There are four main mechanisms by which air filters work: straining, impingement, diffusion, and interception. Straining occurs when larger particles are too large to pass between the filter fibers. Impingement happens when inertia causes larger, denser particles to collide with fibers. Diffusion captures the smallest particles through Brownian motion collisions with fibers. Interception uses an electrostatic charge to pull particles towards fibers. Together, all of these mechanisms operating simultaneously determine the overall efficiency of the air filter over time.

1. The mechanics
of filtration
How does an air
filter actually work?
Leadership in Filtration

2. You know that an air filter
captures dirt particles.

3. You probably also know that dirty
air goes in one side of a filter and
clean air comes out the other.

4. But do you know what happens in
the middle part of this process –
where the air is actually cleaned?
?

5. HVAC air filtration comes down
to four basic mechanisms.
Straining Impingement Diffusion Interception

6. Let’s look at each in a little more detail…

7. Straining
First up is straining –
the simplest form of filtration.

8. Straining
The particle follows the
path of the air flow...

9. Straining
...but is too large to pass between the
media fibers and is caught by the filter.

10. Impingement
Large, high-density particles are
typically caught by impingement.

11. Impingement
As the air travels through the filter
media, it passes around the fibers...

12. Impingement
...but inertia causes the particle to separate
from the air flow and collide with the fiber.

13. Diffusion
The smallest of particles
are captured by diffusion.

14. Diffusion
These tiny particles travel along
irregular paths, similar to a gas.

15. Diffusion
This is known as Brownian Motion and
is usually separate from the air flow.

16. Diffusion
This irregularity increases the likelihood
that the particle with collide with a fiber.

17. +
+
–
Electrostatic
Lastly is interception, which is only
found in filters with a synthetic media.

18. +
+
–
Electrostatic
The particle follows the path
of the air flow...

19. +
+
–
Electrostatic
...but as it nears a fiber, an electrostatic
force pulls the particle in...

20. +
–
Electrostatic
...so that it collides with the
media and is caught.

21. Now, the great thing about these
filter mechanisms is that they do not
operate alone.

22. A filter could be employing thousands
of instances of these filtration
mechanisms at any one time.

23. The efficiency of each mechanism will
fluctuate over the life of the filter...
Efficiency
Time
ELECTROSTATIC
STRAINING,
IMPINGEMENT
& DIFFUSION
ELECTROSTATIC
STRAINING,
IMPINGEMENT
& DIFFUSION

24. Efficiency
Time
ELECTROSTATIC
STRAINING,
IMPINGEMENT
& DIFFUSION
But it’s the cumulative effect of all these
mechanisms that determines the overall
efficiency of the filter.
TOTAL EFFICIENCY

25. What’s important is combining the
mechanisms correctly...

26. ...to achieve the right level of
protection.

27. That’s why when developing new filter media,
we model the efficiencies of each mechanism
over the filter’s entire service life.

28. Typically, this is down to the individual fiber.
And often, these are nanofibers.

29. All this effort is to ensure that you
receive a product that performs just
as you expect it will.

30. Visit our website
to learn more about
the science of
filtration.
Leadership in Filtration