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Fabric Filtration 69

Fig. 2. Mechanisms for particle removal by a filter.

but touches and is captured by the filter. Diffusion consists of random particle motion in
which the particles contact with and adhere to the fiber filters. Diffusion increases as
particle size decreases and is only significant for submicron-diameter particles.
A high A/C ratio (filtering velocity) promotes particle capture by impaction. On the
other hand, an excessive velocity will blow captured material off or through the fabric,
in many cases the only support for the cake. This would reduce collection efficiency. As
for filtering by diffusion, a higher air-to-cloth ratio reduces the residence time available
for particle collection. “Normal” air-to-cloth ratios are about 3:1; “high” air-to-cloth
ratios are 6:1 and above.
New filter fabrics having no buildup of solid material will often exhibit a pressure
drop of 0.5 in. (1.27 cm) H O at normal air-to-cloth ratio ranges. This is called the fab-
2
ric permeability and is often the same for woven and felted fabrics, although woven
2
2
bags usually have a weight of 5–10 oz/yd (170–340 g/m ), and the much heavier and
2
2
fuzzier felted bags have a weight of 10–20 oz/yd (340–680 g/m ). A/C ratios are not
based on theoretical or empirical relationships, but on installation experience of indus-
try and fabric-filter vendors. Recommended A/C ratios usually depend on a specific dust
and a specific cleaning method.
Hand calculations using basic equations give only a general indication of the needed
A/C ratio. In practice, tabulated values are frequently provided and are an approximation.