Efficient but not Necessarily Easier
Many industrial facilities choose to recirculate air from a dust collector inside of their facility rather than exhaust it outside the building. However, doing so presents some engineering concerns that must be addressed, including attention to NFPA 654. Let’s consider 3 reasons why you might consider recirculating the air in your facility despite the potential complications.
Climate Control Costs
Costs related to heating and cooling can make up a large portion of operating expenses for a facility. With so much money spent heating large, open areas inside an industrial facility, it makes sense to consider baghouse operations which can both evacuate expensive heat and return it.
By one estimate, recirculating the air from a 10,000 CFM system inside a plant to 70F from an outside temperature of 10F can save an estimated $1,600 per month! Additionally, plants with high ceilings can often improve heating efficiency by collecting air at the top of the ceiling (hot air) and returning it to the bottom at ground level.
Avoid Regulatory Burdens Posed By Emissions Regulations
Emissions permitting from the EPA or other state air regulatory boards can be a daunting task. The permitting process may involve applications, testing and regulatory red tape. Some have been able to avoid this by not emitting anything outside of the facility. This shifts the dust emissions control issue from a pollution control issue under the EPA to a health and safety issue under OSHA. However, this does not mean plants can become complacent by exhausting their dust collectors back inside. While OSHA does not require emissions testing or permits per se, they do require that minimum indoor air quality standards be met. This may include conducting tests with personnel to establish an 8 hour time weighted average to measure exposure to contaminants. It should be noted that in some states the local air quality board still requires some form of permitting for indoor recirculation for dust collectors.
Avoid Conflicts with Neighbors Over Emissions
Finally, many issues come to the fore when neighbors complain over a plants emissions. Whether real or exaggerated, angry neighbors can cause host of problems for plants, from litigation, regulatory burdens or publicity problems. By containing all emissions within a facility plants can minimize many of these issues.
Caution Needed When Returning Air From Baghouses
While the above benefits give facilities real incentive to recirculate air from their collection systems, there are some factors to consider before making that decision:
- Combustible dust regulations, such as the NFPA 654, place a high burden on dust collectors handling combustible materials. Systems handling extremely combustible materials (e.g. aluminum dust) may have no option but to locate their collectors outside and exhaust to the atmosphere for safety reasons. When recirculating, NFPA 654 puts the burden of proof on the plant, requiring some method of determining the proper operation of the dust collectors and leak detection. Recirculating air in these applications requires a careful study of hazards within each particular system and may require additional prevention and protection strategies as well as extensive indoor dust monitoring to implement effectively.
- OSHA indoor air quality standards have become stricter in recent years, in some cases more so than environmental emissions standards for PM 2.5. For general nuisance dust OSHA specifies 5 mg of nonspecific or nuisance dust (< 10 microns in size) per cubic meter of air. Hazardous materials such as metals, chemicals or even newly reclassified silica dust may have a limit as low as 0.05 mg per cubic meter, more than 100 times stricter than general nuisance dusts. All hazardous materials must contain monitoring technology (such as triboelectric bag leak detectors) and HEPA after filters.
Recirculation of air from an industrial dust collector can provide operational benefits for plants. However, plant engineers should carefully weigh all the benefits against the high requirements for maintaining health and safety standards in such a setup, including compliance with additional sets of regulations including OSHA and NFPA 654.