Effects of Face Velocity, Flanges, and Mannikin Position on the Effectiveness of a Benchtop Enclosing Hood in the Absence of Cross-Drafts

Although enclosing hoods are often deemed the most effective type for industrial processes, there is little published research concerning simple enclosing hoods similar to those used in industry. This study investigated the effects of face velocity, mannikin position, and flange angle on exposure to a mannikin standing at the face of an industrial benchtop enclosing hood. Sulfur hexafluoride (SF6) was released at low velocity inside a plain enclosing hood located on a tabletop. Plastic tubing transported sampled air from the breathing zone (BZ) and other sample sites to an infrared spectrophotometer. For some tests an anthropometrically scaled, 1.02 m (40 in.) mannikin stood in front of the hood opening with its arms straight; for others the mannikin was bent forward with its hands on each side of the source in a “working” position. Flange conditions were either none or a flange with a 30°, 45°, 60°, or 90° angle to the plane of the hood. Face velocities were varied from 0.30 to 0.86 m/sec (60 to 170 ft/min). Cross-draft velocities were random in direction and less than 0.05 m/sec (10 ft/min). The results demonstrated the great importance of face velocity. BZ exposure was greatest at the lowest face velocity (0.3 m/sec) and decreased dramatically as velocity increased. Other variables were of lesser importance. BZ exposure was higher in the “working” than in the “straight” mannikin posture. Flange taper angle was statistically significant but confounded by the gap between the mannikin torso and the edge of the flange. The “no-flange” conditions, however, produced the lowest exposures In every comparison with other flanges.