Bag Dust Collectors (Overview)
BAG HOUSE – As the name implies, “bag houses” employ fabric materials sewn into a “bag” format. Most of these bags are in the form of hollow tubes, although some bags are in relatively flat ‘envelope” formats. In both cases, the fabric is held in its designed form by internal welded wire cages or, in a few instances, by metal coiled into spring-like shapes. The bags are sealed in all but small areas (the ends of tubes, or the tops of “envelopes”), through which process air is drawn by a blower. Air drawn from the bag interiors causes dust-laden air to flow into the fabric, with dust arrested on their surfaces. The accumulation of dust will gradually impede airflow until effective blockage occurs.
Bag House Filter Cleaning – At or prior to airflow blockage, dust is removed from the bags by various means. The most common methods are:
Manual or Mechanical Shaking – The dust collector is shut down completely, either via timer or manual control. Some bag houses require complete shutdown and bags are then manually or mechanically shaken. When the dust (now usually agglomerated into a heavier “cake”) has settled into a storage container, the collector is returned to service.
Reverse Pulse – Here the bags are generally cleaned during dust collector operation. A high-pressure compressed air “pulse” is released into a portion of the bags through the small openings. The combination of the generation of positive-pressure inside of the bags and the shock of the high-pressure pulse flexes the bag surfaces so that agglomerated dust (“cake”) separates from the surface to fall into the storage container. By “pulsing” only a portion the total bag set each time, airflow through the remaining filters and the removal of dust from it can be maintained. This procedure is known as “on-line cleaning”. Such a process is almost always directed and controlled via an automated electronic control system.
Reverse Airflow – This style of cleaning system, like the Manual or Mechanical Shakers, requires dust collector shutdown. It is most often employed where fibrous material, which may tend to resist both shaking and pulsing, is the primary contaminant. Airflow through the bags is simply reversed so that the agglomerated materials, forming more of a “mat” than a “cake, are literally blown from the bag surface to fall into the storage container.