Mechanical Filtration

Mechanical filters have media that the air is forced through by a fan system. Particles in the airstream are arrested by the media. Generally, the denser the media fibers, the smaller the particle that will be captured. The particles are thrown out when the filter is replaced. The actual processes by which particles are captured are listed below.

Mechanical filters have media that the air is forced through by a fan system. Particles in the airstream are arrested by the media. Generally, the denser the media fibers, the smaller the particle that will be captured. The particles are thrown out when the filter is replaced. The actual processes by which particles are captured are listed below.

Straining: Straining is a capture mechanism that applies to all filters and all particle sizes and fibers. The particle fastens between the filter fibers and cannot penetrate through the filter media.

Inertia force: Large particles are filtered with the help of the Inertia force, impaction. Larger particles have a greater mass-moment than smaller particles, which means the particles continue to move almost linearly even though the airflow and the small particles bend to get around. When the particles collide with the filter fibers they stick and are “filtered” out from the airflow.

Interception: In order to be captured a particle must come as close to a fiber so that the distance is at most one particle radius. The particle then makes contact with the fiber and binds molecularly to this with Van der Waals force. In contrast to inertia force, the particles captured through interception are small and they, therefore, follow with the airflow until they come into contact with a fiber.

Diffusion: Very small particles do not behave like larger particles with falling velocities and the like. Instead, they move in the same way as molecules in the air (Brownian movements) and try to reach a state of equilibrium with other small particles and molecules such as acid molecules. This “wandering around” happens even when they are in the middle of a filter medium, which increases the ability to hit a fiber and bind molecularly with Van der Waals force. The filtration efficiency increases the slower the air passes through the filter media (media velocity) as the particles have more time to stick to a fiber. Interception and diffusion occur in fine filters and microfilters where high particle separation is required.

The various methods of capturing particles mean a filter can actually arrest particles smaller than the opening size of the filter media. Generally, particles with an aerodynamic diameter of around 0.3 μm are the most penetrating and capture efficiency improves above and below this diameter.
The filter test standards are different between North America (ASHRAE Standard 52.2) and Europe (ISO Standard 16890 and EN 779:2012.) The table below gives an approximate alignment of the standards.