AHU Zoning (HVAC Design)

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 Air Handling Units (AHUs) are the heart of cleanroom HVAC design, responsible for supplying clean, filtered, and pressurized air into controlled environments. In pharmaceutical, biotech, and healthcare facilities, proper AHU zoning is critical to maintaining product quality, patient safety, and regulatory compliance.

 The primary objective of AHU zoning is to ensure that each cleanroom or classified area receives air of the required cleanliness, pressure, and temperature, without the risk of contamination from adjacent areas. Zoning divides the facility into sections based on cleanliness class (Grade A, B, C, D as per EU-GMP or ISO Class standards). Each zone is served by a dedicated AHU, designed to match the cleanliness and operational requirements of that space.

• Dedicated AHUs for Each Grade:
  AHUs serving Grade A/B sterile areas (critical zones) must not be shared with Grade C/D or non-classified areas. This prevents cross-contamination and ensures high reliability.
   
• Pressure Cascade:
  A carefully designed pressure hierarchy ensures air always flows from cleaner to less clean zones.
   
  • Highest pressure → Grade A (aseptic core).
     
  • Medium pressure → Grade B (support areas).
     
  • Lower pressure → Grade C/D (background & preparation areas).
     
  • Lowest pressure → unclassified corridors.
    This cascade prevents ingress of unclean air into critical environments.
     
• Air Change Rates:
  Higher-grade areas require higher air change rates (e.g., 240–600 ACPH for Grade A, 30–60 ACPH for Grade C/D), directly linked to AHU capacity and filter design.
   

• Pre-Filters: Installed at AHU inlet to capture coarse dust and extend life of final filters.
   
• Intermediate (Fine) Filters: Typically F7–F9 filters for additional particle removal.
   
• HEPA Filters (H13/H14): Installed at terminal outlets or within AHUs to achieve particle-free air. These are mandatory for Grade A/B cleanrooms, where sterility must be maintained.
   

• Redundancy: Critical areas often use 100% fresh air or have backup AHUs for reliability.
   
• Ducting & Segregation: Ductwork must be isolated between zones, avoiding crossover between sterile and non-sterile systems.
   
• Energy Efficiency: While maintaining segregation, energy recovery systems (heat exchangers, variable air volume) may be considered, but never at the risk of contamination.
   
• Control & Monitoring: Building Management Systems (BMS) continuously monitor temperature, humidity, pressure differentials, and particulate levels in each zone.
   

International guidelines such as EU-GMP Annex 1, US FDA cGMP, and ISO 14644 emphasize AHU zoning to ensure sterility assurance. Deviations in zoning or pressure cascade can compromise product safety and lead to regulatory non-compliance.

AHU zoning ensures segregation of air systems between critical and non-critical areas, establishes a pressure cascade to control airflow direction, and integrates HEPA filtration for sterile environments. Proper zoning design is a cornerstone of HVAC systems in pharmaceutical cleanrooms, safeguarding both products and patients.