Fume Hood Performance: Face Velocity Variability Inconsistent Air Volume Systems

Abstract

A 3-year survey of 366 bench-type fume hoods in working laboratories in conventional, constant air volume settings showed that face velocities varied greatly from unit to unit and over time. Fume hoods with bypasses performed better than those without; however, even newly fabricated bypass hoods exhibited large variations (i.e., 60 to 200 ft/min, across the face). These variations were due to several factors; however, face velocities at 100 ± 10 ft/min at working sash heights in the range of 20 to 40 cm (8 to 16 inches) were attainable. The use of “smoke” showed poor containment, especially at face velocities below 85 ft/min (0.425 m/s) or above 130 ft/min (0.65 m/s) and when the hoods were obstructed by large items placed on the work surface. Auxiliary/supplemental air created unstable face velocities and poor smoke patterns. The analysis of 3 years of fume hood monitoring showed clearly the need for and importance of a maintenance program where the fume hood lower slots are cleaned and fans, ducts, dampers, and hoods are checked periodically.