Environmental Engineering Reference
In-Depth Information
individual's sense of well-being and alleviate allergy symptoms. Because
such health claims cannot be definitively established, manufacturers cannot
promote negative ion generators for such purposes. After being forced from
the market in the early 1960s by the Food and Drug Administration (FDA),
these devices were reincarnated as air cleaners. They have been shown to
be relatively effective in removing smoke from a 1200 ft
test room under
3
static conditions.
Air cleaners of this
type are portable devices which are larger, and more expensive and effective
than desktop units. They utilize extended-media high-efficiency filters,
HEPA filters, or two-stage electronic systems. A portable electronic air
cleaner can be seen in
Figure 12.10b
.
Air flow rates are dependent on the
filter type, blower capacity, and fan horsepower. These devices utilize good
quality high-efficiency filters/filtration systems and should be relatively
effective in cleaning particles from air in a single room with an air volume
up to 3200 ft
c.
Portable extended-media and electronic cleaners.
).
As they are widely recommended by allergists, the use of such portable
high-efficiency air cleaners is common. They have been demonstrated to be
relatively effective in reducing smoke particle concentrations in a single room
under static conditions. An approximate 70% reduction in particle levels was
observed when an air cleaner with a HEPA filter was placed in the bedrooms
of 32 allergy patients for a 4-week period. These studies indicated that
portable high-efficiency devices can be used to reduce particle levels in a
single room when operated continuously. However, portable electronic air
cleaners operated continuously in closed rooms may produce considerable
odor (from O
(
circa
90 m
3
3
and other products) and even irritation effects.
3
In-duct air cleaning devices are widely used in
North American residences. In-duct devices employ either extended-media
filters or electronic systems (
Figure 12.10a
)
. In-duct systems are installed in
cold air returns immediately upstream of furnace blower fans. They are
designed to clean the air of an entire residence and, except in very large
residences, have the capacity to provide whole-house air cleaning. These
systems, however, only remove particles when the furnace or cooling system
blower fan is activated. In most cases, operation is intermittent since it
responds to heating and cooling needs. In-duct systems need to be run
continuously to achieve the air cleaning performance they are designed for.
Most in-duct filtration media and electronic air cleaners have dust spot
efficiencies of 90% or better.
The installation of an in-duct cleaning system can significantly reduce
airborne particle levels by the initial action of moving contaminated air
through the filter/filtration system and by its continuous recirculation. For
maximum cleaning effectiveness, it is desirable to use a high-efficiency fil-
ter/filtration system and a high recirculation rate. The effect of two different
recirculation rates on cleaning efficiency can be seen in
Figure 12.13
.
d.
In-duct cleaners.
