E-4
ENGLISH
PART I GENERAL DESCRIPTION ON AIR CLEANING
When you are trying to repair the air purifiers, make sure you understand the technologies used by the air purifier and the
potential issues that may be inherent in that technology which could effect the efficiency of the air purifier and your health. Most
of air purifiers may use multiple technologies in their design.
Mechanical filtration
Capturing particles in a filter via physical mechanisms without electrostatic forces
characterizes this air cleaning method. These units are capable of capturing small
particulate matter. They utilize a filter media with very high efficiency ratings. The
Letters in the word HEPA stand for High Efficiency Particulate Arrestance. HEPA filters
are composed of a mat of randomly arranged fibres. Key metrics affecting function are
fibre density and diameter, and filter thickness. The air space between HEPA filter
fibres is much greater than 0.3 microns. A common misperception is that fibrous filters
(the most extensively used in mechanical filtration) work like a sieve, with particles
becoming trapped within the spaces between the fibers. What actually occurs is that
once the particles make contact with the fibers, they remain attached due to strong
molecular forces between the particles and fibers. As a result, the particles become a
part of the filter structure and contribute to a filter's efficiency by creating resistance of
air flowing through the filter. The filter can capture 99.97% of particles as small as 0.3
microns. The disadvantage of HEPA filters is that the need for a powerful fan leads to
increased noise and energy costs compared to less efficient filtration systems, and
replacement filters are generally quite expensive. Critical design details are important
in achieving high air cleaning performance. If an air purifier using a HEPA filter is not
properly designed, air will bypass the filter as static pressure pushes against the filter
and allow particle escape. Most air purifiers on the market include HEPA filters as a
sales and marketing tactic, but fail to achieve HEPA performance.
Electronic filtration
Electrostatic Precipitators, most commonly referred to as electronic air
purifiers, operate on electrostatic attraction to a collecting section usually
called a electronic cell. A typical two-stage electronic cell has an ionizer
section (the front portion of the cell) and a collector section (the back portion
of the cell). The ionizer wires are maintained at several thousand volts by the
high voltage power supply, which produces a corona that releases electrons
into the air stream. These electrons attach to the dust particles and give them
a net positive charge. The collector section is composed of alternate parallel
plates which are charged oppositely (positive and ground) and the charged
particulate adheres to the collecting surfaces. The air velocity between the
plates needs to be sufficiently low to allow the dust to fall and not to be re-
entrained in the air stream. The electronic cell is capable of removing
particles in the range 0.01 micron to 10 microns and can achieve efficiencies
around 95 percent. Electronic cell has a very low pressure drop and high
efficiency on very small particulate. Electronic air purifiers become less
efficient as the collecting cell becomes dirty. As the surface of the collecting
area becomes heavily coated, the charge to attract the particulate is less
effective. The electronic cell must be cleaned regularly to maintain maximum
efficiency. The advantages of electronic air purifiers are they generally have
low energy costs because of low air pressure drop, the airflow through the
units remains constant with use and the electronic cell is reusable, which
avoids long-term filter replacement costs. Electronic air purifiers also can
create ozone as a byproduct of ionization. However, these devices are not
considered to be "ozone generators" as the level of ozone created is
generally low and ozone production can be reduced by adjusting the high
voltage power supply.
The two-stage electronic cell of 200T.
For more information, see PART III PRODUCT
INFORMATION in this manual.
HEPA filter is composed of a mat of
randomly arranged fibres.
Fibres
Ionizer
terminal
Collector
terminal
Ground
terminal
Ceramic
insulators
2. AIR CLEANING TECHNOLOGY
