It seems logical that a filter finds the smallest particles the hardest to stop. For borosilicate glass microfibre media that is not true. Particles well below 0.1 µm are captured very efficiently — and so are particles above 1 µm. The weak spot lies in between, at around 0.3 µm. This size range is called the most penetrating particle size, or MPPS.
Two families of capture mechanisms
Larger particles and droplets carry enough momentum to leave the gas streamlines. Above about 1 µm they hit fibres by inertial impaction; between roughly 0.3 and 1 µm they follow the gas flow but pass close enough to a fibre to touch it — direct interception, which happens when a particle comes within half its diameter of a fibre. Both mechanisms become weaker as particles get smaller.
Very small particles behave differently. Below about 0.3 µm their mass is so small that they no longer follow the conventional laws of moving bodies. They move randomly in all directions — Brownian motion — and this random path greatly increases the chance of contact with a fibre. The smaller the particle, the stronger its motion and the higher its chance of capture.
Why test methods use 0.3–0.6 µm
Because the MPPS is the worst case, the standard test methods for high-efficiency filters — the DOP test and the sodium flame test — challenge filters with particles in the 0.3 to 0.6 µm range. They are designed to rate a filter under its most severe conditions. A filter that performs well here performs better both above and below this range.
| Particle size | Dominant mechanism | Capture tendency |
|---|---|---|
| 20–50 µm | Gravitational settling (at low velocity) | High |
| > 1 µm | Inertial impaction | High, rises with size and velocity |
| 0.3–1 µm | Direct interception | Medium |
| ≈ 0.3 µm | Transition zone (MPPS) | Lowest |
| < 0.3 µm | Diffusion / Brownian motion | High, rises as size falls |
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Velocity moves the balance
The size ranges are not fixed. When gas velocity increases, gravitational settling becomes less important for large particles — they stay suspended — while inertial impaction becomes more effective. Diffusion, in contrast, needs residence time: at higher media velocity, small particles have less time to wander onto a fibre. This is one reason why a housing that is too small for the actual flow can lose efficiency, and why we size housings on actual flow at operating conditions (see sizing filters for high-pressure gas).
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- Larger particles and droplets carry enough momentum to leave the gas streamlines.
- Because the MPPS is the worst case, the standard test methods for high-efficiency filters — the DOP test and the sodium flame test — challenge filters with particles in the 0.
What this means for element selection
- Fine oil aerosol from compressors sits largely in the sub-micron range — choose a final coalescer such as RF-C-51230-HE.
- High-purity duties (analysers, instrument gas, semiconductor) use UX or AX grades as the final stage.
- Heavy aerosol loads are split: an ST or PF pre-coalescer first, then HE — this protects the fine stage and keeps differential pressure low.
- Dry particles are handled by RF-P particulate elements on the same microfibre principle.
The full background is summarised in our Technical Note "Why borosilicate glass microfibre?" (PDF); all element and housing datasheets are in the download library.
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