1. NAFA Guide To Air Filtration
Fourth Edition
Chapter 3
Impingement Filters
presented by the
National Air Filtration Association

2. Impingement Filters
Impingement- The process in which larger particles are removed from an air stream because of their (weight or size) inertia.
Mechanical filters
whose principal
mechanism of dust
capture is impingement.
Particles in the airstream that cannot change direction because they are so large and heavy are captured by impingement.

3. Impingement Filters
Most impingement filters are characterized by relatively high media velocities and by low pressure drops.

4. Impingement Filter Categories
Panel Filters
Washable Metal-Panel Filters
Roll Filters
Slide is self explanatory
Lint Filters

5. Panel Filters
Fixed-media air filters in which the face velocity and the media velocity are essentially the same.
Most impingement filters are panel filters, that is, they have the same square foot of media as the fame in which they are held. For example, a 24 x 24 x 2 inch metal washable filter has how many square foot of face area? (answer 4) and hence the velocity of the air traveling through the filter media is traveling at the same velocity as the face velocity (4 square foot of media in the filter). This contrasted to extended surface pleat that has 12 square foot of media. Now we must divide the velocity by the square footage in order to get the media velocity. In this example, if our cfm is 2000, the panel filter face and media velocity would be how much? (500 fpm). The pleat would also have 500 fpm face velocity but would have only how much media velocity? (answer rounded).

6. Panel Filters
Face velocity-the speed of the air as it approaches the filter face.
Media velocity-the speed of the air as it moves through the filter media.
Any questions on the difference between these two?

7. Panel Filters: Materials of Construction
Metals
Spun-glass fiber media
Synthetic Fibers
Foam

8. Metal Washable Filters
Metal washable filters are normally used with heavy duty loading of large particles and are intended to be washed. Aluminum and steel are the most frequently used materials construction however, other materials are used for special applications such as bronze for ship-board use. Metal washable filters should be used with a non-petroleum/water soluable spray-on adhesive to increase their holding capacity. They also may have several layers of graduated density media in the pack.

9. Spun-glass media Used extensively in Impingement Filters
Usually adhesive coated to improve retention
Media density varies ¼” to 5”
Can be used in pads or cardboard frames
Spun-glass panel filters have been around since the early 1950’s. They were produced to meet the need for a throwaway filter that would capture large lint and dust particles in early non-fan powered HVAC systems. Even in the 2” industrial variety, they rarely achieve more than a MERV 4 efficiency and therefore, do not meet ANSI/ASHRAE Standard 62.1 & 62.2 requirements for commercial and residential facilities of MERV 6 minimum.

10. Synthetic Fibers
Polyester- Most popular material – ring and link panels, pocket filters
Also Polypropylene, Nylon
or other synthetic
(usually meant to be
washed)
Polyester is the most popular material for synthetic throwaway filters. Polypropylene and other materials are used for “passive electrostatic” filters that are washable. Adhesives are usually added to synthetic fibers to increase their ability to hold particles onto the fiber.

11. Synthetic Fibers Fiber weight is measured in denier.
As a rule smaller denier fibers will do a better job capturing small dust particles.
Dual denier media is commonly
used. It incorporates a larger
fiber on the air entry side and
a smaller fiber on the exit side.
Synthetic fiber size is expressed in “denier” even though this is a measurement of weight, not size. For example, a 40 denier fiber is 66 µm in size while a 6 denier fiber 25 µm. Progressive density panels utilize a larger fiber on the air entry side and small fibers on the air exit side. Needling is also technique used with synthetic fibers to cause the fibers to become closer in the media.

12. Characteristics of Synthetic Fiber Media
Fiber distribution consistency is the most important consideration when making synthetic media.
Tackifiers are commonly applied to the air exit side of some synthetic medias to prevent dust migration.
Electrostatic charge is present in some types of synthetic media to aid in particle capture.

13. Foam Usually polyurethane Classified by pore size
The more pores per inch (PPI) the higher the efficiency
Can be washed and reused
Slide is self-explanatory

14. Impingement Filter Media Performance
Determined from independent filter testing
ASHRAE standard 52.1 (now obsolete)
ASHRAE standard 52.2 (MERV)
Although ANSI/ASHRAE 52.1 is still around as a standard, there are almost no labs doing this testing. ANSI/ASHRAE 52.2 has not yet incorporated the dust loading step from 52.1 into Since filters such as metal washable and fiberglass are MERV 4 or less, it is really too expensive to have these filters tested just to let the consumer know they are low efficiency. Above MERV 4, 52.2 testing protocol is used.

15. Method of Renewal
Filter banks using panel filters are renewed by washing or replacing dirty filter media.
The use of disposable
materials has become
the predominant method of renewal.

16. Filter Holding Systems
There are many different types of filter holding systems in use. Filter pad holding frames and side access housings are shown here. For other types of holding devices and systems, see the Installation, Operation and Maintenance of Air Filtration Systems text produced by NAFA.

17. Roll Filters
Devices in which clean filter media is unrolled at one end,exposed to a dirty air stream, and advanced at intervals to keep the pressure drop through the exposed air stream within a desired operating range.
Prone to many mechanical problems, automatic roll filters are an old technology that is infrequently specified today, yet many of these remain in service.

18. Roll Filters: Media Advance and Control
A motor driven system.
Manually operated chain drive.
Can be controlled by an adjustable timer.

19. Roll Filter Media
Fiberglass roll (infrequent)
Synthetic fiber rolls

20. Lint Filters Use the automatic roll filter principle. Filter material
Glass media with spun-bond backing
Thin 1,2, or 3 ply cellulose media

21. Grease Filters
Devices installed in kitchen range hoods to prevent the accumulation of grease in hood exhaust ducts.
Usually made of steel
or aluminum
Grease filters do not
aid in smoke removal.
They can be washed.
Must meet NFPA Std. 96
Grease filters are installed over range hoods. The only legal filter that can be now be used in commercial applications is the baffle-type filter, although many metal mesh grease filters may still be in use as they are grandfathered. The National Fire Prevention Association (NFPA) Standard 96 and tested under UL 1046.

22. Sample Questions
The velocity of the air through any one square foot of media in an extended surface filter is called?
Media Velocity
Having larger fibers on the air entering side and smaller fibers on the air exit side is called?
Graduated density

23. Thank you.
Other Questions