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72 Lawrence K. Wang et al.

Fabric shaking combines stress in a normal direction to the dust–fabric interface (ten-
sion), stress directed parallel to the interface (shear), and stress developed during the
warping, binding, or flexing of the fabric surfaces. Mechanical cleaning studies (10) indi-
cate that dust removal efficiency is a function of the number of shakes, shaking frequency,
shaking amplitude, and bag movement acceleration. In general, more dust is removed
each time the bag is shaken. However, after about 100 shakes, very little extra dust can be
removed, and 200 shakes are recommended as being optimum. At this point, often a max-
imum of only about 50% of the dust is removed. The shaking frequency is significant in
that a resonance frequency can be set up when the fabric is mounted as a bag in a bag-
house. More dust is removed at the resonance frequency, but, otherwise, it appears that
the higher the frequency, the greater the amount of dust that is removed. In the shaker
amplitude range 0–2 in. (0–5.08 cm), dust removal is increased with increased amplitude.
Filter capacity increases with bag shaking acceleration, up to 10 g. Beyond the accel-
eration range of 1.5–10 g, residual dust holding varies approximately with the inverse
square root of the average bag acceleration. Other factors also affect fabric cleaning and
filter capacity. These include initial bag tension, amount of cake deposited on the fabric,
and cohesive forces binding dust to the fabric. The initial bag tension values should
range between 0.5 and 5 lb (2–20 N).
f
Overcleaning requires additional energy and causes undue wear on the bag fabric.
However, undercleaning a filter (e.g., by shaking less than the recommended 200 times),
decreases system filtration capacity and adversely affects operating costs.
The amount of cleaning by pulsed-jet air varies directly with the rate of rise of the
pressure differential across the bag. This should range from 1000 to 4000 in.
(2500–10,000 cm) H O pressure drop per second. Residual resistance values after
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cleaning also depend on the dust–fabric combination. Mechanical shaking often aug-
ments the reversed-airflow cleaning of bags. This is especially applicable to woven
fabric bags. Dust removal in woven bags during reverse flow is usually attributed to
bag flexure. Reverse-flow cleaning is, in general, not a satisfactory cleaning technique.
In fact, data indicate that in combined shaking–reverse-flow systems, mechanical
shaking is responsible for essentially all of the cleaning. The main role played by the
reverse air appears to be prevention of projection of dust into the clean air side of the sys-
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tem. Reverse-air cleaning velocities typically range from 4 to 11 ft/min with 0.3–3 ft of
gas required per square foot of bag area.
Selection of a cleaning method depends on the type of fabric used, the pollutant col-
lected, and the experiences of manufacturers, vendors, and industry. A poor combination
of filter-fabric and cleaning methods can cause premature failure of the fabric, incomplete
cleaning, or blinding of the fabric. Blinding of a filter fabric occurs when the fabric
pores are blocked and effective cleaning cannot occur. Blinding can result from moisture
blocking the pores, increased dust adhesion, or high-velocity gas stream embedding
of particles too deeply in the fabric. The selection of cleaning method may be based on
cost, especially when more than one method is applicable. Cleaning methods are dis-
cussed individually below (13,14), with Table 6 containing a comparison of methods.
A summary of recommended A/C ratios by typical bag cleaning method for many
dusts and fumes is found in Table 4. These ranges serve as a guide, but A/C ratios may