Understanding HEPA Style Codes

So, why do we need a style code system for HEPA filters? For most filter types, such as a pleated filter, the only variables may be the filter size and possibly the efficiency. However with HEPA filters, there are many other potential variations that you can have with a filter.

The style code contains a series of unique characters that reference specific characteristics of the filter in question. For example, the AstroCel I filter has 40 standard sizes, 15 cell sides, 5 types of media, 4 types of separators, 11 types of tests, and more. Likewise, the AstroCel II filter has 34 standard sizes, 17 cell sides, 3 types of media, 7 media depths, and 11 types of tests. Style codes can therefore have millions of potential combinations.

AAF Flanders Style Codes

The style code consists of 11 total characters, representing 9 different fields. The size and cell side design both use 2 characters, while all other fields use a single character. The fields consist of:

  • Filter Size - For the AstroCel I filter this is the HxWxD. For the AstroCel II filter this is simply the HxW.
  • Media Type - Media type is typically categorized as HEPA (99.99% or less) and ULPA (>99.99%), with a few other less common types as well.
  • Cell Sides - This field includes both the cell side type, such as gasket seal or gel seal, and the cell side material, such as galvanized, aluminum, or other.
  • Separators (A’cel I) / Pack Depth (A’cel II) - This field differs with the product type. For the AstroCel I filter this code defines the separator type, such as HCX or standard, or if vinyl coating is required. For the AstroCel II filter, this field defines the pack depth, which varies from 2” to 4”.
  • Bond Material - For the AstroCel I filter, this is basically defined by the temperature requirement regarding which bond is needed. The AstroCel II filter has limited options for the bond material.
  • Gasket/Gel Material - Gasket or gel material is simply which material is required, whether it is a gasket seal or a gel seal cell design.
  • Gasket/Gel Location - This defines the location of the gasket/gel, such as the air leaving side or the air entering side.
  • Acceptance Level - Acceptance level is the exact efficiency requirements for the filter. This field goes hand-in-hand with the second field, which defines the media type.
  • Faceguard/UL 586 Label - This is a dual field that defines if a faceguard is required and if so, what material and location is required. This field also identifies if a UL 586 label is required.

Sample Code

Style Code Sheet Samples

This is a copy of the AstroCel I Filter style code sheet showing all the potential options for each field. Note that nearly each field has an indicator for a “special” requirement. As an example, for the media this would be a Z. If a Z is given, then there would have to be a specific definition for what special requirement is needed. This would also require that the filter be built as an NSP (non-standard product).

This is the style code sheet for the AstroCel II filters.

There is also a separate sheet for the TM modules.

Rules and Common Errors

There are some rules that must be used in making up style codes. Not all combinations are capable of being made. Just because you can make up the style code, doesn’t mean it can be built.

Media/Acceptance Level Mismatch

  • Example: 14A89A2P0M2
  • The first A sets the media type as a 99.99% media. But, the M acceptance level defines the efficiency as 99.9995%, which cannot be achieved with the 99.99% media. So, the first A would need to be changed to an E.

Bond/Cell Side Mismatch

  • Example: 15A74J5Y2A0
  • In this case, the second 5 specifies a 500 degree bond material and the Y is the high temp gasket material. However, the 74 cell sides is particle board and can only tolerate 200 degrees F. So, the filter can be built, but it would not be able to handle 500 degrees F.

Cell Side/Media Pack Mismatch

  • Example: 29A89F2T2H0
  • Another common error for the AstroCel II filters is asking for a pack depth that is deeper than the cell side. This is not physically possible to build. In the case of 29A89F2T2H0, the 89 cell sides is 2-3/4” thick. However, the F media pack depth is 4”. You would have to either use a thicker cell side, or reduce the media pack depth to 2”.


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