Page 86 - Air Pollution Control Engineering

02_chap_wang.qxd 05/05/2004 12:40 pm Page 66
66 Lawrence K. Wang et al.

precoats to react with and adsorb sulfur oxides; and activated carbon can be introduced
to remove odors.
There is another basic service distinction between process and nonprocess work.
Process functions may include the removal of material from air-conveying systems in
which product collection is the primary function. A nonprocess application would be the
removal of nuisance dust, where only a small amount of the product would actually
encounter the filter. However, because of pollution control considerations, the same
care and attention should be paid to nonprocess applications that have been given
process collectors in the past.
The fifth and final classification of fabric filters is by application. These classes
include temperature solids concentration type of pollution in the inlet gas moisture content
suction, pressure applications size of filter and filter efficiency. The use of glass fiber
media makes it possible to operate filters at temperatures up to about 550°F (288°C). A
number of different fabric filter media and their characteristics are given in Table 3. Work
is currently in progress to develop higher-temperature media, as indicated in the table.
Dust loading is defined as the concentration of solids in the inlet gas stream.
Obviously, as dust loading increases, the amount of cake will increase for a given
volumetric flow rate of gas. In order to maintain the necessary gas approach velocity
and be able to operate an intermittent filter for a reasonable filter cycle time, it may be
desirable to reduce the inlet dust loading. One method of doing this is to install
mechanical collection devices in front of the fabric filter to remove large-diameter solid
material. Gas conditioning, which can consist of introducing air as a diluent, could, in
effect, reduce dust loading. However, this process is used more often to reduce inlet
temperature and/or humidity.
It is a wise precaution to operate gas cleaning filter systems above the dew point
temperature. It has been pointed out that if some dusts become wet, they will bridge and
mud (plug) the filter. Methods of keeping the system above the dew point include insu-
lating the filter, heating either the filter and/or the gas, and using warm, dry dilution gas.
Fabric filters can be used in systems that operate at either positive or negative
6
2
pressures. Some systems are operated at pressures over 200 psi (1.38 × 10 N/m ),
and vacuum systems commonly operate at up to 15 in. (0.38 m) Hg. The most common
operating range is ±20 in. (0.508 m) H O.
2
3.3. Efficiency
Fabric filters are extremely efficient solids removal devices and operate at nearly 100%
efficiency. Efficiency depends on several factors (10,11):
1. Dust properties
a. Size: particles between 0.1 and 1.0 µm in diameter may be more difficult to capture.
b. Seepage characteristics: Small, spherical solid particles tend to escape.
c. Inlet dust concentration: The deposit is likely to seal over sooner at high concentrations.

2. Fabric properties
a. Surface depth: Shallow surfaces form a sealant dust cake sooner than napped surfaces.
b. Weave thickness: Fabrics with high permeabilities, when clean, show lower efficiencies.
Also, monofilament yarns, without fibrils protruding into the yarn interstices, show
lower efficiencies than “fuzzier” staple yarns having similar interstitial spacing.