Page 168 - Root Cause Failure Analysis

156 Root Cause Failure Analysis

The gas entering the filter must be kept above its dewpoint to avoid water-vapor con-
densation on the bags, which will cause plugging. However, fabric filters have been
used successfully in steam atmospheres, such as those encountered in vacuum dryers.
In these applications, the housing generally is steam cased.

Reverse-Flow-Cleaned Filter Reverse-flow-cleaned filters are similar to the
shaker-cleaned design, except the shaker mechanism is eliminated. As with shaker-
cleaned filters, compartments are taken off-line sequentially for cleaning. The primary
use of reverse-flow cleaning is in units using fiberglass-fabric bags at temperatures
above 150°C (300°F).

After the dirty-gas flow is stopped, a fan forces clean gas through the bags from the
clean-gas side. The superficial velocity of the gas through the bag generally is 1.5 to
2.0 ft per minute, or about the same velocity as the dirty-gas inlet flow. This flow of
clean gas partially collapses the bag and dislodges the collected dust, which falls into
the hopper. Rings usually are sewn into the bags at intervals along their length to pre-
vent complete collapse, which would obstruct the fall of the dislodged dust.

Reverse-Pulse-Cleaned Filter In the reverse-pulse-cleaned filter, the bag forms a
sleeve drawn over a cylindrical wire cage, which supports the fabric on the clean-gas
side (Le., inside) of the bag. The dust collects on the outside of the bag.

A venturi nozzle is located in the clean-gas outlet from each bag, which is used for
cleaning. A jet of high-velocity air is directed through the venturi nozzle and into the
bag, which induces clean gas to pass through the fabric to the dirty side. The high-
velocity jet is released in a short pulse, usually about 100 milliseconds, from a com-
pressed air line by a solenoid-controlled valve. The pulse of air and clean gas expand
the bag and dislodge the collected dust. Rows of bags are cleaned in a timed sequence
by programmed operation of the solenoid valves. The pressure of the pulse must be
sufficient to dislodge the dust without ceasing gas flow through the baghouse.

It is common practice to clean the bags on-line without stopping the flow of dirty gas
into the filter. Therefore, reverse-pulse bag filters often are built without multiple
compartments. However, investigation has shown that a large fraction of the dislodged
dust redeposits on neighboring bags rather than falls into the dust hopper.

As a result, there is a growing trend to clean reverse-pulse filters off-line by using
bags with multiple compartments. These sections allow the outlet-gas plenum serving
a particular section to be closed off from the clean-gas exhaust, thereby stopping the
flow of inlet gas. On the dirty-side of the tube sheet, the isolated section is separated
by partitions from neighboring sections where filtration continues. Sections of the fil-
ter are cleaned in rotation as with shaker and reverse-flow filters.

Some manufacturers design bags for use with relatively low-pressure air (i.e., 15 psi)
instead of the normal 100 psi air. This allows them to eliminate the venturi tubes for