paper cartridge type dust collector and the following。

a。 Replaceable explosion vents on the collector hoppers in accordance with NFPA-68。

b。 Platforms leading to all elevated access hatches。

c。 Fan located on the clean side of the collector。

d。 Place dust collectors outside of the building for all blasting applications。 NFPA 65 specifically requires that the air pollution equipment be located outside when blasting on aluminum or aluminum alloys。

6-3。6 Recirculation。  Do not recirculate exhaust air when operations generate toxic materials。  If exhaust air recirculation is permitted, design the system in accordance with the ACGIH IV Manual, ANSI Z9。4 (section 6。3) and ANSI Z9。7, 29 CFR 1910。1025 (lead), and 29 CFR 1910。1027 (cadmium)。 The outdoor air volumetric airflow rate must be sufficient to keep the contaminant below 25 percent of the MEC。

6-3。7 Media Reclamation。 Design in accordance with 29 CFR 1910。94(a)(4)(ii) and ANSI Z9。4, section 6。2。 Do not integrate the exhaust ventilation system with the media recovery system。

a。 Protect the media recovery system and ductwork from moisture and rainwater intrusion to keep the media from caking and plugging up the system。