Filtration Science

Inertial impaction

Large particles (>10 µm) following the fluid streamline cannot change direction quickly enough to navigate around fibers, impacting and being captured. Dominant mechanism for coarse particle removal.

Interception

Mid-range particles (2–10 µm) following streamlines pass close enough to fiber surfaces to make contact and be captured. Efficiency increases with lower face velocity (more time for contact) and smaller fiber diameter.

Brownian diffusion

Sub-micron particles (<1 µm) are displaced from streamlines by Brownian motion (thermal energy), increasing contact probability with fibers. Diffusion efficiency increases at lower face velocity and is the dominant capture mechanism for HEPA-class filtration.

Depth filtration vs surface filtration

Depth filtration captures particles throughout the media thickness, distributing loading and maximizing dirt holding capacity. Surface filtration captures particles at the media face, enabling easy cleaning (pulse-jet systems) but lower capacity before blinding.

Most Penetrating Particle Size (MPPS)

The particle size with minimum capture efficiency — typically 0.1–0.3 µm — where neither inertial/interception nor diffusion mechanisms are dominant. Filter efficiency curves show a minimum at MPPS. Per ISO 29463 (HEPA/ULPA), filters are rated at MPPS.

Inertial impaction dominant range

>10 µm

Interception dominant range

2–10 µm

Brownian diffusion dominant range

<1 µm

MPPS range[ISO 29463]

0.1–0.3 µm

Cellulose β₁₀(c)[ISO 16889]

2–10

Synthetic microfiber β₁₀(c)[ISO 16889]

50–200