Environmental Engineering Reference
In-Depth Information
Figure 12.12
Relationships between filter loading and resistance and resistance and
volumetric air flow. (From
ASHRAE
Equipment Handbook
, ASHRAE, Atlanta, 1983,
chapter 10. With permission.)
developed. This market included a variety of products which can be cate-
gorized on the basis of cost, potential air cleaning capacity, and efficiency.
1.
Residential and consumer air cleaners
At the low end of the market are
devices that range in cost from $20 to $100. They are small desktop devices
that use dry, loosely packed, low-density filters located upstream of a high
velocity, low air resistance axial fan. Some units utilize electrostatically
charged electret filters (thin plastic materials imprinted with high voltage
charges). In most cases, such devices have insufficient capacity to clean air
in even a single closed room. Performance tests conducted in a 1200 ft
a.
Fan and filter desktop cleaners.
(33.6
3
m
) room have indicated that tobacco smoke removal effectiveness of such
devices under static chamber conditions (i.e., no air flow in or out of the
space) is little better than using no device at all.
3
The simplest and least expensive devices
generate ions that diffuse out into building air and attach to particles which
plate out on building surfaces. More advanced models are designed to elim-
inate the “dirty wall effect” associated with simple ion generators. In such
devices, an attempt is made to draw charged particles into the air cleaner
(by using a suction fan), where they can be deposited on an electrostatically
charged panel filter. In other ionizers, a stream of negatively charged ions
is generated in pulses, and charged particles theoretically are drawn pas-
sively to the ionizer, which contains a positively charged cover.
Negative ion generators have long been used as health-promoting
devices, particularly by a small population of medical practitioners who treat
allergy patients. There is limited evidence that negative ions may affect an
b.
Negative ion generators.
