Cleanroom Airflow：design，Control and More

System Design & Functionality

Recirculating airflow systems extract and filter air within the cleanroom, reintroducing it after passing through HEPA (High-Efficiency Particulate Air) or ULPA (Ultra-Low Penetration Air) filters. Key characteristics include:

• Air change rates typically between 15-60 ACH for non-critical applications
• Energy efficiency due to reduced outside air intake
• Use of ceiling-mounted diffusers and floor-mounted returns
• Pressure control via supply-exhaust air balance

Applications & Standards

Commonly used in:

• Medical device assembly (ISO Class 7-8)
• Pharmaceutical packaging areas
• Electronics component handling
• Microbiological laboratories (non-sterile)

Design must comply with ISO 14644-4 for operational consistency and EU GMP Annex 1 for pharmaceutical applications.

Operational Principles

One-pass airflow systems supply 100% fresh air, passing through the cleanroom only once before exhausting. Key features:

• Air change rates exceeding 200 ACH for critical environments
• Elimination of recirculated air to prevent contaminant buildup
• Use of HEPA/ULPA filters in supply air streams
• Requires significant energy input for conditioning 100% fresh air

Critical Applications

Essential in environments where contaminant control is paramount:

• Semiconductor wafer fabrication (ISO Class 1-5)
• Sterile pharmaceutical manufacturing (Grade A/B)
• Biological safety cabinets (BSC Class III)
• Microelectronics lithography processes

Compliance with SEMI F20 for semiconductor facilities and FDA cGMP for sterile drug production is mandatory.

Design Components

• Partial recirculation with controlled fresh air intake
• Zoned airflow patterns for varying cleanliness needs
• Combination of diffuser-return and unidirectional flow sections
• Adaptive control systems for dynamic load adjustment

Optimal Use Cases

• Large-scale cleanrooms with diverse process requirements
• Research and development facilities with changing needs
• Medical device manufacturing with mixed criticality areas
• Pharmaceutical facilities requiring multiple cleanliness classes

Flow Characteristics

• Reynolds number > 4000 indicating turbulent flow regime
• Velocity fluctuations creating eddies and vortices
• Particle transport via random eddy motions
• Potential for particle deposition on surfaces due to flow separation

Turbulent flow in cleanrooms is typically associated with:

• Air velocities > 0.5 m/s in non-unidirectional systems
• Obstructions such as equipment or personnel movement
• Suboptimal diffuser/return placement

Control Measures

Mitigation strategies include:

• Streamlining equipment design to reduce flow obstruction
• Implementing proper personnel gowning and movement protocols
• Using computational fluid dynamics (CFD) for airflow modeling
• Maintaining minimum air change rates of 20 ACH to dilute turbulence

While turbulence is generally undesirable, controlled levels are acceptable in lower-class cleanrooms with proper design considerations.

Design Principles

Laminar flow systems are designed to create unidirectional airflow with minimal turbulence. Key design considerations:

1. Flow Direction: Vertical (ceiling-to-floor) or horizontal (wall-to-wall)
2. Filter Coverage: 100% ceiling coverage with HEPA/ULPA filters for vertical flow
3. Velocity Control: Maintaining 0.3-0.5 m/s (60-100 fpm) for optimal particle removal
4. Return Air Design: Perimeter floor returns or raised floor plenums for uniform flow
5. Equipment Placement: Streamlined design to avoid flow obstruction

Design Challenges & Solutions

Pressure Differential Standards

• Positive Pressure: 10-15 Pa (0.04-0.06 in. wg) relative to adjacent areas
• Negative Pressure: -10 to -15 Pa for containment of hazardous materials
• Graded Pressures: Stepwise pressure drops between cleanliness zones
• Airlock Pressures: 5-10 Pa buffer zones to minimize contamination ingress

Pressures are measured using differential pressure transducers with accuracy ±0.5 Pa for critical applications.

Control Strategies

1. Supply-Exhaust Balance: Modulating supply fans to maintain pressure setpoints
2. Airflow Tracking: Using airflow stations to monitor and adjust air volumes
3. Automatic Dampers: Motorized dampers for dynamic pressure correction
4. Pressure Relief Vents: Passive vents to prevent overpressurization during equipment operation
5. Alarm Systems: Visual/audio alerts for pressure deviations beyond set limits

Modern systems integrate BMS (Building Management Systems) for real-time pressure monitoring and adaptive control.

Semiconductor Manufacturing

Laminar flow is essential for wafer fabrication, where particles >0.1μm can cause device failure. ISO Class 1-5 cleanrooms are standard for lithography and deposition processes.

Pharmaceutical Sterile Processing

Sterile drug manufacturing requires Grade A (ISO Class 5) laminar flow for aseptic filling and critical processing steps, complying with EU GMP Annex 1 and FDA cGMP.

Biomedical Devices & Implants

Implantable device production uses ISO Class 5-6 cleanrooms with laminar flow to prevent biological contamination and ensure sterility assurance.

Microelectronics & Optics

Laminar flow is critical for optic lens manufacturing and microelectronics assembly, where particle contamination affects product performance and reliability.

Medical Device Assembly

Non-sterile device assembly (e.g., catheters, diagnostic tools) uses ISO Class 7-8 cleanrooms with turbulent flow, provided particle counts are controlled within process limits.

Pharmaceutical Packaging

Primary and secondary packaging areas for non-sterile drugs operate in ISO Class 8 Cleanrooms with turbulent flow, complying with EU GMP Annex 1 for particulate control.

Biomedical Research Labs

Non-sterile research environments (e.g., cell culture labs, assay development) use turbulent flow cleanrooms (ISO Class 7-8) to control environmental contaminants without requiring laminar flow.

Optical Disk Manufacturing

Production of CDs, DVDs, and Blu-ray discs uses ISO Class 6-7 cleanrooms with turbulent flow, as particle contamination impacts data storage quality but allows for controlled flow dynamics.