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Fabric Filtration 83

junction with heavier more durable fabrics, such as most woven fabrics. Bags with fair
to poor abrasion ratings in Table 3 (e.g., fiberglass) should not be chosen for fabric fil-
ters cleaned by mechanical shaking unless they are treated with a special coating (i.e.,
a backing) before use. Although shaking is abrasive to the fabric, it does allow a dust
cake to remain on the fabric, thus maintaining high collection efficiency (15,22).
2. Reverse-airflow cleaning method: Reverse-airflow cleaning is used to flex or collapse
the filter bags by allowing a large volume of low pressure air to pass countercurrent
to the direction of normal gas stream flow during filtration (16,18). Reverse air is
provided either by a separate fan or by a vent in the fan damper, which allows a back-
wash of air to clean the fabric filters. Reverse-airflow cleaning is usually performed
off-line. It allows the use of fragile bags, such as fiberglass, or lightweight bags, and
usually results in longer life for bags (16). As with mechanical shaking, woven fabrics
are used. Because cleaning is less violent than with pulse-jet cleaning and is performed
off-line, outlet concentrations are almost constant with varying inlet dust loading
throughout the cleaning cycle. Reverse-airflow cleaning is, therefore, a good choice
for fabric cleaning in hazardous air pollutant (HAP) control situations.
3. Pulse-jet cleaning method: In pulse-jet cleaning, a high-pressure air pulse enters the
top of the bag through a compressed air jet. This rapidly expands the bag, vibrating it,
dislodging particles, and thoroughly cleaning the fabric. The pulse of air cleans so
effectively that no dust cake remains on the fabric to contribute to particulate collec-
tion. Because this cake is essential for effective collection on woven fabrics, felted
fabrics are generally used in pulse-jet-cleaned fabric filters. Alternatively, woven fabrics
with a suitable backing may be used. All fabric materials may be used with pulse-
jet-cleaning, except cotton or fiberglass. Previously, mechanical shaking was considered
superior to pulse-jet cleaning in terms of collection efficiency. Recent advances in
pulse-jet cleaning have produced efficiencies rivaling those of mechanical shaking.
Because the air pulse has such a high pressure (up to 100 psi) and short duration
(≤0.1 s), cleaning may also be accomplished on-line, but off-line cleaning is also
employed. Extra bags may not be necessary to compensate for bags off-line during
cleaning. Cleaning occurs more frequently than with mechanical shaking or reverse-
airflow cleaning, which permits higher air velocities (higher A/C ratios) than the other
cleaning methods. Furthermore, because the bags move less during cleaning, they
may be packed more closely together. In combination, these features allow pulse-jet-
cleaned fabric filters to be installed in a smaller space, at a lower cost, than fabric fil-
ters cleaned by other methods. This cost savings may be somewhat counterbalanced
by the greater expense and more frequent replacement required of bags, the higher
power use that may occur, and the installation of fabric-filter framework that pulse-jet
cleaning requires (14,16,18).

Example 4
3
A new 8000-ft /min shaker-type filter installation is being designed to remove iron oxide
from an electric furnace emission. Consider the gas to be air at 110°F with an inlet dust
3
concentration of 0.8 gr/ft (grains per cubic foot). The A/C ratio is 3 ft/min and the mass
mean particle size is approx 1 µm. Other design parameters include the following.
From Table 2 for a 1-µm spherical particle:
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S = specific surface area per unit volume of solids = 1.83 × 10 ft −1
e = porosity = 0.40
Assume that the Kozeny–Carman coefficient k = 5 and
ρ = particle density = (5.18)(62.4) = 323 lb/ft 3
p