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Most Penetrating Particle Size: Filtration Capture Mechanisms



Are you in the midst of properly sizing a filter media for your air or liquid filtration application?  Do you know which particle size stands the best chance of compromising your system by making its way through your filter?  That particle is referred to as the most penetrating particle size and it can create havoc if you’re not careful.  But, if you’re aware of the most penetrating particle size then you can design and optimize your filter’s performance for hassle free operation.

Hi, my name is Ken Milam.  I’m an application engineer here at ThermoPore.  Welcome to Thermo.TV.  In this two part series, we’ll explain why most penetrating particle size is neither the smallest nor the largest particle that will be challenging your filtration media.  To do this, we’ll start by defining and discussing the five basic particle capture mechanisms.  In the second part of this series, we’ll graphically illustrate the role that each mechanism plays on various sized particles and we’ll wrap up the video by explaining why you should use the most penetrating particle size to evaluate the integrity of your filtration system.

Okay, so let’s get started.  In this video segment, we’ll be discussing five basic filtration capture mechanisms that are always acting on a particle as it attempts to follow an air stream through a filter media.  Those mechanisms are sieving, inertial impaction, interception, diffusion, and electrostatic effects.  We’ll delve into each mechanism in more detail later, but for now, recognize that every time a fluid stream (either an air stream or a gas stream) makes its way through a filter or around a filter’s fibril, these five filtration mechanisms collectively work together in an attempt to force the particle into contact with the filter.  Once in contact with the filter, attractive forces (or as I like to think of them, sticky forces) keep the particle from being reintroduced into the fluid stream.  IN other words, filtration occurs when the challenging particles come into contact and subsequently stick to the filter. So, with this in mind, let’s take a look at each mechanism in closer detail.

Let’s start with sieving.  Likely the easiest mechanism to envision – sieving is the easiest to explain.  Sieving occurs when a particle’s size (its diameter) is larger than a filter’s opening size.  As the particle attempts to make its way through the filter, the particle simply gets lodged into an opening that is smaller than its own diameter.  Sieving can occur on the surface of a filter or within the filter’s depth.  In any event, when this occurs, the particle will not be able to travel through the filter.  So sieving describes our first filtration mechanism.

Our second mechanism is inertial impaction.  Inertial impaction is perhaps the second easiest mechanism to envision.  Here’s why.  An airstream has an ability to change its course of direction to move around or through a filter’s three dimension structure because it has very little mass.  Particle with low mass also have low inertia (inertia can be thought of as the amount of force that it takes to alter the course of a traveling object – objects with low inertial values are easy to bump and move around.  Conversely, objects with high inertial values are more difficult to influence).  A dirt particle trying to make its way through a filter system will have mass, and as a result it will also have inertia.  As we said before, particles with high inertial values do not like to change directions.  Or, particles with high inertial values are unable to bob and weave their way through a filter media on account of their high inertia states – therefore, they end up colliding with the filter 3D structure.  They simply aren’t nimble enough to make all the turns and directional changes that are required to navigate through the filter media.  And, as we said before, once they come into contact with the filter media, they become captured…captured by way of our second filtration mechanism, inertial impaction. 

So let’s move onto our third filtration mechanism, interception.  Interception affects 1) particles that are small enough to make their way through the filter’s openings, and 2) particles that have low enough inertia levels to move with the air stream.  In other words, they can bob and weave through a filter’s tortuous path with the air.  However, because of their size (or diameter) they come into contact with the filter.  In other words, imagine a particle that gets caught up in a slip stream that travels too close to the filter’s media.  As a result of this slip stream’s close proximity the filter media, the particle gets captured.  Interception is different from inertial impaction because the particles that are captured via interception have the ability to initially move with the flow steam around a filter – it’s just that they end up choosing a slip stream that travels a distance less than half the radius of the particle.  These particles become captured via our third filtration mechanism – interception.

The fourth mechanism at play is diffusion.  Diffusion results when particles are subjected to Brownian motion.  What is Brownian motion?  Well, it turns out that very, very small particles have a difficult time traveling in a straight line.  Why is this? Well, very small particles get hit and bumped and collide with other gas molecules.  Instead of traveling from point “A” to point “B” in a straight line, the travel path of very small particles is transformed into a zigzag motion.  Fortunately for us, diffusion makes very small particles more susceptible to capture, again by way of our fourth mechanism, diffusion.

The fifth mechanism at play is Electrostatic effects.  Fairly simple to envision, opposite charges are attracted to one another and you’ve probably seen two magnets move towards one another as a result of opposite magnetic charge.  Electrostatics effects are very similar because many particles that are attempting to make their way through the filter also carry charges.  So, if a filter’s charge is opposite the particle’s charge, the attractive forces can alter the path of a particle trying to make its way through a filter media. When a particle’s path gets altered and subsequently captured as a result of an attractive charge, it becomes captured by way of our fifth, and final filtration mechanism, electrostatics.

So, this concludes part one of our two part video tutorial.  Tune in to Thermo.TV for the second part of  the Most Penetration Particle Size tutorial to find out what particle size stands the best chance of making its way through a filter media despite the presence of the these five filtration mechanisms.

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