Environmental Engineering Reference
In-Depth Information
the ambient atmosphere through HVAC systems or natural ventilation pro-
cesses. Particles may include fabric lint and paper dust, tobacco smoke,
organic dusts (see Chapter 5), mineral particles, pollen and mold spores,
industrially and photochemically derived particles, etc.
Airborne particles can be removed from indoor and outdoor air used
for ventilation by application of relatively simple physical principles adapted
from industrial gas cleaning. Air flows through air cleaning devices used for
indoor environments are orders of magnitude less than those used for stack
gases and large industrial local exhaust ventilation systems. Particle/dust
loading rates are significantly lower as well. Airborne dust concentrations
in indoor air rarely exceed 200
. Airborne
particles are typically collected and removed from ventilation air by fibrous
media filters; less commonly by electrostatic precipitation.
µ
g/m
and are usually <50
µ
g/m
3
3
A. Filtration
A wide variety of filter types and filtration systems are used to remove
airborne particles from supply air systems and indoor spaces. Filter panels
are inserted into air-handling units (AHUs) upstream of blower fans in
HVAC systems and domestic heating and cooling systems. These panels
contain a medium which varies from a simple metal grid to the more com-
monly used fibrous mats which are oriented perpendicular to the direction
of air flow. Filter materials may include glass, cellulose, or polymeric fibers,
which vary from <1 to 100
m in diameter. Filter mats vary in density and
depth, with porosities in the range of 70 to 99%. Because of differences in
fiber diameters and mat densities, filters vary in their ability to capture
airborne particles.
µ
1.
Collection processes
Large particles (such as lint) may be collected on filters by sieve action. Most
smaller particles are collected as a result of a number of particle deposition
processes. These include interception, impaction, diffusion, and electrostatic
attraction. Deposition of particles on filter fibers by interception, impaction,
and diffusion processes can be seen in
Figure 12.1
.
Interception occurs when particles follow a streamline within one
radius of a filter fiber (
Figure 12.1a
)
. As a particle collides with the perimeter
of a fiber, it loses velocity and is captured. Interception is important in
collecting particles with diameters >0.1
m, and is a particularly important
collection mechanism in the range of minimum collection efficiency. Col-
lection efficiency increases with increasing filter density and is independent
of particle velocity.
Impaction occurs when particles of sufficient size and mass cannot flow
along fluid streamlines around filter fibers. Due to their inertial mass, large
particles collide with filter fibers and are collected (
Figure 12.1b
).
Collection
efficiency increases with increasing particle size, particle velocity, and Stokes
number (ratio of particle stopping distance to fiber diameter). Impaction is
µ
