1. NAFA Guide To Air Filtration
Fifth Edition
Chapter 4
Extended Surface Filters
presented by the
National Air Filtration Association

2. Extended Surface Filters
The most common method of increasing media area is to pleat the filter media to extend the filter depth.
Extended surface filters have a media area that is greater than the filter face area.
Media velocity is less than face velocity.
Slide is self-explanatory

3. Remember - Formula for Figuring Media Velocity?
Q = VA
V = Q/A
A = Q/V
Where: Q = cubic feet per minute (cfm)
V = velocity in feet per minute (fpm)
A = Area in square feet
Remember the formula for figuring media velocity? (re-explain) Extended surface filter media velocity is always less than the face velocity because we are expanding the “A” in the formula.

4. Extended Surface Filters Are Desirable Because:
Use Interception and Diffusion Principles
Media performs more efficiently at lower media velocities.
Media resistance too high if used as a flat sheet.
Media resistance is, as a general rule, directly proportional to the velocity of air through it.
Extended filter service life
Here are the reasons why this is a good thing…..

5. Types of Extended Surface Filters
Extended surface filters are characterized by the type of media used and the configuration into which it is formed.
Pleat Pocket Cell
We describe extended surface filters by the type of media found in the filter and their design style

6. Types of Media
Cotton/Polyester or polyester - flat media used in pleated panel filters
Lofted Polyester Media – same polyester material found in rolls, rings & links, configured into extended surface filter
Synthetic Fiber Mats – spunbond and meltblown manufactured fibers
Lofted Microglass Media – composed of fine microglass fibers
Wet Laid Microglass – made similar to cellulose paper
Pleated filters usually have a cotton/polyester blended media or just a synthetic media. You could put other types of media in a pleat but the pressure drop would exceed most unit specifications. Lofted polyester media is the same kind of media found in synthetic rolls, rings and links, except it is sewn or heat sealed into a larger extended version, sometimes called a cube filter. You may find these in paint spray operations where capture of larger particles is the goal. Synthetic fiber mats manufactured by extruding the material in a spunbond or meltblown process forms a flat sheet of synthetic media as is Lofted Microglass media composed of fine strands of glass. Lofted microglass media also is the only media color coded to a corresponding efficiency (see colors on page 4.2). Finally, Wet Laid Microglass media is manufactured very much like cellulose paper in that it is mixed into a slurry and dewatered on a screen.

7. Lofted Microglass Mat Type Media – Pocket or Rigid Box Filters
Glass mat is composed of very fine glass microfibers – see page 4.2 for fiber thickness
Requires a backing material to provide physical support.
The efficiencies of mats of different thicknesses are essentially the same.
Mats available in ¼ and ½ inch thickness.
Just a couple of notes on glass and synthetic. Lofted Microglass media fibers are extremely fine which accounts for its ability to capture the smallest of particles. But because it has very little physical strength, it is usually bound to a backing material usually a synthetic scrim material to provide rigidity. It also comes in two different thicknesses – ¼ and ½ inch – with most pocket filters toady being made of the ¼ inch material. Lofted Microglass media is available in MERV efficiencies from 11 to14.

8. Calculating Pocket Filter Media Area
24” x 24” x 22”
6 Pocket
Each Pocket
24 H * 22 D * 2
= 1056 sq in /144 = 7.33 sq ft
Whole Filter
7.3 sq ft * 6 = sq ft
Media Velocity
2000 cfm / sf = 45.4 fpm
24” x 24” x 12”
10 Pocket
Each Pocket
24 H * 12 D * 2
= 576 sq in /144 = 4 sq ft
Whole Filter
4 sq ft * 10 = 40 sq ft
Media Velocity
2000 cfm / 40 sf = 50 fpm
Here are some working examples of calculating square footage of media in a pocket filter (formula on page 4.4) Highlight the division by 144 is because 144 converts square inches into square feet.

9. Standard Filter Mat Colors and Efficiencies (Lofted Microglass Media)
Yellow MERV 14
Pink MERV 13
Orange MERV 11
Tan 50-MERV 9
Synthetics have no standard color coding

10. Synthetic Fiber Mat Type Media
Available in efficiencies comparable to glass media.
Made from various synthetic materials.
Have greater physical strength than glass mats.
May be heat sealed or Sonic welded.
Synthetic fiber mats are physically stronger and usually do not need support for the material to be rigid in a pocket filter. Also, the material does not tend to fracture or tear like microglass media. The mats come in the same efficiencies as lofted microglass. Wet laid microglass media is white in color and looks just like ordinary copy paper. Upon closer inspection however, one can see the fine glass fibers that makeup the media…which comes in MERV efficiencies the from 11 to 15 (this is a misprint in the NAFA Guide as the book fails to account for 95% DOP and 98% 52.2 paper.

11. Style and Media
Pleated filter-cotton/polyester blended or all synthetic-rarely
microglass
Pocket filters - lofted
microglass or synthetic
This is a review of the styles and media…

12. Style and Media Rigid box type-lofted microglass or synthetic
Cell-type-wet laid
microglass
Minipleat V-cell -wet laid
Note that the cell-type filters have the aluminum separators to keep the pleats apart. Minipleat filters have several types of separators but string is probably the most used.

13. Residential Deep Pleated Filters
Deep pleated microglass and synthetic media is also
available for
use in residential
applications
Slide is self explanatory…

14. Filter Installations
The higher the efficiency the better the seal must be to prevent by-pass.
Holding frames must be properly gasketed.
Clamps must provide a tight seal between the filter face and the gasket.
Filter banks must be suitable reinforced to provide rigidity.

15. Side Access Housings Note gasketing on door
Note the door gasketing to prevent bypass. This door gasketing also applies pressure across he bank to hold filters in place. As filter professionals, we can do our clients a service by recommending and installing proper gasketing to maintain filter efficiency in extended surface filter applications.

16. Filter Performance
Best determined from independent filter testing laboratory
ANSI/ASHRAE Standard (MERV)
While there will be more information on filter testing in Chapter 7, know that the best use of filter test data is that from independent laboratories using the full ANSI/ASHRAE 52.2 test.

17. Sample Questions
Lofted Glass Media can be found most often in what style(s) of filters?
Pocket Filters or Rigid Filters
Minipleats have more media than most extended surface filters providing two advantages – what are they?
Lower initial pressure drop & longer life in service
Filter efficiency is best determined by a(n)…?
Independent laboratory ANSI/ASHRAE 52.2 test

18. Thank you.
Other Questions?