Testing Fibrous High Efficiency Filters

Fibrous high efficiency filters offer the unique ability to trap both small and large-sized particles efficiently, while allowing mid-sized particles to pass through. In tests using aerosol disbursement, particles smaller than 0.1 µm and larger than 0.4 µm were effectively captured most. Medium sized particles, defined as 0.1 µm to 0.4 µm in size, were captured least efficiently.

Why? Filters are usually tested with three concepts in mind:

  1. Interception: when the particle comes in contact with a fiber and stays attached to it because the attractive forces between the fiber and the particle are stronger than the forces of disruption of the moving airstream.
  2. Impingement (Inertial Impaction): when a particle’s inertia carries it directly into a fiber.
  3. Diffusion: Zigzagging particles, especially smaller particles, diffuse outward from the airstream to interact with filter materials.

Small and large particles have diffusion and inertial properties and are captured. But medium-sized particles are not always intercepted, flowing with the air right through the filter. This is why TCFS often has to explain a counter-intuitive truth: new air filters are not as efficient as filters that have been in use for “awhile” because they have started “plugging the holes” (the gaps) between fibers.

Fibrous filter materials are arranged so that fibers are generally perpendicular to the direction of the air flow. Particles streaming into the filter may be intercepted, diffused or experience inertial impaction, but the very nature of a filter dictates that there must be spaces between the filter materials to allow air to pass through. When a filter is new, medium-sized particles that don’t bump into filter material, simply pass through. Large particles experience impingement (inertial impaction) and are trapped; while smaller ones, experience diffusion and are trapped; while medium sized particles, may get through.

Choosing the right filter is to a large degree a matter of understanding the science behind filtration. Fortunately, you don’t have to be a rocket scientist to choose the best filter for your needs. The experts at Twin City Filter have decades of experience helping customers choose filters and belts for their manufacturing, industrial, and other needs. We serve customers coast to coast and offer overnight delivery in Minnesota, Wisconsin, Iowa, South Dakota and North Dakota. Twin City Filter Service may be reached at (612) 721-4000.

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