How Many Fan Filter Units Does a Cleanroom Need?

Step 1 — find the required airflow

For ISO 6–8 rooms with mixed (non-unidirectional) airflow, designers work from air changes per hour (ACH). For ISO 5 and cleaner rooms with unidirectional flow, they work from a downflow velocity across the filtered ceiling area, commonly around 0.3–0.5 m/s at the filter face.

Typical design ranges

Actual requirements depend on the process, people, equipment and contamination sources. The room is qualified by particle counts, not by air changes.

Step 2 — a worked example

A room 10 m × 6 m × 3 m high has a volume of 180 m³. For ISO 7 at 60 ACH, the supply airflow is 180 × 60 = 10,800 m³/h.

A 2 × 4 ft FFU (filter about 0.57 × 1.17 m) running at 0.45 m/s face velocity delivers roughly 0.57 × 1.17 × 0.45 × 3,600 ≈ 1,080 m³/h. The room therefore needs about 10,800 ÷ 1,080 = 10 units, before adding a margin for filter loading and for the doors and equipment in your layout.

Step 3 — choose module size and layout

• 2 × 2 ft (570 × 570 mm) units suit small rooms and partial coverage.
• 2 × 4 ft (570 × 1170 mm) units cover more ceiling per unit and reduce the number of connections.
• Place units over the critical process and spread the rest evenly; keep returns low on the walls to sweep particles away.
• Allow for speed control so airflow can be raised as filters load.