Filter School
Module Two

Filter School: Module Two

How air filters function: This is the second module of eight in Camfil's Filter School series about the world of air filtration technology. Module Two describes the mechanisms and principles of air filtration.

With the Filter School series, you can learn some fundamentals of filtration, get some extra education or just bring yourself up-to-date. Or if you are new to filtration, the articles will give you a “crash course” on the subject. The modules are very general in nature and just teach the basics. The subjects are described in popular terms for easy understanding. Let’s get started with Module 2.

Particles and particle filters

The atmosphere contains a complex mixture of air pollutants ranging from solid substances (particulate matter), gases and fumes, to sub- stances in liquid form (haze, fog, droplets) and radiation (see Module 1). Depending on what is to be protected, filtration is needed to remove some pollutants before the air can be used.

The composition and size of atmospheric particles vary considerably. The size varies from a few nanometres (nm) to hundreds of micrometres (μm), making filtration a challenge because of the wide range of particle sizes to deal with. Soot particles, for example, are much smaller than pollen particles.

To get a better understanding of the different sizes, let’s pretend that we take one of the smallest particles soot, for example and enlarge it to 10 millimetres. In the particle world, bigger particles, such as pollen, would have a diameter of about 100 metres and be as large as the London Eye, if placed next to soot. Or imagine putting a roulette wheel ball next to the Globe Arena in Stockholm. Visually, this illustrates the wide range of particle sizes that a particle filter must capture. Keep also in mind that particles come in different shapes and have varying properties.

A filter’s ability to collect particles depends on various physical phenomena, both mechanical and electrical. So how does an air filter function? The following is a brief description of the mechanisms and filtration principles that make it possible to capture a particle or gas.

Filter mechanisms and principles

Different types of filters employ a variety of mechanisms to trap particles. One ordinary type of filter uses filter material (“media”) made of fibre. The most common use fibreglass or polymer fibres. The polymer fibre version is often electrostatically charged.

There are quite a few theoretical and experimental studies on air filtration with fibrous media. A filter comprised of fibres utilises several mechanisms to collect particles, which are described in this article. The overall filtration process – the sum of several different mechanisms – is very complicated. A simplified model is often used to calculate the theoretical particle efficiency of a single fibre (the “single-fibre efficiency theory”).

Gravitational Settling 
Large particles clearly tend to fall to the ground: the bigger the particle, the faster it falls. In the context of filtration, this means that large particles precipitate toward the floor and horizontal surfaces. Most particles trapped by gravitational settling are collected before the filter. Gravitational settling functions in all filters and removes coarse particles.

Total particle efficiency

Particle efficiency is therefore the total result of the various filtration mechanisms. Gravitational settling, straining and inertial impaction will have a greater effect on large particles, while thediffusion effect increases with smaller particles. As a consequence, it is most difficult to filter a specific particle size. Depending on the air velocity and filter media, the particle size 0.1-0.3 μm is the most difficult to collect in a filter. This is called MPPS, standing for Most Penetrating Particle Size.

An interesting detail: with the exception of electrostatic attraction, filtration to remove the standard reference particle in Europe according to EN 779 – 0.4 μm – is not affected by air velocitybecause interception is the main mechanism for collecting this size of particle. Filtration becomes more effective as the velocity increases for particles larger than 0.4 μm but increases at lowervelocities when collecting particles smaller than 0.4 μm.

Gas and molecular filters

We are fascinated by airborne particles and we can remove all atmospheric particles from the air, regardless of their number, size, shape and properties. But gases and molecules pass rightthrough the very best air filters. Molecules are 1,000 to 10,000 times smaller than particles and usually exist in much higher concentrations.

How can we remove these incredibly tiny molecules?

– by simply using the laws that govern the behaviour of gas molecules. For example, gas molecules do not tolerate being in different concentrations in the same space – they try to even out the concentration. When gas molecules strike an adsorbent, such as activated carbon, which has a very large surface, the gas molecules will diffuse (even out the gas concentration) by seeking out the carbon and fastening to its surface.

*) Van der Waals force is the sum of the attractive or repulsive forces between molecules.

**) Brownian motion is a mathematical model used to describe how particles collide with other particles and move in different velocities, in different random directions.