Page 166 - Root Cause Failure Analysis

154 Root Cause Failure Analysis

Once the dust layer has been fully established, sieving probably is the dominant depo-
sition mechanism.

Configuration

A baghouse system consists of the following: a pneumatic-conveyor system, filter
media, a back-flush cleaning system, and a fan or blower to provide airflow.

Pneumatic Conveyor
The primary mechanism for conveying dust-laden air to a central collection point is a
system of pipes or ductwork that functions as a pneumatic conveyor. This system
gathers dust-laden air from various sources within the plant and conveys it to the dust-
collection system. See the beginning of Chapter 9 for more information on pneu-
matic-conveyor systems.

Dust-Collection System
The design and configuration of the dust-collection system varies with the vendor and
the specific application. Generally, a system consists of either a single large hopper-
like vessel or a series of hoppers with a fan or blower affixed to the discharge mani-
fold. Inside the vessel is an inlet manifold that directs the incoming air or gas to the
dirty side of the filter media or bag. A plenum, or divider plate, separates the dirty and
clean sides of the vessel.

Filter media, usually long cylindrical tubes or bags, are attached to the plenum.
Depending on the design, the dust-laden air or gas may flow into the cylindrical filter
bag and exit to the clean side or it may flow through the bag from its outside and exit
through the tube’s opening. Figure 12-1 illustrates a typical baghouse configuration.

Fabric-filter designs fall into three types, depending on the method of cleaning used:
shaker cleaned, reverse-flow cleaned, and reverse-pulse cleaned.

Shaker-Cleaned Filter The open lower ends of shaker-cleaned filter bags are fas-
tened over openings in the tube sheet that separates the lower, dirty-gas inlet chamber
from the upper, clean-gas chamber. The bags are suspended from supports that are
connected to a shaking device.

The dirty gas flows upward into the filter bag and the dust collects on the inside sur-
face. When the pressure drop rises to a predetermined upper limit due to dust accumu-
lation, the gas flow is stopped and the shaker is operated. This process dislodges the
dust, which falls into a hopper located below the tube sheet.

For continuous operation, the filter must be constructed with multiple compartments.
This is necessary so that individual compartments can be sequentially taken off-line
for cleaning while the other compartments continue to operate.