Principles of Partical Filtration

Mechanical air filters remove particles from the air stream because particles come into contact with the surface of fibres in the filter media and adhere to the fibres. The mechanisms by which the particles come into contact with the fibres in the filter media are Straining (sieving), Interception, Diffusion, Inertial seperation and Electrostatic attraction. The first of these mechanisms applies mainly to mechanical filters and is influenced by particle size. Electrostatic filtration is obtained by charging the media as a part of the manufacturing process.

Mechanisms

There are five mechanisms by which particles are filtered from the air by air filter media. They include Straining (Sieving), Interception, Diffusion and Intertial seperation and Electrostatic attraction. Each mechanism has a certain size range where it is the dominant factor for filtration of particles. Inertial separation and interception are the dominant collection mechanisms for particles greater than 0.2 μm, and diffusion is dominant for particles less than 0.2 μm. Electrostatic attraction is obtained by charging the media as a part of the manufacturing process.


Straining

occurs when the opening between the media members (fibers, screen mesh, corrugated metal, etc.) is smaller than the particle diameter of the particle the filter is designed to capture. This principle spans across most filter designs, and is entirely related to the size of the particle, media spacing, and media density.


Inertial seperation

uses a rapid change in air direction and the principles of inertia to separate mass (particulate) from the air stream. Particles at a certain velocity tend to remain at that velocity and travel in a continuous direction. This principle is normally applied when there is a high concentration of coarse particulate, and in many cases as prefiltration mode to higher efficiency final filters.


Interception

In order to be intercepted, a particle must come within a distance from a fibre of one radius of itself. The particle thus makes contact with the fibre and becomes attached. The interception mechanism can be contrasted with the impaction mechanism in that a particle which is intercepted is smaller and its inertia is not strong enough to cause the particle to continue in a straight line. It therefore follows the air stream until it comes into contact with a fibre.


Diffusion

occurs when the random (Brownian) motion of a particle causes that particle to contact a fiber. As a particle vacates an area within the media, by attraction and capture, it creates an area of lower concentration within the media to which another particle diffuses, only to be captured itself. To enhance the possibility of this attraction, filters employing this principle operate at low media velocities and/or high concentrations of microfine fibers, glass or otherwise. The more time a particle has in the "capture zone", the greater the surface area of the collection media (fibers), the greater the chances of capture. Filter manufacturers have two distinct methods of addressing this principle — employ more square footage of fine glass-mat type media or employ less square footage of high lofted glass media.


Electrostatic attraction

Filters utilizing large diameter fibre media rely on electrostatic charges to increase their efficiency of fine particle removal. Large diameter fibre media is normally chosen due to low cost and air flow resistance. However, these filters often lose their electrostatic charge over time because the particles captured on their surface occupy charged sites, thereby neutralizing their electrostatic charge.


Mechanical and Electrostatic filters

As mechanical filters load with particles over time, their collection efficiency and pressure drop typically increase. Eventually, the increased pressure drop significantly inhibits airflow, and the filters must be replaced. For this reason, pressure drop across mechanical filters is often monitored because it indicates when to replace filters.

Conversely, electrostatic filters, which are composed of polarized fibers, may lose their collection efficiency over time or when exposed to certain chemicals, aerosols, or high, relative humidity. Pressure drop in an electrostatic filter generally increases at a slower rate than it does in a mechanical filter of similar efficiency.

Thus, unlike the mechanical filter, pressure drop for the electrostatic filter is a poor indicator of the need to change filters. When selecting an HVAC filter, you should keep these differences between mechanical and electrostatic filters in mind because they will have an impact on your filter’s performance (collection efficiency over time), as well as on maintenance requirements (change-out schedules).


IMAGE NEEDED ABOVE Fine fiber media operates under a mechanical removal principle, and fibers do not lose efficiency over time. Their initial efficiency is indistinguishable from their actual efficiency over life, providing the user with the particle removal performance they have specified.
IMAGE NEEDED ABOVECoarse/electret fibers, because of their large size, are easier and less expensive to produce. Their primary effect of particle capture requires a charge imparted on the fiber during the manufacturing process. As the charge dissipates because of particulate loading, so does the efficiency of the filter. This is a critical condition, as 99% of all particles are under 1 micron in size — the range where these types of filters suffer critical loss of efficiency.