It is a common assumption that filters struggle with very small particles. The truth is more interesting: the efficiency curve is U-shaped, and the lowest efficiency occurs at an intermediate size.
Three mechanisms, three size ranges
Diffusion. Very small particles are buffeted by Brownian motion and have a high probability of colliding with a fibre. The smaller they get, the higher the efficiency.
Interception and impaction. Coarse particles cannot follow the streamline, strike a fibre by inertia, or are simply too large to pass. The larger they get, the higher the efficiency.
The size in between. There is a range where both are weak — the most penetrating particle size (MPPS). High-efficiency filter classification is defined by the efficiency at exactly that size [1].
The practical consequence: rating a filter by its efficiency at 0.3 µm may comply with the standard, but what matters in system design is the size distribution of your own dust.
Velocity also affects efficiency
As the face velocity rises, less time is available for diffusion, so the efficiency on the fine side falls. The same filter at a higher flow rate both loses more pressure and captures less. Fitting a bigger fan is therefore not always the answer.
Modelling
The filter is represented in the flow field as a porous region, with the pressure loss versus flow curve taken from manufacturer data. Resolving the internal structure of the filter is neither necessary nor practical.
The real analysis question is usually not the filter itself but the distribution of the flow reaching it: if the air is not distributed evenly over the filter face, the high-velocity regions both clog earlier and capture less. Filter life is largely set by that distribution.
Fouling changes with time
As it clogs, the resistance rises, the flow falls and the system operating point shifts. See the fan operating point note. The analysis should be run twice, with clean and with dirty filter resistance; the design has to work in the dirty condition too.
References
- ISO 29463 / EN 1822. High efficiency filters and filter media for removing particles from air — Classification, performance testing and marking.
- Ergun, S. (1952). Fluid flow through packed columns. Chemical Engineering Progress, 48(2), 89–94.