Airflow and filtration velocity

Airflow is defined at the source, capturing the dust at the point where it is emitted. There are two approaches depending on the case: capturing at the emission point by ensuring a capture velocity high enough to draw the particle into the hood, or sizing by air changes of the enclosure when the emission is diffuse. The more suction points, the larger the area to ventilate or the lighter the particle to draw, the higher the required airflow. This airflow is the starting point: everything else is sized from it.

Filtration velocity is the relationship between the airflow and the total filter media area. The result is a velocity —how much air passes through each square meter of filter— and it is what determines the filter area (and therefore the number of filters) the unit needs to move that airflow. It is the same criterion for cartridge and bag collectors; in bag filters it is traditionally called “air-to-cloth ratio”, a name that comes from the fabric medium, but as a concept it applies equally to the cartridge, where it is more precisely called air-to-media ratio.

There is no single filtration velocity. The finer, more concentrated or stickier the dust, the lower the velocity must be —and therefore the more filter area is needed for the same airflow. In addition, each technology works in its own range: the cartridge collector operates at a considerably lower filtration velocity than the bag filter, so for the same airflow the area and amount of media are not sized the same in one and the other. Defining the technology (cartridge or bag) is, then, a step prior to sizing.

A filtration velocity that is too high is the most common and most expensive mistake: the filters clog sooner, the pressure drop rises, the effective airflow falls, media life shortens and the fan consumes more energy to move the same air. At the other extreme, a velocity that is too low means an oversized unit: more filter area, higher initial cost and more space occupied than necessary. Sizing well is finding the point that balances the initial cost of the equipment with the operating cost over its life.

Sizing a dust collection system is, at bottom, chaining two decisions: setting the airflow needed at the source and choosing the filtration velocity appropriate to the dust and the technology. With those two defined, the filter area and the final size of the unit follow. That is why it is best to resolve first which type of collector applies, and only then size it.