Air-Clear, LLC, Frazer, Pennsylvania
8.1 DEVICE TYPE
Fiberbed lters are specialized ltration devices that are primarily designed to
coalesce and capture liquid contaminants such as acid mists and aerosols, the vis-
cosity of which is low enough that they ow or can be made to ow from the ber-
The design gets its name from the medium used. It consists of micron-size bers
that are compressed tightly in a mat or bed, which provides the surface area and gas
path thickness needed to capture the pollutant.
These designs are somewhat related to lament/mesh scrubbers in that they uti-
lize target bers in a wet environment. The berbed lter bers, however, are in the
5 to 15 μm diameter range, or a fraction of the diameter of the lament or mesh type
scrubbers. The ber spacing is therefore closer in a berbed lter and, in general, it
can remove smaller diameter aerosols.
Figure8.1 shows a cutaway view of a berbed lter unit. The individual lters
(sometimes called candles, given their shape) are mounted on a tube sheet in either
a hanging or sitting position. The unit shown shows them hanging from a tube sheet.
The small J-shaped pieces under each candle are liquid traps that allow the liquid to
drain but prevent gases from bypassing the lter.
8.2 TYPICAL APPLICATIONS AND USES
The following are brief descriptions of common berbed lter applications. With
one exception, they all involve the collection of liquid droplets. In general, if the
exhaust stream is wet or the particles in the exhaust are liquids, or if a high-efciency
lter that can withstand a high pressure drop is required, then berbeds are a poten-
tial control option (see Figure8.1).
8.2.1 aCiD mist
Collecting acid mist was the rst signicant commercial use of berbed lters and
is still the largest application for them. Most sulfuric acid manufacturing plants
use berbed lters in the absorbing and drying towers to remove SO
acid mist from the air. Fiberbeds are also used to remove residual mist in the
exhaust of wet scrubbers, particularly hydrochloric acid scrubbers, because the
80 Air Pollution Control Equipment Selection Guide
reaction with the scrubbing liquid can be violent and creates a visible emission
from the scrubber. These are typically cool and clean applications, requiring no
preltration or cooling.
If additional berbed surface area is required, a nesting or concentric type lter
can be built. In these designs, as shown in Figure8.2, a berbed is mounted within
another berbed, thus increasing the face area of the medium and slowing the gas
velocity. The reduced gas velocity is said to improve the capture of aerosols and
8.2.2 asphalt proCessinG
Equipment used in asphalt processing include coaters, saturators, converters (blow
stills), storage tanks, and truck loading and unloading facilities. The coaters and
saturators used in roong manufacture often have solids that must be preltered
before the berbeds. Saturator exhaust may also require cooling. Tanks and loading
racks usually achieve adequate cooling through radiant losses in the ductwork and
have little solid particulate. Asphalt converters are also relatively free of solids but
may require cooling. Such a unit is shown in Figure8.3.
8.2.3 plastiCizer/Vinyl/pVC proCessinG
Vinyl and PVC processing, such as calendaring, coating, and curing operations, emit
oily plasticizers and other materials that can cause a substantial exhaust stack plume.
While oven exhaust must usually be cooled to condense the vapors, coater and calen-
dar emissions are often captured by canopy hoods that draw in ambient air that cools
the exhaust. Prelters are usually not required.
FIGURE8.1 Cutaway of berbed lter (Air-Clear, LLC).
Many coating and laminating processes, especially on fabric and vinyl, create visible
emissions that berbed lters can effectively control. The emissions are typically
generated during the drying and curing phase of the operation, so the exhaust is hot
and usually requires cooling to condense the vapors. The cooling coil housing is on
the right side in Figure8.4.
Electronic component manufacturing, such as solder leveling, can create oil mist
from the uxes used. Fiberbeds can also be used as point source collection for acid
mists, reducing the load on house scrubbers and reducing salt formation in the duct-
work. Materials of construction must be carefully chosen because many of the mate-
rials are potentially corrosive.
8.2.6 textile proCessinG
Textile tenter frame ovens and dryers can emit a mixture of pollutants including oils,
resins, waxes, tars, and various solids, producing a prodigious stack plume. This hot,
dirty exhaust requires both cooling and preltration. The mineral oil–based emission
FIGURE8.2 Filter within a lter (Monsanto Enviro-Chem Systems, Inc.).