HEPA Construction:
Although HEPA filter construction
can vary considerably, most HEPA filters consist of a rigid frame
into which a filter pack, constructed by folding a 
continuous
sheet of media into closely spaced pleats, is sealed. Conventional
HEPA media is an all-glass paper which is composed of an extremely
large number of randomly oriented micro-fibers which utilizes
some fairly complex mechanical principles to achieve its effectiveness
on sub-micron particles (see principles of filtration). Because
of the density of the media, air flow is restricted and the pleating
is used to increase the amount of media (surface area) allowing
for a greater ratio of media to air flow. Over 
the
years, filter manufacturers have developed different methods
and materials for maintaining the spacing of the pleats all in
an effort to optimize media surface area and reduce air flow
resistance.
In addition to sealing the
filter pack in the frame, the filter must be sealed when it is
installed in equipment in order to prevent air flow and the sub-micron
particles it contains from by-passing the HEPA filter. Like the
formation of pleats, filter manufacturers have developed different
methods and materials to accomplish this. The use of closed cell
neoprene gaskets is perhaps the most common method of sealing
the HEPA filter installation.
Many different HEPA filter
specifications and standards have been written for the myriad
applications in which HEPA filters have been put to use. From
UL-586 with environmental testing to ASME AG-1 with seismic qualification
for nuclear use, all have been developed to ensure the suitability
of a filter for a particular use.
IEST-RP-CC001.3
The most comprehensive guidelines
for HEPA filter construction is set forth in the Institute of
Environmental Sciences and Technology Recommended Practice IEST-RP-CC001.3.
The IEST Recommended Practice specifies many of the materials
used for HEPA filter construction and describes the various types
of filter packs as differentiated by the method used to maintain
the spacing of pleats. IEST-RP-CC001.3 also defines six levels
of performance and six grades of construction that distinguish
between the types of service the filter units may experience.
For example, Type "A" filter performance uses the Mil-Std-282
penetration test with a minimum efficiency rating of 99.97% on
0.3 microns whereas Type "F" filter performance is
for ULPA filters (ultra low penetration air) with a minimum efficiency
of 99.999% at 0.1 to 0.2 microns. Construction Grades range from
Grade 1 which meets severe environmental and handling tests to
Grade 6 for noncritical or nonsafety-related applications.
IEST recommended practices
are available for purchase from the Institute of Environmental
Sciences and Technology at www.iest.org.
Other Standards:
As was mentioned earlier, there
are many HEPA filter specifications and standards. Some have
been developed by individual companies for their own use, some
are for specific applications or industries. It is also common
to find some specifications or standards misapplied to a HEPA
filter or HEPA filtration, in general. Following is a list of
specifications or standards often found associated with HEPA
filters used in vacuum cleaners and a brief description of its
relevance.
ANSI Z9.2 - Fundamentals
Governing the Design and Operation of Local Exhaust Systems. Although this standard is often applied
to a vacuum cleaner as an exhaust system it does not specify
any HEPA filtration efficiency, construction or test requirements.
C-vac replacement HEPA filters will maintain the proper compliance
of any vacuum operating within the parameters of this standard.
ASME N509 - Nuclear Power
Plant Air-Cleaning Units and Components.
ASME N510 - Testing of Nuclear
Air Treatment Systems.
ASME NQA-1 - Quality Assurance
Requirements for Nuclear Facility Applications.
ASME AG-1 - Code on Nuclear
Air and Gas Treatment.
Written for nuclear power plants and nulear fuel cycle facilities,
this code includes standards for design, fabrication, inspection,
and testing of air cleaning equipment and components. Although
special filters can be manufactured to this code, standard HEPA
filters used in most vacuum cleaners do not meet this code and
the particular configuration of the filter may present other
specific limitations.
Mil-F-51068 - Filters, Particulate
(High-Efficiency Fire Resistant).
A military specification approved for use by all Departments
and Agencies of the Department of Defense, this specification
covers eight sizes and types of HEPA filters which must first
be qualified to be furnished under this specification. Although
some HEPA filters used in vacuum cleaners can be manufactured
with the materials and testing required by this specification,
there are no cylindrical filters included in its scope and most
standard HEPA filters used in vacuum cleaners do not comply.
Mil-F-51079 - Filter, Medium,
Fire-Resistant, High-Efficiency.
A military specification covering one grade of high-efficiency,
fire-resistant, filter medium normally used for compliance with
Mil-F-51068.
UL-586 - Standard for High-Efficiency,
Particulate Air Filter Units.
An Underwriters Laboratories standard for HEPA filters including
construction and environmental test requirements where samples
are tested for minimum performace characteristics. Filters employing
different materials or forms of construction from those detailed
may be determined to be in compliance with the standard if substantially
equivalent. Filters listed by UL under this standard may carry
a UL-586 label to identify compliance.
UL-900 - Test Performance
of Air Filter Units.
Underwriters Laboratories classifies air filters submitted
for testing as to flammability only. This standard investigates
the combustibility and smoke generation of a filter, not the
ability of a filter to remove airborne particles.
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