A quick survey of the three mechanical filtration mechan

摘要

Filters are omnipotent in the practice of contamination control. We filter many things―cleanroom air, ultrapure water, process chemicals and high-purity gasses. We install filters at the outlets of our vacuum cleaners and we filter our process effluents. Sometimes, we even put our people in helmets and filter the air they are exhaling. Our processes often require that we filter particles from fluids. While end users need to "filter" airborne molecular contamination (AMC), this mechanism is not truly "filtration" but instead relies on the removal processes of chemisorption or physisorption (adsorption and absorption). But we'll leave the topic of AMC removal for another column. For now, let's define a fluid as anything that "flows"―gases and liquids. We will also deal solely with fiber filters, where the filter media is composed of numerous separate fibers, as opposed to membrane filters. The capability of a filter to remove particles from a fluid is expressed by describing its Efficency, or equivalently, and its Penetration: 1. Efficiency (E)―Fraction (or percentage) of particles captured by a filter, for a given particle size and velocity; 2. Penetration (P)―Fraction (or percentage) of particles that pass through the filter, for a given particle size and velocity.