Filtration Mechanism of Fine Particle
There are usually four kinds of methods available to separate solids or liquid particles from an aerosol:
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Filtration Mechanism of Fine Particle
There are usually four kinds of methods available to separate solids or liquid particles from an aerosol: 1. Mechanical type – gravity dust collector, impinger, and cyclone 2. Electrostatic force – single-stage electrostatic precipitator, double-stage electrostatic precipitator, etc. 3. Washing – spray scrubber, water film precipitator, Venturi dust collector, etc. 4. Filtration – filling-in filter, bag filter, etc. In the field of air cleaning, the particle concentration is typically very low compared with industrial dust separation applications, and the particle size is small. Hence the reliability of the performance of filters must be assured. Therefore, filtration and separation techniques are often used to remove particles in the airstream and are the focus of this chapter.
3.1
Filtration and Separation
Air filters remove particulate matter via several mechanisms including interception, impaction, and diffusion. According to the position of the capture particles, particulate air filters may be divided into two types: (1) surface filter and (2) depth filter. Surface filters have many forms including metal wire mesh and perforated plate, where particles are captured on the surface. Chemical porous membrane is made from fibrous ester (nitrocellulose or cellulose acetate), which looks like a piece of white paper, and its characteristic follows that of a surface filter. The thickness of this kind of membrane filtration medium is approximately 50 μm. Circular pores with diameter 0.1–10 μm are uniformly distributed on it; the pore size is controlled during the membrane manufacture process. The average number of pores is about 107–108 #/cm2, and the porosity can be as high as 70–80 %. For theoretical purposes the pores are often treated as capillaries. Particles larger than the pore diameter are captured on the surface and the filtration efficiency could be 100 %. It is believed Z. Xu, Fundamentals of Air Cleaning Technology and Its Application in Cleanrooms, DOI 10.1007/978-3-642-39374-7_3, © Springer-Verlag Berlin Heidelberg 2014
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3 Filtration Mechanism of Fine Particle
Fig. 3.1 Surface air filters (a) Single ribbon model (plate); (b) Isolated cylinder model (metal wire mesh); (c) Small pore model (perforated plate)
that the minimum size of particles captured by the membrane could be 1/10 to 1/15 of average pore diameter. Figure 3.1 shows the structure of a surface air filter and the schematic of particle capture. Depth filters can be divided into two with high and low solid fractions (also called low porosity and high porosity). Particles are captured on the surface or inside the medium layer. The solid fraction α can be expressed as: α¼
filter medium densityðsuch as fibrous layerÞ filter medium material densityðsuch as fiberÞ
Depth filters with a high solid fraction (α > 0.2) have various forms, such as granular filling layer (gravel layer, activated carbon layer, etc.), porous filter media and thick filter paper. Their structure and a schematic of the parti
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