Abstract:
Local exhaust ventilation (LEV) in the form of enclosing and capturing hoods is meant to protect workers from unhealthy exposures and to capture and remove airborne hazards before those contaminants can enter the worker's breathing zone or the general working environment. Enclosing hoods typically enclose the source on all but one side in order to reduce the effect of competing air current on the capturing efficiency of the hood. Capturing hoods must capture the contaminant, which is located exterior to the hood, and overcome competing air currents in order to draw the contaminant into the hood and out of the working environment.
The hypothesis tested in this research was that enclosing hoods provide better protectoin to the worker than capturing hoods, and that this advantage increased with increasing crossdraft velocity.
Air was drawn by a fan through the enclosing or capturing hood at constant face or capturing velocities while SF6 was released through a diffuser at a constant known rate. Air was drawn across the face of the enclosing and capturing hoods at a canstant velocity by the wind tunnel fan. An anthropomorphically scaled poseable mannikin was posed in front of each hood in a working posture. The concentration of SF6 was measured at the mannikin's nose and at various site near the hoods.
In all cases examined there was no exposure at the manniken's nose when the capturing hood was challenged. There was exposure at the mannikin's nose when the enclosing hood was challenged. This exposure decreased with increasing crossdraft and increasing face velocity.