Recirculating Air From Dust Collectors (continued)

Fire safety and explosions

If the dust being captured is explosive or flammable, special safety concerns must be addressed. While highly explosive dusts such as aluminum powder should be exhausted outside, many other flammable dusts can be handled with a recirculating dust collector, as long as adequate safety precautions are taken.

Explosions are a big concern. The explosive power of a dust is denoted as Kst, the rate of pressure rise (see Table 2). Both NFPA and Factory Mutual use this value in formulas to calculate the amount of explosion vent area required for a dust collector. Collectors requiring explosion vents should be located outside and vent away from buildings and populated locations.

If the collector must be located inside, the plant engineer can duct an explosion outside through a very short duct (9 ft or less). This design adds back-pressure and usually requires reinforcing the collector to handle the increase in pressure. If it is not feasible to duct to the outside, the collector should be outfitted with an explosion suppression system. An indoor suppression system may cost more than the dust collector itself.

Fire is another safety concern with a dust collector. Welding, metal grinding, and similar operations can cause sparks to enter the collector inlet and start a fire. To prevent this, a spark suppression and detection system should be installed in the inlet ducting to sense and extinguish sparks or flames. Several different suppression systems are available utilizing water, chemicals, or inert gas.

A diversion system that incorporates spark detection and diversion valves called "abort gates" is also available. When a spark is detected, the abort gate diverts the recirculated air stream to the outside, arresting the flame front before it reaches the dust collector. Companies that specialize in this equipment can help design a spark detection and extinguishing system.

Recirculation

After the air has been adequately cleaned and safety issues addressed, how should the air be recirculated downstream of the dust collector? Ideally, to maintain balance and optimize energy conservation, the return air duct should allow distribution to the same areas from where the air was originally exhausted. A common mistake is to draw the air out of one room and recirculate it elsewhere, creating areas of negative and positive pressure.

A well-designed recirculating system not only saves energy; it can actually enhance worker comfort. For example, a system that serves multiple welding stations might consist of one long duct with adjustable diffusers at each station. This design allows personnel to use the diffusers like individual fans, directing the air toward or away from their workstations as desired. The Industrial Ventilation Manual (see "More info" box) expands on this area in great detail, and its use is recommended.

There are two possible return air configurations. A general ventilation system with zone return, used in cold climates, will recover heat from the ceiling and return it to the work area (Fig. 2). This configuration is also useful when the process does not allow the use of source capture hoods. A major disadvantage is that a general ventilation dust collection system requires larger airflows, fans, and filters, resulting in higher equipment and operational costs.

Figure 2 - General ventilation arrangement with zone returns captures ambient air for filtration.

Figure 3 - Source capture ventilation with zone returns uses hoods for capturing dust where it is generated.

In a source capture system with zone return, hoods are added over each workstation (Fig. 3). This is a more efficient air distribution system with lower airflow, fan, and filter requirements. However, it can only be used with stationary processes.

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