Sources of Dust

Many industries, such as chemical, printing, metallurgy, fine ceramics, and welding operations, have processes generating dust. Crushing, blast­ing, material transporting and processing, finishing, and other opera-

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Tions result in dust release into the building space. Other particle sources may include construction and renovation, deteriorated insulation, clean­ing, etc. The dust resulting from each of these processes differs in its chemical and physical characteristics. Particle size, shape, density, and release method influence the distribution of dusts inside the space, and the methods of particle control and air cleaning technology used. Dusts in the work environment vary in particle size from 0.1 |xm to 25 |xm (Fig. 7.3). Respirable particle sizes vary from less than 1 jun to 10 |uri. Large particles with a size of tens of microns deposit on surfaces, while smaller particles remain suspended in the air and migrate in the space with air­flows.

Grinding, polishing, and other finishing operations produce particles imparted with some momentum and thus are considered to be dynamic sources. A particle ejected into still air with an initial velocity will travel some distance (stopping distance) before decelerating to rest due to drag forces. The data from Hinds show the difficulty in throwing even fairly large

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Particles an appreciable distance in still air.9 When the receiving hood is de­signed, any particles that are in the inhalable range should be considered im­movable.10

Granular and other bulk materials processing and conveying, bag empty­ing and disposal, and similar operations air into the process enclosure. In­duced air picks up dust. If the system component, such as a bin, is tight, the induced air will reverse its path and carry the entrained dust back through the upstream opening, as shown in Fig. 7.4.10

Resistance and arc welding operations, and plasma and laser cutting produce fumes by expulsion or evaporation of the base material, coating, and electrode wear. Larger particles deposit on the surrounding surfaces, while smaller particles move upward with convective flows. Specific con­taminants associated with different welding and cutting operations are listed in AWS.5

Air induced by flowing material

Induced air picks up dust

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