Environmental Engineering Reference
In-Depth Information
Table 12.2
Relationship Between Flow Rate
and Electronic Air Cleaner Dust
Spot Efficiency
Flow rate (CFM)
Dust spot efficiency (%)
400
93
550
90
750
85
cies of approximately 99%. In indoor applications, collection efficiencies are
rarely >95%, and are usually less.
As seen in
Table 12.2
,
collection efficiencies are significantly affected by
volumetric air flow rates. As air velocity increases, particles have less time
to be drawn to and deposited on collection plates. As a consequence, collec-
tion efficiency is reduced. Because of the sensitivity of electronic air cleaners
to changes in velocity, it is common to use prefilters, perforated plates, etc.,
to induce some resistance and provide uniform velocity air flow through the
collection system.
In single-stage electronic air cleaners, ionizing wires are placed between
collection plates. Because of reduced travel path lengths, single-stage clean-
ers are less efficient than two-stage units. They have the advantage of requir-
ing less space in AHUs.
Electronic air cleaners are available as modular, free-standing units;
modular units that can be suspended from a ceiling or mounted on a wall;
in-duct units installed in residential heating and cooling systems; or units
of various sizes placed in AHUs of mechanically ventilated buildings. A
residential in-duct and a portable free-standing electronic air cleaner are
pictured in
Figure 12.10
.
Resistance to air flow in electronic air cleaners is very low; as a conse-
quence, volumetric air flows are constant. Because of low air flow resistance,
energy requirements for fan systems are also low. Maintenance is usually
limited to cleaning collector plates. It is both desirable and common to use
dust stop filters as prefilters to maintain high collection efficiencies for small
particles and reduce the need for cleaning.
Electronic air cleaners have one major disadvantage. Because of the high
voltages used, they produce ozone (O
). Ozone production varies among
products and product vendors. It is common to smell the sweet odor of O
3
3
in residences where electronic cleaners are in continuous operation (portable
and in-duct units). The presence of O
in buildings is of concern because of
its known health effects, its ability to oxidize fabric dyes and crack rubber
and soft plastics, and its ability to initiate indoor chemical reactions.
3
2.
Charged-media ionizing cleaners
In this type of air cleaner, dust in an airstream is charged by passing it
through a corona discharge ionizer and then collected on a charged-media
filter.
