Workplace Safety Guides

   Design of Local Exhaust Systems


Now, let's look at a few good practices to follow when designing, building, and operating a local exhaust system. Be sure trained personnel are used, such as ventilation engineers or industrial hygienists. Remember to enclose the source as much as possible. Or, bring the hood as close as possible to the source. The hood must be compatible with the worker and work practices. If it interferes with the work, productivity will be lost. Or the worker may alter the hood, making it less effective as an emission control. This receiving hood over an induction furnace was cut by employees because they could not see inside to check the melt.

Adding a flange to the hood will normally increase its effectiveness by reducing air flow from areas behind the hood where no contaminant exists, and improving velocity distribution in front of the hood, where contaminants are generated. Flanging can reduce air flow requirements by as much as 50 percent, resulting in lower capital and operating costs for the system.

A duct should be of the proper diameter and constructed of appropriate material. For example, when ventilated air contains sulfuric acid, stainless steel duct is often used. Be sure to maintain an adequate transport velocity in the duct or the duct may fill up with dust. An alternative is to provide clean-out openings at intervals throughout the duct work. This, of course, increases maintenance requirements.

Avoid 90 degree Ts in the duct work. Turbulence results from changes in the air velocity and direction causes pressure losses in the system, which must be overcome by the fan. Branches should enter at gradual expansions. At angles of 45 degrees or less to maintain duct velocities while minimizing turbulence. Avoid two branches entering directly opposite each other to reduce turbulence loss. Avoid sharp elbows such as this mitered elbow. The magnitude of pressure losses caused by elbows depends on the radius of the elbow curvature compared to the duct diameter. Where space permits, elbows should have a centered line radius of two to two and a half duct diameters.

Replace duct work when it wears out due to erosion and impact damage from airborne particles. Maintain all duct work in good working order. During design, remember to check with your local air pollution control agency to obtain a permit. The agency can give you guidance for selecting an air cleaner. The major types of particulate air cleaners include fabric filter devices such as bag houses, electric static precipitation, and cyclone separators. Gasses and vapors are often removed from the air stream by scrubbers.

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