A: John A. Constance, The Engineers Collaborative, says:
Sometimes, dust collection systems don’t work well. I’ve encountered situations where systems don’t reduce the airborne dust that personnel are exposed to and systems that are constant maintenance problems. Even if a dust collection system is properly designed and engineered, problems can still occur because of management decisions, maintenance issues, or technical problems. I’ve experienced similar dust collection system issues in the field that can be corrected to reduce production downtime, cut operating costs, and, hopefully, get the system operating effectively.
Canopy hood for dust control fails again. A benchtop hand-milling operation was exposing the technician to excessive dust. In-house design produced a canopy hood with a 4-inch duct connection to the existing dust collection system, installed above the technician’s head. I was called in to find out why the canopy hood wasn’t doing its job. The in-house designer wanted to increase the exhaust airflow to the canopy hood for better dust capture. I explained that all increased exhaust airflow was likely to do would be to pull more dust up past the technician’s breathing zone, increasing the technician’s exposure to the dust. I asked maintenance personnel to remove the 4-inch flexible duct from the canopy hood and place it close to the milling feed and discharge points. Once the duct was repositioned, you could see fugitive dust get pulled right into the exhaust duct. After that, a simple local exhaust ventilation hood connecting to the 4-inch exhaust duct was fabricated for permanent installation at the mill.
Improper filter cleaning makes things worse. While designing a central dust collection system in another plant, I noticed a technician servicing a self-contained bag-dump station. The access door protecting the cartridge filters was removed and the filters were being cleaned in place manually with a compressed air hose. The technician explained that the filters plugged frequently and that this was the best way to clean them, although it didn’t seem to help much. In fact, although some dust was being removed from the filter, the compressed air was also driving dust into the filter media, which increased plugging of the filter media. I noticed that the pressure to the automatic filter cleaning system built into the bag-dump station was at 40 psig. The solution was to ask maintenance to increase the pressure to 90 psig. That change, combined with new filter cartridges, solved the plugging problem. Manual filter cleaning was no longer necessary and the filters lasted longer.
John A. Constance is a consulting engineer for the The Engineers Collaborative and a member of PBE’s Editorial Advisory Board.