ASHRAE Journal - May 2013 - 84

ENGINEER’S NOTEBOOK
HEPA or ULPA Filtration

Air-Distribution Design
By David A John, P.E., Member ASHRAE

hanges made recently to critical environments standards affect the
way that design engineers specify and lay out air-distribution systems

in operating rooms, cleanrooms, and more. The changes target improvements in safety and environmental quality. This column explores changes to
cleanroom classification and the role the Institute of Environmental Sciences
and Technology (IEST) plays in the classification of HEPA and ULPA filters.
Cleanroom Class Numbering

Our industry commonly classifies
cleanrooms by class number, such as
“Class 100” or “Class 10,000” spaces.
The class number was deﬁned by the
General Services Administration (GSA)
standard FED-STD-209E with the class
number denoting the number of particles
0.5 µm or larger permitted per cubic
foot (cubic meter) of air. Though still
widely in use, this standard was ofﬁcially
cancelled by the GSA in 2001, based on
a recommendation by IEST Working
Group CC100.
Instead, the IEST, assigned by the
GSA as the Preparing Activity organization for FED-STD-209E, recommended that ISO 14644, Clean Rooms
and Controlled Environments (14644-1:
Classiﬁcation of Air Cleanliness, and
14644-2: Speciﬁcations for Testing and
Monitoring to Prove Continued Compliance with ISO 14644-1), supersede
FED-STD-209E (Table 1). ISO 146441 and 14644-2 are parts of a group of
standards developed by ISO Technical
Committee 209, Cleanrooms and Associated Controlled Environments.

MERV and Critical Environments
The Minimum Efficiency Reporting
Value (MERV) is a measure used to describe the efﬁciency with which particulate
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ASHRAE Journal

ﬁlters remove particles of a speciﬁed size
from an airstream. The higher the MERV
designation, the better the efﬁciency of
removal, particularly for smaller particles.
ASHRAE Standard 52.2, Method of
Testing General Ventilation Air-Cleaning
Devices for Removal Efﬁciency by Particle
Size deﬁnes MERV 1 through 16, while
MERV 17 through 20 (HEPA and ULPA
ﬁlters) is in the purview of IEST Recommended Practices IEST-RP-CC001.5 and
IEST-RP-CC034.3 (Table 2).

MERV 1–16 and Standard 52.2-2012
MERV Levels 1 through 16 are determined using Standard 52.2-2012, which
establishes a test procedure for evaluating
the performance of air-cleaning devices
as a function of particle size. The 2012
revision to Standard 52.2 combined 52.1
and 52.2 with the objective of improving
the technical accuracy of ﬁlter testing.
By combining the two standards, the
Standard 52.2 committee plans to make
future changes that will make testing
more technically accurate, change the
MERV table ranges, narrow the ambient
conditions allowed and further refine
the instrumentation speciﬁcations with
a goal of reducing the variability of the
data produced when using the standard.
MERV should be stated with the air
velocity at which the ﬁlter was tested. For
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example, if the ﬁlter was tested with an air
velocity of 492 fpm (2.50 m/s) and was
found to be MERV 8, the ﬁlter’s MERV
would be MERV 8 at a velocity of 492 fpm.
Standard 52.2-2012 tests are to be
conducted at one of seven airﬂow rates,
which are found in Section 8.1.1 of the
standard.

MERV 17–20
Standard 52.2-2012 does not address
high efﬁciency particulate air (HEPA)
ﬁlters or ultra low penetration air (ULPA)
ﬁlters (MERV 17 through 20), and they
are assigned MERV based on their
performance in accordance with IEST
Recommended Practices (RP).
IEST Recommended Practice
The testing of HEPA and ULPA (MERV
17 through 20) filters is included in
the Recommended Practice IEST-RPCC001.5, HEPA and ULPA Filters. Its
scope states that the RP covers basic provisions for HEPA and ULPA ﬁlter units.
Filters that meet the requirements of this
RP are suitable for use in clean air devices
and cleanrooms that fall within the scope
of ISO 14644 and for use in supply air and
contaminated exhaust systems that require
extremely high ﬁlter efﬁciency (99.97% or
higher) for submicrometer (µm) particles.
The RP describes 11 levels of ﬁlter performance and six grades of ﬁlter construction and recommends that a speciﬁcation
include the level of performance and
grade of construction required. It further
recommends that the speciﬁcation should
include the ﬁlter efﬁciency required.
HEPA and ULPA Filter Leak Tests
HEPA and ULPA f ilters are leaktested using the descriptions in IESTRP-CC034.3, HEPA and ULPA Filter
Leak Tests. Its scope states this RP covers
deﬁnitions, equipment, and procedures
May 2013

ASHRAE Journal - May 2013

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