FFU (Fan Filter Unit): Structure, Quantity Calculation Formula, and Selection for Cleanrooms

If HEPA and ULPA are the "heart" of a cleanroom, the FFU - Fan Filter Unit is the "circulatory system" — supplying air, filtering it, and maintaining differential pressure across every zone. A large electronics or pharma cleanroom project can need several hundred to several thousand FFUs, so understanding their construction and sizing them correctly is the key driver of both capex and opex.

1. What is an FFU?

An FFU - Fan Filter Unit is an air-filtration device with an integrated fan motor. In other words, an FFU = fan + HEPA/ULPA filter + housing, mounted directly on the cleanroom ceiling (or floor).

FFUs are used to supply ultrapure air to cleanrooms, laboratories, healthcare facilities, and micro-environments by removing harmful airborne particles during recirculation. Units are installed in ceiling or floor grids. Large cleanrooms need correspondingly large numbers of FFUs — in some cases hundreds to thousands.

Depending on the cleanroom class, FFUs are fitted with HEPA (H13/H14) or ULPA (U15-U17). In addition to the filter and fan, the FFU contains an air-distribution element that produces an even outlet — laminar flow for high-class cleanrooms.

2. Basic FFU construction

A standard FFU has 5 components:

• Housing — typically powder-coated steel, aluminium, or 304 stainless steel. Stainless housings are used for VHP/H₂O₂ sterilisation environments and GMP pharma plants. Aluminium is lightweight, saving ceiling load.
• Inlet protective grille — keeps foreign objects out of the fan.
• Centrifugal fan — two main motor types: traditional AC or EC (Electronically Commutated), which saves 30-50% electricity with stepless control.
• Diffuser — ensures the outlet airflow is flat and even.
• HEPA or ULPA filter — mounted on the underside with airtight gaskets to prevent bypass.

Fan speed is usually controlled in 2 ways:

• Direct switch on the FFU body.
• Computer control over a LAN — common on large projects, allowing monitoring of differential pressure, fan efficiency, and HEPA-clogging alerts.

3. Cleanliness depends on choosing HEPA or ULPA

An FFU's cleaning power depends directly on its internal filter:

• HEPA H13/H14 — removes particles ≥ 0.3 µm with 99.97% efficiency (US) or 99.95-99.995% (EN).
• ULPA U15-U17 — removes particles ≥ 0.12 µm with 99.999% efficiency or better.

Some premium FFUs integrate a particle counter or differential-pressure sensor to signal when the HEPA needs replacement.

4. Formula for sizing FFUs for cleanrooms

This is the most important part of cleanroom design. The basic formula:

nFFU = (V × N) / S

Where:

• V — Room volume (m³)
• N — Air changes per hour (ACH)
• S — Airflow per FFU (m³/h)

A concrete example

Calculate the number of 575 × 1175 × 240 mm FFUs needed for a room: 50 m long, 20 m wide, 2 m floor-to-ceiling.

Step 1 — Volume: V = L × W × H = 50 × 20 × 2 = 2,000 m³

Step 2 — Choose the required air changes: Assume N = 54 per hour (close to a Grade B cleanroom).

Step 3 — Airflow of one 575×1175 FFU: A 575×1175 FFU typically delivers 18 cmm (cubic metres per minute). S = 18 × 60 = 1,080 m³/h

Step 4 — Apply the formula: nFFU = (2,000 × 54) / 1,080 = 100 units

So the room needs 100 FFUs.

5. Choosing ACH (N) by cleanroom class

Setting N depends on the cleanroom class and requirements. Typically:

Cleanroom class	Air changes per hour
Grade A (Class 100 / ISO 5)	60
Grade B (Class 1,000 / ISO 6)	40
Grade C (Class 10,000 / ISO 7)	30
Grade D (Class 100,000 / ISO 8)	20

Note: ACH is only one parameter. Grade A cleanrooms in sterile compounding also require laminar velocity of 0.36-0.54 m/s at the working plane — so the actual FFU count may exceed the formula's result.

6. How to pick the right FFU

a. By ceiling module size

The two most common module sizes in Vietnam:

• 575 × 1175 mm — standard for 600 × 1200 mm grid ceilings, ~1,080 m³/h.
• 1175 × 1175 mm — square module, ~2,000 m³/h.

b. By housing material

• Powder-coated steel — economical, used in standard electronics cleanrooms.
• Aluminium — lightweight, easy to suspend, mildly corrosion-resistant.
• 304/316 stainless steel — mandatory for GMP-WHO/EU pharma plants and chemical-sterilisation environments.

c. By motor type

• Traditional AC — cheap, common for small projects.
• EC motor — saves 30-50% electricity, stepless control, long service life, recommended for large projects. Capex is higher but pays back in 2-3 years.

d. By filter grade

• FFU + HEPA H13 — Grade D cleanroom (Class 100,000).
• FFU + HEPA H14 — Grade C cleanroom (Class 10,000).
• FFU + ULPA U15 — Grade A/B cleanroom (Class 1,000 or below).
• FFU + ULPA U17 — semiconductor fabs at Class 1-10.

7. Installation and operation notes

• The T-grid ceiling must support ~25-40 kg per FFU plus the negative-pressure load when the fan runs.
• Dedicated power circuits — group FFUs by zone for easier maintenance.
• Whole-room air balancing after installation: measure velocity at multiple points and trim each FFU's speed.
• Plan HEPA/ULPA replacements: 3-5 years for HEPAs, 5-10 years for ULPAs, depending on environment.
• On-site HEPA leak testing (DOP/PAO): mandatory at 100% for GMP, every 12 months for electronics cleanrooms.

Conclusion

FFUs drive both the capex and opex of a cleanroom. Applying nFFU = (V × N) / S correctly, picking the right HEPA/ULPA grade, and choosing the right housing material allows a project to meet ISO 14644 / GMP while saving billions of VND in electricity and maintenance over a 10-year lifecycle.

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About Green Filter

Green Filter supplies and installs 575×1175 and 1175×1175 FFUs in a wide range of configurations — HEPA H13/H14 or ULPA U15/U17, steel/aluminium/stainless housings, AC or EC motors. Green Filter's technical team will help size the FFU count from your drawings and offer long-term warranty.

📞 Contact Green Filter for FFU sizing and a project quotation: [insert hotline / email / website]

See also: What is a HEPA Filter? · ULPA Filters U15/U16/U17 · Energy savings with EC-motor FFUs.