Fabric filters diffusion collection

Each is discussed in Sec. 17 of this handbook under Gas-Solids Separations. The effectiveness of conventional air-pollution-control equipment for particulate removal is compared in Fig. 22-25. These fractional efficiency curves indicate that the equipment is least efficient in removing particulates in the 0.1- to 1.0-pm range. For wet scrubbers and fabric filters, the very small particulates (0.1 pm) can be efficiently removed by brownian diffusion. The smaller the particulates, the more intense their brownian motion and the easier their collection by diffusion forces. Larger particulates (>1 pm) are collected principally by impaction, and removal efficiency increases with particulate size. The minimum in the fractional efficiency curve for scrubbers and filters occurs in the transition range between removal by brownian diffusion and removal by impaction. 

Fig. 9.3. Acid mist removal candle filter being installed atop a stainless steel H2SO4 making tower. It is one of many. Exiting gas passes inward through the candle fabric and out the top of the candle - then out of the tower. The acid mist is caught in the candle fabric by impact, diffusion and Brownian forces (Brink, 2005 Friedman and Friedman, 2004 Lee and Byszewski, 2005 Ziebold and Azwell, 2005). The large total area of the candles gives a low gas velocity through the fabric, which allows 99+% capture of the mist. The captured mist trickles down the fabric and drips back into the tower or into collection pipes (Outokumpu 2005).

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