I had a distributor that asked about the difference in filtration for the Heavy Duty HEPA Vac and the Heavy Duty Dry Vac. Besides the obvious thing that one is an “innie” and the other is an “outie” (HEPA filter is on the inside and Dry Vac filter is on the outside), there is a difference in the level of filtration. Model 901357 HEPA media is 99.97% efficient at 0.3 micron and Model 6804 media is 95% at 0.1 micron. What does this all mean?

In the simplest term, filtration is a method of separating. 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 method of capturing the particles are done by 3 major mechanical methods:

1. Inertial Impaction – Larger particles that are captured because they cannot follow the air stream around a fiber. It would be similar to hitting a golf ball into an oak tree. The tree is mostly open with air, but that golf ball always seems to hit that one little branch. The mass of the golf ball will not allow it to change direction as it comes towards the branch. Thus, we are looking for a lost ball. Captured!
2. Direct Interception – Medium sized particles that can follow the air stream around some of the fibers. Because of the tortuous path and the inertial mass, they can contact fibers as they try to wind their way through the filter. This is similar to an extremely curvy road, and a vehicle with no brakes. As you start, you can keep on the road, but as the curves become sharper and tighter, your inertia will have you sliding off the road. Captured!
3. Diffusion – Small particles with little mass are bounced around by the air molecules until they hit a fiber. It would be similar to a little clown car traveling on icy roads with truckers. If everyone had bumper pads, the little clown car will be bounced forward and backwards, side to side, and around and around. Whoops, you are off the road. Captured!

When we take an efficiency curve by particle size of an air filter, it resembles an inverted bell. In Diagram 1 above, you notice how each filtration mechanism is used to capture the particles by size. If you look closely, you will notice that the 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. Say what? 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 forces of other molecules but not enough mass to create a large inertia passing by or around the fibers of the filter.

Now lets apply these variables to our two types of filters. With Model 901357 HEPA media, the marker is 99.97% at 0.3 micron. Model 6804 media is 95% at 0.1 micron. If we apply the inverted curve analysis like in Diagram 1 to each of the filter medias, we can estimate the efficiency curve. As you can see in Diagram 2, the 901357 HEPA is more efficient at capturing particles than the 6804. So, when would you use the Heavy Duty Dry Vac or the Heavy Duty HEPA Vac? It is dependent on your dirty application and environment. If you require very fine filtration and/or very clean exhaust air, then I would recommend the Heavy Duty HEPA Vac. For most general industry environments, the Heavy Duty Dry Vac will work just fine. If you are still not sure, you can always contact our Application Engineers at EXAIR.

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