7 - Design of Exhaust Hoods

Abstract

The initial point of entry of a contaminant into the local exhaust ventilation system is the hood. The exhaust hood, which may consist of only a simple round or rectangular opening or may be specially designed for the process, is designed to provide the necessary velocity of air at the point of contamination to control the contaminant and draw it into the system. Proper design of the exhaust hood is crucial in the operation of a local exhaust ventilation system. If the contaminant is not initially controlled, the local exhaust system does not perform its desired function. There are three types of exhaust hoods that can be installed. In the enclosed hood, the source is enclosed either totally or partially and adequate velocity is supplied to prevent the escape of the contaminant from the enclosure. In the exterior hood, the hood is installed exterior to and away from the source of the contaminant. Adequate velocity is provided to capture the contaminant at its farthest distance from the hood and to draw the contaminant into the hood. This velocity is called the “capture velocity” of the hood. The receiving hood can be defined as the hood that receives the contaminant that is generated at the source. For the receiving hood to function properly, it must provide an adequate rate of flow to remove the contaminant from the hood as it is received to assure that the contaminant does not spill.