ABSTRACT
The "canopy hood" in its common usage includes any exhausted opening that is located above a source of contamination, regardless of its shape or height above the source it is intended to service. It may take the form of a large positive-flow roof ventilator directly above a large-scale heat process; a suction grille in a ceiling above a hot, fuming process; or a low-hanging pyramid-shaped hood above a tank. A low-hanging canopy hood above a cold process is essentially the same as a poorly constructed booth or enclosure, having four open faces instead of a single-plane opening. Canopy hood installations frequently have a common failing. One category of canopy hood includes those that are mounted above horizontal hot surfaces with vertical distances not much greater than three feet. Deflection may occur due to the viscous drag of the exhaust air streaming toward the hood or it may result from turbulent mixing of the hood-induced stream with the hot air.
