HEPA Air Filter - (High Efficiency Particulate Air Filter) Review

Filtration Table of Contents

A HEPA or High Efficiency Particulate Air Filter is a high performance filtration device capable of removing particulate matter with minimum collection efficiency of 99.97% for a 0.3 micrometer (micron) diameter of a thermally-generated dioctylphthalate (DOP) particle. The particulate removal efficiency may also be measured with an aerosol and must have a maximum clean-filter pressure drop of  1.0 in w.g. when testing the filtration device at rated air flow capacity. Particlates 0.3 microns are the most difficult to filter from a airstream and are considered the Most Penetrating Particle Size (MPPS). Particles that are larger or smaller are filtered with even higher efficiency.

HEPA filters are typically manufactured with a mat of randomly arranged fibers and of various materials. The elements of the filter that effect performance are the fiber density, diameter, and linear thickness. The air space (effective density) between HEPA filter fibers is usually larger than 0.3 microns. HEPA filters are designed to trap pollutants and particles by trapping or sticking these elements to the filter fibers using one of the following techniques.

Interception HEPA Filter: Particulates are directed through the filtration unit in a specific flow area to come within one radius of a fiber and adhere to the filter fiber.

Impaction HEPA Filter: Particulates larger then the space between filter fibers are directed through curving contours with the air stream and are forced to embed into the filter fibers directly. The performance improves by systematically reducing the fiber separation, narrowing the airflow passage which increase air flow velocity.

Diffusion HEPA FIlter: Filtration unit has baffles or other mechanisms that agitate and increase the collision of the particulates with with gas molecules, which causes the flow to be impeded and delayed within the filter unit. Diffusion is best when particulates are below 0.1 μm diameter. Impaction and interception function best when the particulates are larger than 0.4 μm. Diffusion, interception and MPPS work best for particulates 0.3 μm.

Pleat HEPA Filter