How the mechanisms work behind a fine particle filter for respiratory protection

A fine particle filter is often compared to a sieve. If the holes in the sieve are 3 mm wide, all particles smaller than 3 mm will be allowed to pass through. However, certain fine particle filters, such as the P3 filter cartridges from Sundström, filter particles that are smaller than the smallest hole of 3 mm. Our respiratory protection specialist Jan Willem de Winter explains how that’s possible.

Electrostatic and mechanical filters

Fine particle filters are divided into electrostatic and mechanical types. The electrostatic types collect particles because the filter material has a static charge, just like a statically charged piece of PVC tubing attracts scraps of paper. The disadvantage of this filter material is that the static charge fluctuates considerably due to a whole range of influences such as temperature, humidity level, air speed, age of the filter and the quantity of dust collected. Electrostatic filter material is used in disposable masks and filter cartridges in cheaper half-face masks. Mechanical types work like a type of sieve, where they are still able to capture particles that could theoretically pass through the smallest of gauzes. To understand how that’s possible, let’s take a look at the inside of the filter.

The four principals of a mechanical filter

How a mechanical filter works is based on four principles:

A – Trapping:

If a particle is too big to pass through the fibres, it is trapped and captured. The more particles are trapped, the narrower the space between the fibres and the more efficient the filter becomes.

B – Resistance:

Depending on the shape, size and density of the particle, it has a certain resistance. The course of a small heavy particle in an air flow will change less rapidly than that of a large light particle. If an air flow changes course, the particle “flies” straight ahead and collides with the filter fibre, where it is restrained and captured.

C – Interception:

When particles pass through the gauze of the filter material, they touch the fibres of the filter material, ensuring that they are halted and captured.

D – Diffusion:

Ultra-small particles make irregular movements because they are colliding with air molecules all the time. Because they don’t follow the air flows through the filter, they collide with the fibres in the filter material and are halted and captured. This is an important mechanism that ensures that ultra-small particles such as nanoparticles are filtered.

Fijnstoffilter werking

Sixteen times better than the minimum requirement prescribed in the standard

The combinations of these mechanisms provide a particular curve with the size of particles on the horizontal axis and the filter efficiency on the vertical axis. The size of particles where the efficiency is lowest is referred to as MPPS and stands for Most Penetrating Particle Size. For our Sundström SR 510, SR 610 and SR 710 P3 filters, that equates to 0.4 µm for a particular maximum air flow. Our filters are 99.997% efficient for that size of particle, which is already over 16x better than the minimum required by the European standard for P3 filters. Larger and smaller particles are therefore filtered even more efficiently!

Sundström tests each fine particle filter for MPPS. Only when the filter efficiency satisfies their own standard of 99.997% (16x better than the standard for P3) will the filter leave the factory.

Many mechanisms are therefore involved in the filtering of fine particles. That explains why nanoparticles, for example, despite their extremely small dimensions, are nevertheless captured extremely efficiently by particular P3 filters.

Want to get to know more?

Do you want to know more about the operation of particulate filters, or can we help you with other issues regarding respiratory protection? Please contact us, call 088 – 130 6030. Or use the contact form below.

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