Here we will be discussing about different methods of particulate dust filtration. Particulate filters play an important role in maintaining the Indoor Air Quality. Particulate Filtration mainly works by the principle of Mechanical Filtration and Electrostatic Filtration.
In Mechanical Filtration the dust particles are removed by capturing it on filter media, the material that comprises the filter element. Mechanical Filtration is mainly influenced by particle size to be captured. There are Four main principles of Mechanical Filtration.
1. Straining
Straining occurs when the size of the dust particle that needs to be captured is bigger that the distance between two adjacent strains of the media fibre. Hence the dust will be trapped in between the fibres and hence filtered out. This principle is prevalent in Coarse filters like metal mesh filters. Impingement filters may be coated with adhesives to improve their bonding with dust particles. Adhesives also prevent the dust particle from being dislodged due to velocity or vibration.
2. Impingement
Impingement separation is also known as inertial separation. As air passes through a filter, the filter media fibre causes hindrance to the smooth flow of air and hence the air has to make directional changes to get through the filter media. When the air makes directional changes inside the filter media, large dust particles in the air stream will not be able to make these directional changes due to their weight, speed and size. The large particles follow their straight-line path (due to inertia) and collides with the fibres and hence gets captured. This filtration technique is used in Coarse filters and media fibres may in some cases be coated with adhesives to retain the dust particles.
3. Interception
Interception occurs when a dust particle follows the airstream, but still comes in contact with the filter fibre as it passes around it. Interception does not depend upon inertia of the particle to bring it in contact with the fibre. Interception is fibre size dependent and most effective when the size of the fibre is closest to the size of the particle. The particle is retained in the fibre by a force known as “Vander Valls forces” which is kind of adhesive force.
4. Diffusion
Diffusion explains the capture of Micro fine dust particles and is better explained by “Brownian Motion”. The very fine dust particles do not follow the air stream, but behaves and moves around like gas molecules rather than like particles. These small dust particles are bombarded by other air molecules. This erratic movement increases the probability of particles coming in contact with fibres and hence they get captured. The particle is retained in the fibre due to “Vander Valls forces”. The filters using this principle works at low operating velocity and uses high concentration of micro fibres.
2. Electrostatic Method of Filtration
In Electrostatic filtration the media fibres get a positive charge either when the air passes through the media fibres or charge is imparted to the filter fibres during the manufacturing process. This charge attracts the dust particles on to the fibres and thus the dust particle is retained in the fibre. Over a period of time the electrostatic charge starts to diminish as the fibre gets coated with dust particles and the charge starts to weaken. The dust particles act like an insulator on the filter fibre over a period of time.
(All above information & Data are derived from Book records of Filtration Standards & Other related standards, research papers and its related webpages)