How to Select the Right FFU for ISO Class 5, 6, 7, 8 Cleanrooms?
Understanding ISO 14644-1 Cleanroom Classification
ISO 14644-1 is the internationally recognized standard for cleanroom air cleanliness classification. It divides cleanrooms into classes from ISO 1 to ISO 9 based on airborne particle concentration.
The cleanliness class is determined by the maximum allowable concentration of particles at specific sizes. For example, an ISO Class 5 cleanroom requires that the concentration of particles ≥0.5μm does not exceed 3,520 particles per cubic meter.
In practice, cleanrooms must be verified in one of three occupancy states :
As-built: Facility completed, no equipment or personnel
At-rest: Equipment installed and operating, no personnel
Operational: Equipment operating with personnel working-this is the most realistic verification state
ISO Class 5-8 Particle Concentration Limits
Below are the core specifications for ISO Class 5 to Class 8 cleanrooms. These data form the foundation for FFU selection:
| ISO Class | Max Concentration ≥0.5μm (particles/m³) | Max Concentration ≥5.0μm (particles/m³) | Typical Applications |
|---|---|---|---|
| ISO 5 | 3,520 | 29 | Semiconductor fabrication, sterile filling |
| ISO 6 | 35,200 | 293 | Medical devices, precision coating |
| ISO 7 | 352,000 | 2,930 | Food packaging, laboratories |
| ISO 8 | 3,520,000 | 29,300 | Electronics assembly, general manufacturing |
Note: The data in this table represent cumulative concentrations. For ISO Class 5, the 3,520 particles/m³ at ≥0.5μm includes all particles larger than 0.5μm.
FFU Selection Parameters by ISO Class
Based on the latest ISO 14644-1:2025 practices, the FFU selection parameters for different cleanroom classes are as follows:
| ISO Class | FFU Coverage | Average Airflow (m/s) | Air Changes per Hour (ACH) | Recommended Filter Grade |
|---|---|---|---|---|
| ISO 5 | 60-80% | 0.35-0.55 | 300-600 | H14 / U15 |
| ISO 6 | 40-60% | 0.30-0.45 | 150-300 | H13 / H14 |
| ISO 7 | 25-40% | 0.30-0.45 | 60-150 | H12 / H13 |
| ISO 8 | 15-25% | 0.30-0.45 | 20-60 | H11 / H12 |
Key Explanations:
FFU Coverage: The proportion of ceiling area occupied by FFUs. Higher coverage enables higher cleanliness classes .
Air Changes per Hour (ACH): The number of times the total air volume in the cleanroom is replaced per hour. ISO Class 5 requires 300-600 ACH .
Recommended Filter Grade: Based on EN 1822 standards. H13 filters achieve ≥99.95% efficiency at 0.3μm, H14 ≥99.995% .
How to Calculate FFU Quantity?
Calculating the number of FFUs is a critical step in selection. Here is the standard formula:
Number of FFUs = (Cleanroom Area × Target Coverage) / Effective Coverage per FFU
Practical Examples ：
| Scenario | Calculation | Result |
|---|---|---|
| ISO 5 Semiconductor Fab | Area 100㎡ × 70% coverage = 70㎡ ÷ 0.65㎡ per FFU | ≈108 units → +15% redundancy → 123 units |
| ISO 7 Lithium Battery Plant | Area 200㎡ × 30% coverage = 60㎡ ÷ 0.65㎡ per FFU | ≈92 units → +10% redundancy → 101 units |
Why add redundancy?
Add 5-10% FFUs in high personnel traffic areas
Add additional FFUs near equipment heat sources to compensate for airflow disturbances
Reserve 20% spare capacity for future upgrades
Which Filter Grade to Choose?
The core of an FFU is the high-efficiency filter. According to EN 1822 standards, different filter grades have different efficiency specifications:
| Filter Grade | Efficiency (MPPS) | Typical Applications |
|---|---|---|
| H13 HEPA | 99.95% @ 0.3μm | ISO Class 6-7 cleanrooms, pharmaceutical, hospitals |
| H14 HEPA | 99.995% @ 0.3μm | ISO Class 5 cleanrooms, sterile filling |
| U15 ULPA | 99.9995% @ 0.12μm | Semiconductor wafer fabs, nanotechnology |
| U16 ULPA | 99.99995% @ 0.12μm | 5nm/3nm chip manufacturing |
Selection Tips:
Avoid over-engineering: Using H14 filters for ISO Class 8 cleanrooms increases resistance by 25% and raises energy costs
Follow standards: Select the filter grade that corresponds to your required cleanroom class
Selecting the right FFU requires comprehensive consideration of four core elements: cleanroom class, FFU coverage, filter efficiency, and validation testing. By following ISO 14644-1 standards and basing decisions on actual process requirements rather than blindly pursuing higher grades, you can achieve optimal cleanliness while optimizing costs.
If you are planning a new cleanroom project or need to upgrade an existing system, welcome to contact us for professional selection advice. We offer custom size FFUs and OEM services to meet the special requirements of different projects!
