Environmental Engineering Reference
In-Depth Information
Figure 12.8
HEPA filter.
to 91.4 cm) or more, depending on the application. Increasing pleat number
and filter depth are used to increase filtration efficiency, air volumes that can
be cleaned, or both. The filter medium may be rigid enough to be self-
supporting. In many high-efficiency filtration appliances, the filter medium
is held in place by a combination of rigid corrugated metal spacers and a
thick adhesive application between the filter medium and its surrounding
panel, case, or box.
As indicated, extended-media filters provide higher performance than
dry-panel filters. They are classified as medium, high, and very high effi-
ciency. Medium-efficiency filters have dust spot efficiencies (described later)
in the range of 40 to 60%. They consist of 5 to 10
µ
m diameter fibers that are
in
” (6 to 12 mm)-thick mats. High-efficiency filters have dust spot
efficiencies in the range of 80 to 90% or more, with fiber diameters in the
range of 1 to 4
1
/
to
1
/
4
2
m.
Very high particle cleaning efficiencies are achieved by HEPA filters (col-
lection efficiencies of 99.97% or greater at a 0.3
µ
m DOP test [see DOP smoke
penetration tests]). A HEPA filter is illustrated in
Figure 12.8
.
Their small fiber
diameters and high packing densities favor the collection of very small par-
ticles (circa 0.01
µ
m) by Brownian motion. Electrostatic forces cause small
particle agglomeration and adherence to media fibers. In some applications,
filter surfaces are coated to discharge static build-up so that they may be
cleaned by a pneumatically operated pulse cleaning system and reused.
µ
Filter fibers are often coated with vis-
cous, low-volatility oils to enhance particle collection and retention. Viscous-
medium panel filters have high porosities and low resistance to air flow.
They also have very low collection efficiencies for particles commonly found
in indoor air, but are very efficient in collecting fabric dusts and very large
particles (>10
c.
Viscous-media panel filters.
m). Typical operating velocities through such filters are in
the range of 300 to 600 fpm (91.4 to 182.8 mpm).
µ
