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Air Movement and Control

All exhaust systems are comprised of hoods, duct segments, and special fittings leading to an exhaust fan. A complex system is merely an arrangement of several simple exhaust systems connected to a common duct. There are two general classes of duct system designs: tapered systems and plenum systems. The duct in a tapered system gradually gets larger as additional flows are merged together, thus keeping duct velocities nearly constant. If the system transports particulate (dust, mist, or condensable vapors), the tapered system maintains the minimum velocity required to prevent settling. The duct in a plenum system (see Section 5.7) is generally larger than that in a tapered system, and the velocity in it is usually low. Any particulate in the air stream can settle out in the large ducts. Figures 5-4 and 5-5 illustrate design alternatives. Regardless of which system is used, the following procedure will result in a workable system design. I. Select or design each exhaust hood based on the toxicity, physical, and chemical characteristics of the material and the ergonomics of the process and determine its design flow rate, minimum duct velocity, and entry losses (see Chapters 3 and 10). Note that minimum duct velocity is only important for systems transporting particulate, condensing vapors, or mist and to prevent explosive concentrations building up in the duct (see Section 5. I 8 for a discussion on economic velocities for non-particulate systems).