How HEPA and Activated-Carbon Filters Work

1. Interception - the principal filtration mechanism

First, it is important to understand that dust, TVOC, formaldehyde, solids, liquids, bacteria, and pollen are treated alike by a filter whenever they can be classified as particles: they are intercepted.

Fibers are arranged in a complex pattern within the filter layer, creating countless openings. When a particle of a given size travels along an airflow streamline - or moves under inertia, random Brownian motion, or a field force - and approaches a fiber surface, it is intercepted and deposited if the distance from the streamline, which is also the particle's centerline, to the fiber surface is equal to or less than the particle's radius. This is called the interception effect. The sieving effect is one form of interception.

2. Inertial effect

As dust-laden air passes through a HEPA fiber layer, the complex arrangement of the fibers forces the airflow streamlines to make repeated sharp turns. When a particle has sufficient mass or speed, which may be treated as the airflow speed, its inertia prevents it from following the streamline around a fiber in time. It therefore leaves the streamline, approaches the fiber, collides with it, and is deposited. If inertia carries the particle not directly into the fiber surface but into the range of the interception effect, the particle is captured by the combined action of both mechanisms.

3. Diffusion effect

Collisions caused by the thermal motion of gas molecules produce Brownian motion in particles, and the effect becomes more pronounced as particle size decreases. At room temperature, this motion can span distances several to dozens of times the spacing between fibers, giving small particles more opportunities to reach and settle on fiber surfaces. For particles larger than 0.3 micrometers, Brownian motion weakens and is generally insufficient to move them away from the streamline and into a fiber.

4. Gravitational effect

As particles pass through the fiber layer, gravity can make them leave the streamline and settle on a fiber. Because air takes far less than one second to pass through a fiber filter, especially a filter-paper element, a particle smaller than 0.5 micrometers passes through the layer before it can settle onto a fiber. Gravitational settling can therefore be ignored for such particles.

5. Electrostatic effect

For various reasons, including friction and the presence of positive and negative charges in the air, both fibers and particles may become electrically charged. This creates an electrostatic effect that attracts particles.