Issue link: https://www.e-digitaleditions.com/i/1252179
June 2020 / 33 capture the remaining particles. When a lter begins operating, particles build up on the outside of each lter element, which becomes the "dirty" side of the lter. As particles enter the lter unit, they form a cake on the outer lter surface, which is why this layer is appropriately named lter cake. This lter cake catches even more particles, growing thicker over time, and provides some of the most important ltration. A vari- ety of small and large particles actually helps achieve a proper and porous lter cake. And while combining separator devices can be bene cial to some processes, placing a cyclone in front of a dust collector is generally not recommended because a combination of small and large particles in the airstream improves a dust collector's performance. Placing a cyclone in front of a dust collector would result in the cyclone removing the larger particles, leaving only ne particles for the dust collector. The result of that is undesirable because a lter cake made of only ne particles is denser and has more resistance than a lter cake made with a variety of particle sizes. For the particles not caught in the outer lter cake, another level of ltra- tion occurs in the lter fabric, in which the smallest particles follow the tortuous path of air through the fabric but are caught when they touch the lter's bers. Thus, particle size and particle size distribution are the most important parameters to be tested for these separation mechanisms. Another important consideration is called can velocity. Below the lter section, the air and particles ow upward at a relatively slow velocity. Once the air and remaining solids reach the bottom of the bag or cartridge lter, all of the air is squeezed into the much smaller interstitial area between the lters. This critical location is the point at which the highest upward veloc- ity, also called can velocity, occurs. Above this point, velocity diminishes because the air exits the dirty por- tion of the lter unit (the lter element's exterior) and starts owing through the clean portion. Material is supposed to be ltered on the lter ele- ment's exterior, then fall off and drop to the bottom when the lters are pulsed clean. However, if the can velocity at the lter element's bottom is too high, the upward air velocity will keep the material from drop- ping. Particles will then quickly re-entrain on the lters instead of falling to the bottom, and the lters may quickly blind over or become clogged with too much material. Blinding occurs when the spaces between the lter's bers get clogged with bits of material. This builds up over time and eventually clogs the fabric completely so that solids and air can't pass through. Accordingly, testing your material's terminal velocity is A dust collector unit relies on several mechanisms to separate particles from the airstream. Most of these units are designed so that the air-material stream enters the unit below the lters. This allows gravity to do much of the work, as a majority of particles will settle out of the air and won't reach the lters. Termi- nal velocity testing, as described earlier, is the most important consideration for this method of separation. In addition to gravity, a round lter-receiver with tangential inlets can separate the material from the air using cyclonic action in which particle friction against the sidewall helps slow the particles so their speed falls below terminal velocity and they aren't carried up into the lter with the air. After separation via gravity, all of the air and some of the particles move up into the lter section where the dust collector uses several more mechanisms to FIGURE 6 Bin-vent lter can be any shape FIGURE 7 Filter-receiver needs to have a cylindrical shape Clean air out Air-material in