Page 165 - Root Cause Failure Analysis

DUST COLLECTORS

The basic operations performed by dust-collection devices are (1) separating particles
from the gas stream by deposition on a collection surface, (2) retaining the deposited
particles on the surface until removal, and (3) removing the deposit from the surface
for recovery or disposal.

The separation step requires (1) application of a force that produces a differential
motion of the particles relative to the gas and (2) sufficient gas-retention time for the
particles to migrate to the collecting surface. Most dust-collections systems are con-
stituted of a pneumatic-conveying system and some device that separates suspended
particulate matter from the conveyed airstream. The more common systems use either
filter media (e.g., fabric bags) or cyclonic separators to separate the particulate matter
from air.

BAGHOUSES
Fabric-filter systems, commonly called bug-@fer or bughouse systems, are dust-col-
lection systems in which dust-laden air is passed through a bag-type filter. The bag
collects the dust in layers on its surface and the dust layer itself effectively becomes
the filter medium. Because the bag’s pores usually are much larger than those of the
dust-particle layer that forms, the initial efficiency is very low. However, efficiency
improves once an adequate dust layer forms. Therefore, the potential for dust penetra-
tion of the filter media is extremely low except during the initial period after startup,
bag change, or during the fabric-cleaning, or blow-down, cycle.

The principal mechanisms of disposition in dust collectors are (1) gravitational depo-
sition, (2) flow-line interception, (3) inertial deposition, (4) diffusional deposition,
and (5) electrostatic deposition. During the initial operating period, particle deposi-
tion takes place mainly by inertial and flow-line interception, diffusion, and gravity.

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