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Fabric Filtration 81

to alternate between long filtering periods and short cleaning periods. During cleaning,
accumulated dust on the bags is removed from the fabric surface and deposited in a hop-
per for subsequent disposal.

Fabric filters collect particles ranging from submicron to several hundred microns in diameter,
at efficiencies generally in excess of 99% Routinely, gas temperatures can be accommodated
up to about 500°F, with surges to approx 550°F. Most of the energy use in a fabric-filter sys-
tem derives from the pressure drop across the bags and associated hardware and ducting.
Typical values of pressure drop range from about 5 to 20 in. of water column.

Example 2
Fabric filters are often categorized by the cleaning method for removing the dust cake.
Three common types include (1) shaker filters, (2) reverse-air filters, and (3) pulse-jet
filters. Describe and discuss (1) general cleaning methods and (2) the three types of
fabric filter.
Solution:

1. General cleaning methods: As dust accumulates on the filtering elements, the pres-
sure drop across the bag compartment increases until cleaning of the bags occurs.
Cleaning is usually controlled by a timer or a pressure switch set at the specified max-
imum pressure drop. At this point, the bags in the compartment are cleaned to remove
the collected dust, and the cycle is then repeated. The two basic mechanisms for bag
cleaning involve flexing the fabric to break up and dislodge the dust cake, and reverse
airflow through the fabric to remove the dust. These may be used separately or togeth-
er. The three principal methods used for fabric cleaning are mechanical shaking (man-
ual or automatic), reverse airflow, and pulse-jet cleaning. The first method uses only
the fabric flexing mechanism; the latter two methods use a combination of the reverse-
airflow and fabric flexing mechanisms.
2. Three types of fabric filters:
a. In a shaker filter (see Fig. 3), the bags are hung in a framework that is oscillated
by a motor-controlled timer. In this type of system, the baghouse is usually divid-
ed into several compartments. The flow of gas to each compartment periodically is
interrupted, and the bags are shaken to remove the collected dust. The shaking
action produces more wear on the bags than other cleaning methods. For this rea-
son, the bags used in this type of filter are usually heavier and made from durable
fabrics (13,26).
b. In a reverse-airflow filter, gas flow to the bag is stopped in the compartment being
cleaned and a reverse flow of air is directed through the bags. This approach has the
advantage of being “gentler” than shaking allowing the use of more fragile or
lightweight bags (13).
c. The third type of baghouse, pulse-jet fabric filter, is by far the most common type
for Superfund applications. In this type of system, a blast of compressed air
expands the bag and dislodges collected particles. One advantage of pulse-jet fab-
ric filters is that bags can be cleaned on line, meaning fewer bags (less capacity)
are required for a given application (26).

Example 3
Discuss (1) mechanical shaking cleaning methods, (2) reverse-airflow cleaning methods,
and (3) pulse-jet cleaning methods in detail.