Tag: shop vacuums

To become more aware of our Heavy Duty HEPA Vac Systems, I am going to dive into the media and filtration efficiency.  The model 901357 HEPA filter has an efficiency level of 99.97% at 0.3 micron.  What does this mean?

In the simplest of terms, filtration is a method of separation.  With air filters, we are separating particles from air.  Particles, due to their mass and size, behave in certain ways as they pass through a fibrous media.  The three main methods for capturing particles are done by:

1.  Inertial Impaction – Generally with particles over 2 microns in size.  Large particles that are captured because they cannot pass through the openings of the media.  An example, a fly being captured by a spider web. 
2. Direct Interception – Generally with particles between 0.2 micron and 2 microns in size.  These medium-sized particles follow the air stream around filter fibers and openings.  Because of the tortuous path, the winding turns and the inertial mass of the particle causes it to contact a fiber and get captured.  An example, a sports car going too fast on a curvy road.  The momentum can push you off the edge of the road and into the trees.    
3. Diffusion – Generally particles less than 0.2 micron in size.  Small particles with very little mass that are bounced around by the air molecules.  The smaller size and weight, the more movement the particle has.  This will increase the chance of hitting a fiber and getting captured.  An example, walking home drunk and being attacked by bees.  You will randomly run everywhere and end up in the neighbor’s bushes.

In Diagram 1 above, you notice how each filtration mechanism is used to capture the particles by size.  When you combine the efficiencies of each mechanism, you will get the total efficiency of a filter.  The particle sizes in Diagram 1 are limited above the vapor range.  This inverted “Bell” curve is a typical characteristic for fibrous filters.   If you look closely, you will notice that the MPPS, (Most Penetrating Particle Size) is between 0.2 and 0.3 micron.  That means that if you go larger than 0.3 micron or smaller than 0.2 micron, the filter is more efficient.  That is correct.  With the examples above, a particle in the range of 0.2 to 0.3 micron has enough mass to resist the “bounce” force from other molecules and is light enough to minimize inertia forces, allowing it to pass by and around the fibers of the filter.

Now let’s apply these variables to our HEPA filter.  With the 901357 HEPA media, the efficiency is 99.97% at 0.3 micron.  Knowing the characteristics of a filter, we can apply the inverted curve analysis like in Diagram 1.  As you can see in Diagram 2, the 901357 HEPA efficiency curve will show the efficiency of capturing particles at different sizes.   

Of course, there are different grades of filtration, and they are classified by ISO and European standards.  The chart above shows the different types and grades.  The EXAIR HEPA filter would fall into the H13 class.  HEPA stands for High Efficiency Particulate Air filter, and ULPA stands for Ultra Low Penetrating Air filter.  You can tell by the number of 9s after the decimal to determine the downstream cleanliness.  These standards use the MPPS to test the efficiency and qualify the filtration rating.  For any application that has very fine particles and/or requires very clean exhaust air, the Heavy Duty HEPA Vac System should be recommended. If you have any questions, you can contact an Application Engineer at EXAIR.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

Photo: Reference standard EN 1822 or ISO 29463