Recirculating Air From Dust Collectors
Whether dust collectors are used in a plant to control indoor air quality (lAQ), keep equipment clean, and/or recover high-value process dusts, many plants are considering recirculating the air back into the plant downstream of collectors instead of exhausting it outdoors. When using recirculating dust collection systems, special safety and performance concerns must be addressed.
Benefits
There are at least three positive reasons to recirculate indoor air.
Less regulatory paperwork
When contaminated air is exhausted outdoors, the EPA must be satisfied that the exhausted air is in compliance with current standards, a process that involves time-consuming permit applications, testing, and regulatory paperwork. By containing the air totally inside a building, the plant engineer deals with OSHA instead of the EPA - a less daunting prospect, even though OSHA indoor air quality standards are becoming increasingly stringent.
OSHA does not require permits or collector testing, but they do require a plant to meet certain indoor air quality standards, no matter how compliance is acheived.
OSHA may require the use of a personal monitor on an individual in the workplace and perform an 8-hr. time-weighted average (TWA) test to make sure contaminants are below allowable levels. Unlike the EPA, they will not test the dust collector or require that a permit be filed for the system.
In some states, the local EPA may still want to permit an indoor collector. Plant engineers should check with local agencies to find out their position on recirculating dust collectors.
Substantial energy savings
When a dust collector is designed to recirculate heated or cooled air back through the plant, the cost to replace that conditioned air is eliminated - an expense that can be substantial. Consider the example of a 10,000-cfm dust collector where the outside temperature is 10° F. Use of a recirculating dust collection system could save an estimated $1600 per month during the winter - the approximate cost to heat an equivalent amount of replacement air to 70° F, based on an energy cost of $0.60 per ccf (100 cu ft of natural gas), an 8-hr day, and a 5-day work week.
Dust collecting systems for welding shops with high ceilings can often improve the efficiency of a heating system by taking hot air off the ceiling and delivering it at ground level.
Being a good neighbor
An indoor dust collection system is not subject to unneeded scrutiny by commercial or residential neighbors. Outdoor systems and exhaust stacks can be a frequent source of community concern and potential complaints. These issues are eliminated with a recirculating system.
Safety Issues
Some types of contaminants must be exhausted outdoors. This includes combustion gases and unusual gas stream constituents that cannot be adequately handled by a particulate removal system. When in doubt about a given application, ask a supplier or consultant with dust collection engineering expertise three key questions:
- How can OSHA emission requirements be achieved?
- How can fire safety/explosion concerns be met?
- How should the air be recirculated for energy efficiency and worker comfort?