Most HEPA units are used in the open-face
configuration. When used in this manner, the filter is
secured firmly to a rigid framework by a pressure device such
that a leak-free seal exists between the unit and the
framework. The HEPA filter may also be placed completely
within an enclosing casing that is equipped with pipe access
at both ends for attachment to existing ventilation ducts.
Enclosing casings may be metal or plywood, but care must be
taken to ensure the casing material is compatible with
Underwriters Laboratories, Inc., (UL) requirements for
resistance of the filter to heated air and flame. The
enclosing casing forms the leak free pressure boundary in
addition to the filter case, and care must be taken to ensure
that it is treated as an encapsulated design for both
performance and leak-acceptance testing. Enclosed HEPA units
have significantly higher resistance to airflow than the
open-faced design because of the added restrictions of the
duct transitions.
Enclosed filters are sometimes referred to as
encapsulated (pipe-connected, closed-face, or self-contained)
HEPA filters. They are not recognized by applicable codes
(i.e., AG-12) and standards and fail to meet all the
requirements contained in DOE Standard DOE-STD-3020-976. The
most serious deficiency is failure to meet the requirement
for uniform velocity across the filter face. This can
invalidate the in-place filter leak test.
