Environmental Engineering Reference
In-Depth Information
Table 12.3
Performance of Dry Media Filters
Arrestance
(%)
Dust spot efficiency
(%)
Media type
Fine open foams/textile nonwovens
70-80
15-30
Mats of glass fiber/multi-ply cellulose/wool felt
85-90
25-40
Mats of 5-10
µ
m fibers, 6-12 mm thick
90-95
40-60
Mats of 3-10
µ
m fibers, 6-12 mm thick
>95
60-80
Mats of 1-4
µ
m fibers, mixtures of fiber types
>95
80-90
Source:
From ASHRAE, Air cleaners, in
Equipment Handbook
, ASHRAE, Atlanta, 1983, chap. 10.
1.
Arrestance
Arrestance is a measure of the ability of a filter to collect relatively large
particles. It is a measure of the performance of panel filters that are used to
protect mechanical equipment or serve as prefilters where low-efficiency
cleaning is acceptable.
Arrestance values are determined by aerosolizing a standard dust mix-
ture upstream of the filtration system. The ASHRAE dust mixture includes,
by weight, 72% standard air-cleaner fines, 23% molacco black, and 5% No.
7 cotton linters ground in a mill. This dust mixture is designed to take into
account the large variability in composition and size of particles that enter
HVAC system AHUs. Arrestance values are based on the weight of standard
dust collected on or in the filter compared to the weight of the standard dust
mixture aerosolized upstream of the filter. Arrestance values for a number
of filter media/filters are found in
Table 12.3
.
Note that, in general, arrestance
values are relatively high, and increase with increasing filter density and
smaller fiber size. Because arrestance focuses on weight, it is a measure of
large particle (>10
µ
m) cleaning effectiveness.
2.
Dust spot efficiency
The dust spot efficiency test is a better indicator of filter performance over
a broad range of particle sizes. It is employed to measure the performance
of medium to high efficiency extended-media filters and electronic air clean-
ers. It measures discoloration differences (as determined by optical density)
observed on glass fiber filter tape samples collected both upstream and
downstream of filter/filtration units. Dust spot efficiency of the filtration
unit is expressed as percent reduction of the optical density of downstream
compared to upstream samples.
Dust spot efficiencies for a range of filter media/filters are also summa-
rized in
Table 12.3
.
Note the significant differences between arrestance and
dust spot efficiency values. High dust spot efficiencies are associated with
filter media with small fiber diameters and high packing densities. Though
dust spot efficiency is a relatively good measure of overall dust collection
efficiency, it is subject to some error due to differences in optical properties
of collected particles. Darker particles absorb more light and therefore result
