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Most Penetrating Particle Size (Part 2 of 2)



For every given filtration application, there exists a particle size that is the most difficult to filter or capture.  This particle size, commonly referred to as the most penetrating particle size, is least affected by filtration capture mechanisms.  You might wonder, "Why is this the case and how can you use this information to better select a filter media for your specific application?"  We’ll answer these questions in this second of a two part video series. 

Hi, my name is Ken Milam.  I’m an application engineer here at ThermoPore.  Welcome to Thermo.TV.  In this video, we’ll graphic show the contribution that our five filtration capture mechanisms have on various sized particles.  Then, we’ll reveal the likely size of your filters most penetrating particle size. Lastly, we’ll explain how this information can best be used to validate the integrity of your filtration system.

Let’s get started with a quick review of our particle capture mechanisms.  In the first part of this video series we introduced the following terms:  sieving, inertial impaction, interception, diffusion, and electrostatics.  Now let’s talk about the particles that each mechanism is able to influence.

I’ve got a graph that depicts particle capture efficiency on the “Y” axis and particle size on the “X” axis.  Both the particle size and the capture efficiency increase as you travel away from the graph’s origin.

What I’d like to do is draw a curve for each of our filtration capture mechanisms.  I’ll end up with five curves on this graph – but let’s look at each mechanism one at a time.  Let start with sieving – sieving was defined as the mechanism that captures or filters particle that are larger than the opening’s in our filter.  It’s safe to say, that this mechanism has a greater influence on larger particles – and virtually no effect on small particles – so we would draw a curve that looks like this to demonstrate the effect of sieving. 

Again, this curve graphically represents that sieving’s filtration efficiency contribution increases as a particle’s size increases.  It also suggests that its filtration contribution is minimal for particles small in size.  So let’s repeat this same process for the remained of our filtration mechanisms.

With inertial impaction, we discussed the notion that particles with mass have inertial tendencies that tend to make them unable to make their way through a filter media.  So, as you might expect, the filtration contribution made by inertial impaction is similar to sieving – i.e., larger particles with more inertia tend to get captured via inertial impaction.  So, this mechanism will have a greater influence on larger particles and less of an influence on smaller particles.

Interception was the next mechanism that we discussed.   As you probably recall, interception influences particles that escape capture by our first two mechanisms.  Yet, the particle's diameter needed to be large enough to make contact with the filter media.  Therefore, interception has more influence on larger particles and less of an influence on smaller particles.  So let’s draw our interception curve so that it’s similar to our first two - but slightly offset. 

Next we introduced diffusion.  Diffusion described capture via a particle’s Brownian motion.  For a particle’s trajectory to be influenced by other gas molecules, you can probably infer that we’re talking about a mechanism that affects very small particles.  Further, you can probably speculate that as the particle gets larger and heavier, that its Brownian motion decreases.  So, diffusion then will heavily influence small particles and it will make little to no influence on larger particles.  So our curve will look something like this.

The same can be said for electrostatics, our fifth and final capture mechanism – electrostatics tend to influence smaller sized particles more so than larger sized particles.  So, again, we can draw the fifth capture mechanism curve so that it demonstrates a higher influence on smaller sized particles. 

Now, the important concept to realize at this point in time is that these five filtration capture mechanisms are always present.  They’re always attempting to influence a particles capture.  In other words, these filtration capture mechanisms are additive, so the resulting curve that can be drawn to summarize their influence will look something like this.

Notice that there are filtration capture mechanisms that affect the smallest particles and the larger particles – it’s just that their combined contribution is smallest in the .1 micron to .5 micron region.  So now what does this mean for you?  Well, your test and evaluation of your filter’s performance should take into consideration particle sizes that stand the best chance of compromising your filtration system. 

If you filter is going to need filtration capable of filtering particles less than 10 microns, then you need to pay careful attention to the filter’s capability to function and perform when it’s being challenged by particles that range from .1 to .5 microns.  By ensuring that your filtration media is suitable for use with particles that fall into that range, then you’re effectively ensuring even better performance against particles that fall outside that range.

Well I hope this installment of Thermo.TV has helped provide you with some insight into filtration capture mechanisms and I also hope that we’ve been able to explain the science behind the existence of a Most Penetrating Particle Size.


That’s it for now – but stay on the look out for later installments of Thermo.TV.  As always, if you have any additional questions or if there are some topics that you’d like to see added to the Thermo.TV channel – give us a call or drop us a line.  For now, I’m Ken Milam saying thanks for watching this installment of Themro.TV – we’ll see you next time.

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