Enclosed HEPA Filter Review and Application

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.