Page 170 - Root Cause Failure Analysis

158 Root Cause Failure Analysis

This can be a real problem for baghouses that rely on automatic timers to control
cleaning frequency. The use of a timing function to control cleaning frequency is not
recommended unless the dust load is known to be consistent. A better approach is to
use differential-pressure gauges to physically measure the pressure drop across the fil-
ter media to trigger the cleaning process based on preset limits.

Fan or Blower
All baghouse designs use some form of fan, blower, or centrifugal compressor to pro-
vide the dirty-air flow required for proper operation. In most cases, these units are
installed on the clean side of the baghouse to draw the dirty air through the filter media.

Since these units provide the motive power required to transport and collect the dust-
laden air, their operating condition is critical to the baghouse system. The type and
size of air-moving unit varies with the baghouse type and design. Refer to the O&M
manuals, as well as Chapters 8 (Fans, Blowers, and Fluidizers) and 10 (Compressors)
for specific design criteria for these critical units.

Performance
The primary measure of baghouse-system performance is its ability to consistently
remove dust and other particulate matter from the dirty airstream. Pressure drop and
collection efficiency determine the effectiveness of these systems.

Pressure Drop
The filtration, or superficial face, velocities used in fabric filters generally are in the
range of 1-10 ft per minute, depending on the type of fabric, fabric supports, and
cleaning methods used. In this range, pressure drops conform to Darcy’s law for
streamline flow in porous media, which states that the pressure drop is directly pro-
portional to the flow rate. The pressure drop across the fabric media and the dust layer
may be expressed by

where

Ap = pressure drop (in. of water);
V’ = superficial velocity through filter (ft/min);
o = dust loading on filter (lbdft’);
K, = resistance coefficient for conditioned fabric (inches of water/foot/
minute) ;
K2 = resistance coefficient for dust layer (in. of water/lbdft/min).

Conditioned fabric maintains a relatively consistent dust-load deposit following a
number of filtration and cleaning cycles. K, may be more than ten times the value of
the resistance coefficient for the original clean fabric. If the depth of the dust layer on