PARTICULATE REMOVAL 841
2) The cake should be removed uniformly.
3) Some residual cake is necessary after cleaning for
efficient filtering.
4) The cleaning operation should not result in a large
dispersion of dust because either a refiltration
operation would be necessary or the efficiency of
the filter would suffer.
5) The cleaning method should not result in exces-
sive bag damage.In general, the filter cloth is cleaned by shaking, gas flow
reversal, or some combination of the two methods.
Mechanical shaking from the upper fastening is the most
common method of cleaning the filter bags. Amplitudes vary
from less than 1 in. to a few inches, and the frequency is
usually several cycles per second. During the cleaning, the
filtration is stopped, otherwise the dust will have a tendency
to work through the bags and result in internal abrasion. For
this purpose, the baghouse is built in several compartments
to allow the isolation of one compartment at a time for clean-
ing. One disadvantage of this method of cleaning is the rela-
tively large amount of fabric flexure involved especially at
the top of the bags.
The second method of cleaning the bags is by flow rever-
sal. If the dust cake is loosely adhering to the bag, then only
a low-pressure reversal of fast flow will be necessary for
cleaning; however, partial collapse of the bag can be done
for the more adherent cakes. Although this method avoids
some of the problems associated with bag flexure during
shaking, it does have several disadvantages. Reverse flow
without bag collapse requires that some support structures
be placed within the bags (rings, screens etc.) which leads
to difficulty in changing bags and chafing effects. Also, the
reverse air used for cleaning has to be refiltered which results
in a larger baghouse for a given filtration capacity.
A combination of the above two methods has been
devised which is known as pulse cleaning. In this method, a
pulse of compressed air is released from nozzles which resultsin a combination of flow reversal and fabric deformation.
A unique feature of pulse cleaning is the short time required
for cleaning. Therefore pulse cleaning is useful for equipment
which must handle very high dust leadings. One of the major
disadvantages of the method is its tendency to over-clean,
especially for baghouses equipped with woven bags.
Other variations are available such as reverse jet clean-
ing which employs a jet of compressed air supplied from a
ring which travels up and down on the outside of the bag.
Occasionally, sonic vibration is used to aid in the cleaning.
Baghouse Construction The type of baghouse construc-
tion depends on the moisture condensation and temperature
considerations. Below a temperature of about 135F, an open-
sided design can be used (inside filtration) which is the least
expensive type. A closed baghouse is used at higher tempera-
tures, and above 160F, the structure is usually insulated.
Materials of construction are selected with regard to
corrosiveness of the gas handled. Steel, aluminium, and
asbestos cement are some typical materials of construction.
Performance of Fabric Filters The two parameters which
are important in determining the performance of the fabric
filters are its efficiency and pressure drop. The efficiency is
a measure of how effectively the device is performing its job.
The pressure drop is one of the major variables which deter-
mines operating cost because it affects the fan power neces-
sary to move the gas through the filter, and also determines
the frequency and duration of the cleaning cycle.
There are several factors affecting filter efficiency. The
major one is particle size. In general, particles having diam-
eters 1 m will be filtered with a 99% removal efficiency.
Efficiencies are about 90% for particles 1 m. In a typical
baghouse application, most of the particles passing through
the filter are of sub-micron size.
The amount of dust buildup on the filter medium also
affects the efficiency of collection, because a particle has a
greater probability of being trapped by one of the filtration
mechanisms discussed previously if there is a greater amount
of cake. Thus, cleaning should not be too thorough.TABLE 2
Relative properties of some common fabricsFiberMax. temp.
(dry) FResistance to
moist heatResistance to
abrasionResistance to
mineral acidsResistance to
alkalis
Cotton 180 G G P G
Wool 200 F F F P
Rayon 200 G G P F
Nylon 200 F E P G
Polypropylene 200 F E G E
Acrylic 260 G G G F
Polyester 275 F E G F
Nomex 400 E E F G
Teflon 400 E F E E
Glass 550 E P E PE excellent; G good; F fair; P poor.C016_002_r03.indd 841C016_002_r03.indd 841 11/18/2005 1:06:47 PM11/18/2005 1:06:47 P