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Custom Fan Filter Unit (FFU) Sizing

As the leader in custom engineered Fan Filters Units (FFUs), we are often presented with the challenge of fitting a FFU in a restricted height area. As the semiconductor equipment market continues to pack more equipment in less space, the challenge of getting clean air to the key zones while staying within the space allotted is increasingly challenging.

The following blog presents various options, along with the pros and cons of low profile HEPA/ULPA/PTFE fan filter units

There are two forms of Fan Filter Units (FFUs) than can be used in low profile applications and each has it’s place depending on the customers goal for air cleanliness and air uniformity, while fitting in the height available.

1) Standard FFUs (Fan plenum on top of a HEPA/ULPA FILTER)

2) Side Blower/distribution membrane

Low Profile FFU


Standard 9” overall height Airflotek Euro

In our standard 9” FFU the uniformity of a 2’X4’ PTFE version is +4.8%/-4.4% @ 98 FPM. (6 spots measured with a Velgrid and using the Relative Standard Deviation uniformity calculation method) All results stated in the following slides are in relation to these numbers.

Shorten the FFU to 8”

The FPM drops by 3% to 95 FPM while the uniformity is negatively affected only slightly to +4.5%/-6.5%, still within most customer uniformity specifications.

Shorten the FFU to 7”

The FPM drops by 6% to 92 FPM while the uniformity is increasingly negatively affected  to +8.1%/-5.9%, still within most customer uniformity specifications.

Shorten the FFU to 6.25”, leaving the fan ¼” from the top of the filter

The FPM drops by 8% to 90 FPM while the uniformity is further negatively affected to +7.0%/-5.2%, still within most customer uniformity specifications.

For Low Profile FFUs

1)The FFU can be constructed so that the main body of the FFU is shorter than the area where the fan wheel is, saving an additional ¾” in the main body. (and so a height of 5 ½” overall and 6 ¼” where the fans are-see next slide)
2)Using smaller fans to save an additional 1 ¼” for a FFU height of 5” across the entire top
3)A combination of 1) and 2) allowing a height of 4.4” in the main FFU body and 5” where the fans are located

Mixed height FFU (different heights for the fan section and main body of FFU

HEPA ULP PTFE different heights for the fan section and body of FFU

The standard layout for a Fan Filter Unit requires a minimum of 5” of height, plus an additional 2 ¾” above the FFU for air inlet. In some applications it is necessary to make HEPA/ULPA filter air to the side and blow it into a distribution chamber with a CEG membrane across the bottom. In this manner a total height of 2” can be achieved in the area where the filtered air exits. Getting the specified uniformity out of this technique often requires development time, but it can be made to deliver uniformity to meet most applications.

Side Blower/Distribution  Membrane 

Sample test rig for a Side Blower/distribution membrane application

Wedge Shaped Distribution Plenum FFU unit

Contact TES-Clean Air Systems today

STUDY: Low Profile FFU – Short FFU – Restricted-Height FFU

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