As shown in Figure 5.16.11, in outside collectors, dusty gas flows radially inward through cylindrical bags held open by a metal frame inside them. The bags are typically 3 m tall and 200 mm in diameter. Dust collects on the outside bag surfaces. Clean gas passes out of the top of each bag to a plenum.
Cleaning outside collectors usually involves injecting a pulse of compressed air at the outlet of each bag. This pulse snaps the bag open and drives the collected dust away from the bag surface into the hopper. Because pulse-jet cleaning takes a fraction of a second, it can be done online without interrupting the gas flow to a compartment.
Pulse-jet filters generally use thick felt fabrics to reduce dust penetration even when the dust cake is not thick. Filtration velocities through a pulse-jet filter are several times higher than through a reverse gas filter; therefore, pulse-jet filters can be smaller and less expensive. Operating at high filtration velocities can lead to an excessive pressure drop, dust penetration, and fabric wear. These filters are most commonly used to control industrial dust.
Pulse-jet cleaning can be ineffective when the dust is fine. The particles are driven beneath the fabric surface and are not easily removed. As fine dust accumulates below the surface, the pressure drop across the filter gradually increases. If the pressure drop becomes too high, the blinded bags must be replaced. With online cleaning of pulse-jet filters, as little as 1% of the dust on a bag can fall to the hopper after each cleaning pulse (Leith, First, and Feldman 1977).
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