Fundamentals of Gas Filtration:
Concept of Most Penetrating Particle Size

Understanding the ability of filter media to collect particles from a gas stream passing through it is key to successful filter design and operation.  As was discussed in the prior “One Minute Filtration” article entitled Fundamentals of Gas Filtration: Particle Capture Mechanisms, particle capture is dependent on several particle capture mechanisms.  As the gas flows through the filter media, particles are primarily removed from the gas stream via the particle collection mechanisms of diffusion, interception, inertial impaction and electrostatic deposition.  Lesser importance mechanisms include sieving and gravitational sedimentation.  The particle capturing effectiveness of each mechanism is primarily dependent on the particle size, gas velocity and physical characteristics of the filter media (e.g., filter thickness, porosity, fiber diameter, etc.). 

In actual operation, the collective action of all particle capture mechanisms leads to an overall (total) particle collection curve.  A generic filter efficiency curve is shown in the attached graph.  The graph also illustrates the degree of particle capture via each mechanism.  All filter materials exhibit the same basic total curve as illustrated.  However, the effectiveness of each mechanism and thus the resulting overall (total) degree of particle capture can vary widely depending on the design of the filter, media characteristics, and filter operation conditions.  The effect of particle size on particle capture is well documented in the scientific literature.

For particles in the vicinity of the most penetrating particle size (MPPS), the dominant particle collection mechanisms are diffusion and inception.  The combination of these two mechanisms leads to an overall filter efficiency curve that first deceases with increasing particle size, and then increases with further increases in the particle size.  Or from the viewpoint of particle penetration through the filter, the overall particle penetration curve first increases with increasing particle size, and then decreases with further increases in the particle size.   Of particular importance is maximum penetration point, which is also the point of minimum particle capture efficiency.  The corresponding particle size is referred to as the MPPS.  Still larger particles are captured via the mechanisms of inception and inertial impaction.  By understanding the efficiency in the vicinity of the MPPS, one then knows that the efficiency will be even higher for any other particle size.

Filter efficiency (particle penetration) curves for all filter materials exhibit the same basic shape as shown in the attached figure, with the level of particle capture and the location of the MPPS are dependent on the filter media and its operating conditions.  In general the location of the MPPS is typically in the range of 0.1 to 0.3 um; thus leading to the use of the traditional 0.3 um DOP test for filter efficiency. 

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