Environmental Engineering Reference
In-Depth Information
Figure 12.5
Dry panel-type filter.
filters are designed and widely used to clean air in small spaces such as
rooms in residences. Filtration systems which require high performance
characteristics (HEPA) are used in clean rooms in the manufacture of phar-
maceuticals and semiconductors, in nuclear power plants and other nuclear
facilities, and in hospital operating rooms. They are also used in portable
home air cleaning devices.
Commercially available filters include wire mesh units used in older
HVAC systems and cooking range hoods, and the more commonly used
fibrous media filters. In the latter case, filters are described as dry, viscous-
media, or charged-media filters (see electronic air filters). They may be single-
use or renewable panels.
Dry-type panel filters (
Figure 12.5
)
have high
porosities and low dust spot efficiencies. As a consequence, their use is
limited to dust-stop filters in HVAC systems and home heating and cooling
units. They collect large particles such as lint by sieve action, impaction, and
interception. They are typically used in systems with air velocities in the
range of 200 to 700 ft/min (fpm) (60.9 to 213.3 m/min [mpm]). Newly
exposed filters have low associated pressure drops (0.05 to 0.25” H
a.
Dry-type panel filters.
O, 12.4
to 62 pascals). Typically, they are allowed to reach pressure drops of 0.50 to
0.75” H
2
O (124 to 186 pascals) before being replaced. Collection efficiency
increases significantly as filters become soiled. Replacement is required
because of the associated increased resistance to air flow. Filter media used
in dry-type panels include fiberglass, open cell foams, nonwoven textile
cloths, and cellulose fibers.
2
Extended-surface media filters
have been developed to overcome the air resistance/pressure drop problems
associated with use of thick, high-density, high-efficiency filtration media.
Filter surface area is extended by pleating the filter medium; this significantly
increases the surface area available for particle collection (relative to the flat
b.
Extended-surface dry-type filters.
