Recirculating filtered air from a dust collection system back into the facility is a fairly common process. This can conserve heat generated from within the plant, balance out air pressure and airflow as well as provide other functions.
However, plants need to exercise great caution in evaluating whether this approach is appropriate for their plant. Recirculating air may expose workers to whatever contaminates remain in the air stream after the pollution control devices. For good reason, local, state and federal workplace safety regulations do not allow recirculation of air for many applications involving hazardous materials, such as lead, heavy metals, organic poisons, radioactive compounds, etc.
For facilities where recirculated air can be used, some wonder exactly how a bag leak detector might fit in - and what type of monitoring is indicated to prevent health issues or other problems such as damage to capital equipment.
Ensuring Air Quality Crucial For Recirculating Air Into Facility
Only clean air can be recirculated within a facility that has workers present. If any contaminates remain these could create an unsafe work environment for workers. Exposure to these compounds can have severe health effects. Additionally, if the collected materials contain combustible dust(s) then the dangers of reintroducing dust-laden air into the facility where it can settle and form accumulations pose a real danger of feeding a combustible dust fire or explosion.
Only the use of a fabric filter dust collector with high efficiency filters can achieve the required level of collection efficiency. However, if any of the filters sustain rips, tears, puncture holes or if technicians misalign the bags during install the required performance may be compromised. Even just one misaligned bag and cage or one small hole can result in emissions rising above permissible levels for recirculation.
The standard NFPA 654 section 188.8.131.52.3 paragraph (6) states: "The system includes a method for detecting air-material separator malfunctions that would reduce collection efficiency and allow increases in the amount of combustible particulate solids returned to the building." What this is saying is that even though you may use a fabric filter dust collector, you need to provide a method of detecting failures - in other words something like a triboelectric detector for bag leak detection to protect the facility from the possible dust excursion.
Additionally, most regulatory bodies require plants to pass the processed air through an further level of filtration using a HEPA rated filter before reintroducing it back into the plant. These HEPA filters are not designed for continuous loading and cleaning like dust collector filters. Keeping them free from large dust loads is crucial for them to operate effectively. If the main dust collector suffers a leak this heavy dust load can then pass to the HEPA after filter causing it to plug.
Operators can use triboelectric detectors to monitor collector performance and keep close watch on any potential leaks that may occur within the system. This enables operations and maintenance personnel to catch potential problems quickly and take immediate action to correct them before they become serious. In systems using a HEPA after filters, without tribo detectors plant personnel likely will only notice leaks after the HEPA after filter plugs and requires a shutdown to replace it.
Recirculating air from a dust collection system can provide valuable benefits where possible but carries an elevated risk. Using a triboelectric detection system can help you make it a success and mitigate the risks of accidental exposure and damage to your facility and its equipment.