Handbook of air conditioning and refrigeration / Shan K

15.68 CHAPTER FIFTEEN
High-Efficiency Air Filters
High-efficiency air filters including MERV 13, 14, 15, and 16 are used to remove particles of 0.3 to
1 �m such as bacteria, viruses, cooking oil fumes, tobacco smoke, and other smoke.They have a 0.3
to 1�m particle size efficiency E 1 < 75 percent (corresponding to a dust spot efficiency of 80 to 90
percent) up to E 1 > 95 percent. The filter media are often made of glass fibers of submicrometer and
micrometer diameter. They are often in the form of a pleated mat in a cartridge, as shown in Fig.
15.38, or in the form of a bag filter. The air velocity flowing through the filter media is lower, and
the minimum final pressure drops across the filter media are 1.4 in. WC (350 Pa). High-efficiency
air filters are often protected by low- or medium-efficiency prefilters to extend their service life.
High-efficiency filters are widely used in air systems for hospitals, high-demand commercial buildings,
and precision manufacturing workshops.
Ultrahigh-Efficiency Air Filters
Ultrahigh-efficiency filters include high-efficiency particulate air (HEPA) filters, ultralow penetration
air (ULPA) filters, and gaseous adsorbers and chemisorbers. Activated carbon adsorbers are
discussed in Sec. 15.16, and chemisorbers are discussed in Sec. 24.5.
HEPA filters have an efficiency of 99.97 percent for dust particles � 0.3 �m using the DOP test
method. ULPA filters have an efficiency of 99.999 percent for dust particles � 0.12 �m using the
DOP method. A typical HEPA filter is shown in Fig. 15.38. The dimensions of this filter are 24 in.
by 24 in. by 11.5 in. (600 by 600 by 287 mm). The filter media are made of glass fibers of submicrometer
diameter that are formed into pleated paper mats. Some of the larger fibers act as the
carrier of the web. The performance of the filter medium is often assessed by an index called the
alpha value �, which can be calculated as follows:
� �
(2 � log P)100
25.4 �p t
(15.51)
where �p t � pressure drop of the filter medium, in. WC (Pa). Both penetration P, in percent, and
pressure drop �p t are measured at an air velocity flowing through the filter medium of 10.5 fpm
(0.05 m/s). The � value is usually between 10 and 11. Because of the development of new media
with lower pressure drops, an � value of 13 or even higher can now be achieved. The surface area
of the filter medium may be 50 times the face area of the ultrahigh-efficiency filter, and the rated
face velocity may vary from 190 to 390 fpm (0.95 to 1.95 m/s) for ultrahigh-efficiency filters at a
pressure drop of 0.65 to 1.35 in. WC (162 to 337 Pa) for clean filters. The face velocity of highcapacity
ultrahigh-efficiency filters can be raised to 500 fpm (2.5 m/s).
The filter media themselves have efficiencies higher than that of the mounted filter. Sealing of
the filter pack within its frame and sealing between the frame and gasket are critical factors that
affect HEPA and ULPA filter penetration and efficiency. Penetration of dusts represents aerosol
passing through the medium and through pinholes in the filter medium, as well as leaks between the
pack and the frame and between the frame and the gasket.
To extend the service life of an ultrahigh-efficiency filter, it should be protected by either a
medium-efficiency filter or two filters: a low-efficiency filter and a medium-efficiency filter located
just upstream from the ultrahigh-efficiency filter. The removal of large particles in the prefilter reduces
the dust load and prolongs the life of the ultrahigh-efficiency filter.
HEPA and ULPA filters are used to remove air contaminants such as unattached viruses, carbon
dust, combustion smoke, and radon progeny of particles in sizes � 0.3 �m. They are widely used in
clean rooms and clean spaces for the microelectronics industry, pharmaceutical industry, precision
manufacturing, and operating theaters in hospitals.