About Air Filtration

Air is essential for the survival of mankind. Only slight changes in its combination will make it useless for humans and animals. While being exposed to atmospheric pressure, the intake of air containing less than 12 percent oxygen as well as more than five percent carbon dioxide is dangerous to living beings. Over a longer period of time, changes in the mixture of air might have a profound impact on health.
Humans use up about 30 liters of oxygen per hour. Hence, our need for air is likewise small: 150 liter/h or 0.15 m³/h. However, because we also produce carbon dioxide our body requires about 5 m³/h of fresh air in order to keep the amount of carbon dioxide below a life-threatening level.  When installing an air conditioning (AC) system it is sufficient to determine the amount of air needed. It will usually be set at 15 - 20 m³ per person and hour. However, larger volumes of air might be necessary for managing warmth and cold or draw off polluted air.

Keeping the air clean of dust and aerosols is not only important for maintaining buildings and their interior, it also guarantees healthy inhabitants and their well being.

How filters work

Air filter are used to extract particles out of airflows entering ventilation systems as well as contaminated or polluted air, e.g. in nuclear power plants, laboratories and isolation wards. Lately, air filter are also commonly found in industrial processes, in high-tech areas, or other applications with low occurrences of dust. Not considering carbon filters – which are used to absorb gas and odor – filters are classified as follows: Coarse dust filter, fine dust and high efficiency filter, HEPA-, and ULPA-filter. They are classified due to their filter media, it‘s particle holding capacity, as well as the effect used for absorbing – hence, due to their area of application. Furthermore, there are electro filters, which work on the electrostatic level. However, these filters play only a minor part in the field of ventilation technology due to reasons of safety and costs of ownership.

Here we concentrate on fibre based filters because most of our products base on such. The filter's ability to retain particles depends on physical and mechanical characteristics such as diffusion, interception, impaction, and filter effect. Electrostatic effects between particles and fibers are also significant.

Interception Effect: Small, light elements are able to be carried past the fiber by the airflow. If the particle‘s center gets closer to the fiber than the particle‘s diameter [Dp], it gets caught and sticks to the fiber.
The speed of the air stream has no effect on interception as long as it doesn‘t change the fiber‘s shape. The bigger the particle, the smaller the fiber and the gap between them, the more effectively interception works. Meaning: The filter media should contain lots of small fibers of the same diameter as the particle to be adhered.

 

Diffusion Effect: Particles below 1 µm in size don‘t follow the airflow past the fibers. They are influenced by the Brownian motion: Molecules in the air make these small particles obtain a zigzag motion. When touching the fibers they will adhere to it. The possibility that these particles attach themselves to a fiber increases with a decreasing amount of speed and decreasing particle and fiber size.

Screening Effect: Particles that are bigger than the passage between two fibers are blocked by them.

Impaction Effect: Heavier particles‘ moment of inertia is too big for them to follow the airflow running around the fiber.  Those particles keep following their original path and therefore impact the fiber on it‘s air side. Inertia increases with the speed of the airflow, particle size and a decreasing fiber size.

 

Electrostatic Effect: Electrostatic fields are installed as plate condenser as an active filtration element. Alternatively, they can be preloaded onto the fibers of synthetic filter media. Thus, around the fibers or collectors respectivly an electric field will form, which will attract complementary charged particles. Precharged electrostatic within the fibers will abate after the filter’s initiation.
External influences may benefit or weaken this effect.

 

A fibre filters efficiency is depending on all these effects. There is a particle size range beteen 0,1 to 0,3 µm where, depending upon the specific filtermedia, the combination of these effects work worst. Such particles are to big for true diffusion effect but too lightweight and small to be filtered by impaction, screening or interception. This particle size is called MPPS (Most Penetrating Particle Size). High performance filters such as EPA, HEPA and ULPA filters have their efficiency stated in this particle range determining it as the absolute minimum efficiency. So particles bigger and / or smaller than MPPS are filtered with higher efficiency rates. Filters for EN779 are rated at particle size 0,4 µm because this particle size is reflecting the most given size spectrum for naturally generated particles. Nowardays the unnatural particles in smaller size generated by diesel engines etc. dominate in the overall spec.

 

Filtration efficiency & Classification of Filters

Due to various precipitation efficiencies and areas of application, diverse testing methods are being applied to different filter:

 

Filter Class EN 779	Average Arrestance A (%)	Example Particle	Size
G1	A < 65	Hair	20 - 200 μm
G2	65 < A < 80
G3	80 < A < 90
G4	90 < A	Pollen	10 - 100 μm

 

Filter Class EN 779	Average Efficiency(%)	Example Particle	Size
M5	40 < E < 60	Pollen	10 - 100 μm
M6	60 < E < 80	Spores	10 - 25 μm
F7	80 < E < 90 / M.E.: 35%	Toner	5 - 20 μm
F8	90 < E < 95 / M.E.: 55 %
F9	95 < E         / M.E.: 70 %

 

Filter Class EN 1822	Integral Efficiency MPPS	Example Particle	Size
E10	85 < h	Toner	5 - 20 μm
E11	95 < h	Oilfog	0,3 - 5 μm
E12	99,5 < h	Bacteria	0,2 - 25 μm
H13	99,95 < h	Tobacco smoke	0,01 - 1 μm
H14	99,995 < h

 

Filter Class EN 1822	Integral Efficiency MPPS	Example Particle	Size
U15	99,9995 < h	Tobacco smoke	0,01 - 1 μm
U16	99,99995 < h	Virus	0,002 - 0,05 μm
U17	99,999995 < h