Microbial barriers as filtration systems

Filtration theory predicts that materials which are permeable to air and gases employ three mechanisms to remove particles from the air stream:

Interception

 

 

Interception. This occurs when a filter fibre splits the air stream that a particle is following.  The particle continues on its original path and collides with the fibre. Interception is therefore a constant particle removal mechanism that is a function of the material’s fibre structure. It is independent of both the particle’s mass and its velocity.

 

 

 

 

 

Inertial Impaction

 

 Inertial Impaction. This occurs when a particle, as a result of its mass, deviates from the air stream flowing around a fibre and collides with it. The effectiveness of this method of capture is directly related to the mass of the particle and the speed of the air stream. The higher the velocity and the mass of the particle, the greater the chance of it colliding with a fibre.

 

 

 

 

 

 

Diffusion

 

Diffusion. This is the interception of a particle with a fibre as a result of random particle movement (Brownian motion) and, for some materials, electrostatic attraction. The effectiveness of this capture mechanism is inversely related to the mass of the particle and the velocity of the air stream. The lighter the particle and the slower its velocity, the greater the chance of capture.

 

 

 

 

 

 

All three of these mechanisms are in operation at all flow rates and for all particle sizes. However, larger particles moving at higher flow rates are more likely to be trapped by inertial impaction, whereas lighter ones moving at slower speeds are more likely to be caught by diffusion.

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