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Types of
HEPA Filters; Winter 2004 issue
of Air Media
Author(s):
Alan C. Veeck, CAFS, NAFA Staff
Top of
Page
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How Many Types of HEPA Filters are there?
According to the Institute of Environmental Science and
Technology, (IEST) with regard to performance, there are 6
types – A, B, C, D, E & F. Each has its own distinct
characteristics as shown by chart 1 below.
Chart 1
Recommended
test and minimum rating for filter types A through F1
|
Filter type |
Penetration Test |
Scan Test (see note) |
Comments |
Minimum efficiency rating |
|
Method |
Aerosol |
Method |
Aerosol |
|
A |
MIL-STD 282 |
Thermal DOP |
None |
None |
|
99.97%*
at 0.3 μm |
|
B |
MIL-STD 282 |
Thermal DOP |
None |
None |
Two flow leak
test |
99.97%
at 0.3 μm |
|
C |
MIL-STD 282 |
Thermal DOP |
Photometer |
Polydisperse DOP |
|
99.99%
at 0.3 μm |
|
D |
MIL-STD 282 |
Thermal DOP |
Photometer |
Polydisperse DOP |
|
99.999%
at 0.3 μm |
|
E |
MIL-STD 51477 or
MIL-STD F51068 |
Thermal DOP |
Photometer |
Polydisperse DOP |
Two flow leak
test |
99.97%
at 0.3 μm |
|
F |
IES-RP
CC007 |
Open |
Particle
Counter |
Open |
|
99.999%
at 0.1 to 0.2 μm |
Note: Either of the two test methods or an alternative method
may be used for filter types C, D, E and F, if agreed upon
between the buyer and seller. Equivalency of the alternative
test method should be determined jointly by the buyer and
seller.
* Test on
encapsulated filter
Because of the fragile nature of HEPA filters, they may be
damaged in transit or upon installation by
less-than-knowledgeable technicians. Types of damage include
fractured frames from stacking in trucks, punctured media due
to objects penetrating the shipping carton, filters that are
dropped when being handled, filters stored incorrectly, and
finally filter media integrity damaged upon installation. In
order to assure that factory tested HEPA filters do not have
pin-hole “leaks” and that gasketing and frames are leak free,
in-place testing with heterogeneous (cod) dioctyl phthalate
(DOP) or suitable substitute is highly recommended. This
process introduces cold DOP upstream at a concentration of
10-29 micrograms/liter of air and utilizes a forward light
scattering photometer to scan downstream to detect leaks
greater than 0.01% of upstream concentration.
A
HEPA filter is only as good as its gasketing and framing when
installed and because all HEPA filters are used in critical
applications, in-place HEPA filter testing should be
mandatory.
Footnotes:
1
Chart 1 - NAFA Guide to Air
Filtration, 3rd Edition, pp 5.6. IEST-RP-001-86,
IEST-RP-CC001.3.
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