Understanding how air filters work is beneficial when it comes to understanding your air mapping and air handling requirements.
How Air Filters Work
An air filters ability to retain particles depends on its physical and mechanical characteristics. Electrostatic effects between particles and fibres are also significant.
The five main effects which are used to filter particles from air are described here.
1. INTERCEPTION EFFECT
Small, light elements are able to be carried past the fibre by the airflow. If the particle‘s centre gets closer to the fibre than the particle‘s diameter [Dp], it gets caught and sticks to the finer. The speed of the air stream has no effect on interception as long as it doesn‘t change the fibre’s shape. The bigger the particle, the smaller the fibre and the gap between them, the more effectively interception works. Meaning: The filter media should contain lots of small fibres of the same diameter as the particle to be adhered.
2. DIFFUSION EFFECT
Particles below 1 µm in size don‘t follow the airflow past the fibres. They are influenced by the Brownian motion: Molecules in the air make these small particles obtain a zigzag motion. When touching the fibres they will adhere to it. The possibility that these particles attach themselves to a fibre increases with a decreasing amount of speed and decreasing particle and fibre size.
3. SCREENING EFFECT
Particles that are bigger than the passage between two fibres are blocked by them.
4. IMPACTION EFFECT
Heavier particles‘ moment of inertia is too big for them to follow the airflow running around the fibre. Those particles keep following their original path and therefore impact the fibre on it‘s air side. Inertia increases with the speed of the airflow, particle size and a decreasing fibre size.
5. ELECTROSTATIC EFFECT
Electrostatic fields are installed as plate condenser as an active filtration element. Alternatively, they can be preloaded onto the fibres of synthetic air filter media. Consequently, an electric field will form around the fibres or collectors , which as a result will attract complementary charged particles. Precharged electrostatic within the fibres will abate after the air filter’s initiation. External influences may benefit or weaken this effect. Heavier particles‘ moment of inertia is too big for them to follow the airflow running around the fibre. Those particles keep following their original path and therefore impact the fibre on it‘s air side. Inertia increases with the speed of the airflow, particle size and a decreasing fibre size.