How Filter Captured Particles ?

*HEPA and ULPA filter collect particles by five different mechanisms: sieving, impaction, impingement, diffusion and electrostatic attraction. Sieving, impaction, and impingement collect very large particles while diffusion and electrostatic attraction capture very small particles.

Sieving: particles that are too large to fit between fibers are trapped on the surface of the filter media by the processes of sieving. As particle size decreases impaction and impingement works to capture particles

In impaction particles have too much inertia to follow the air movement around the fiber. The particles crosses streamlines and strikes the surface of the fiber. When the particle touches the surface of the fiber, it is captured by van der Waals forces.

In impingement, particles follow the streamlines. If the streamline approaches within one particle radius of the fiber, the particle touches the fiber and is collected by van der Waals forces.

In diffusion, the particles are so small that they are not bombarded uniformly on all sides by collisions with air molecules. The collisions that are not canceled out by collisions in the opposite direction impart motion to the particles superimposed on the bulk motion of the volume of air carrying the particle. This results in random motion (Brownian motion) of the particle around the primary streamline defining the particle’s average path. The smaller the particle, the greater the tendency to take the off-centerline walk or diffuse and be captured by van der Waals attraction when the particle touches the surface of a fiber.

Nearly all naturally generated particles are charged either positively or negatively and nearly all have some imbalance of electrical charge. The charged particles become attracted to charges on the surface of filter fibers. The charges on the fibers can occur because fibers are excellent collectors of ions from the air. In fact, because of the very small diameter of air ions, they are collected very efficiently by HEPA and ULPA filters. Charges on the surface of filter fibers attract oppositely charges particles. Electrostatic attraction is very strong for small particles but decreases with increasing particle size.

Particle filtration by electrostatic attraction and diffusion is effective for very small particles, those less than about 0.01 micrometers in diameter, but decreases as particle size increases. Impaction and impingement are effective for collection of large particles, those above 10 micrometers in diameter, but decrease as particle size decreases. Therefore, these mechanisms must combine to produce an overall filtration efficiency curve that has an observed minimum somewhere between 0.1 and 1.0 micrometers in diameter. This minimum in particle collection efficiency occurs at the maximum penetrating particle size. This is the size at which HEPA and ULPA filters are rated.

**Understanding How HEPA and ULPA Filters Capture Particles**

HEPA (High-Efficiency Particulate Air) and ULPA (Ultra-Low Penetration Air) filters are designed to trap airborne particles using five primary mechanisms: sieving, impaction, impingement, diffusion, and electrostatic attraction. Each of these methods targets different particle sizes, ensuring a broad range of contaminants are effectively captured.

The sieving process captures larger particles that are too big to pass through the gaps between the filter fibers. As particles get smaller, impaction and impingement come into play. During impaction, particles with significant inertia cannot follow the airstream around the fibers. Instead, they collide with the fiber surface and are held there by van der Waals forces. Impingement, on the other hand, occurs when particles closely follow the airstream. If the path brings them within one particle radius of a fiber, they touch the fiber and are also secured by van der Waals forces.

Diffusion becomes the key mechanism for capturing extremely small particles. These tiny particles experience random motion, known as Brownian motion, due to uneven collisions with air molecules. This random motion increases the likelihood that these particles will meet and adhere to the filter fibers.

Electrostatic attraction plays a crucial role, especially for small particles. Most airborne particles carry a slight electrical charge, and the fibers in HEPA and ULPA filters can accumulate ions from the air, creating opposite charges. When a charged particle comes near a fiber with an opposite charge, it is drawn to and captured by the fiber. This mechanism is particularly effective for particles smaller than 0.01 micrometers.

Each of these filtration mechanisms has a range of particle sizes it is most effective against. Impaction and impingement work best for larger particles, generally those above 10 micrometers, while diffusion and electrostatic attraction excel with smaller particles under 0.01 micrometers. The combined effect of these mechanisms results in an overall filtration efficiency, with the least effective filtration occurring for particles between 0.1 and 1.0 micrometers in size. This range, where the filtration efficiency is at its minimum, is also the basis for the rating of HEPA and ULPA filters.