FAHU in Positive/Negative Pressure Control

1. Why Pressure Control Matters

• Buildings are not just about temperature; they also need controlled air movement between spaces.
• FAHUs supply 100% outdoor air, which is the primary lever for establishing pressure differentials.
• By adjusting supply air vs. exhaust air volumes, FAHUs help maintain:
  • Positive pressure in clean/safe areas.
  • Negative pressure in contaminated/hazardous areas.

2. FAHU in Positive Pressure Control

Definition:

Positive pressure exists when supply airflow > exhaust airflow, causing air to flow outward when doors or leaks occur.

FAHU Role:

• Delivers a continuous stream of conditioned outdoor air into the zone.
• Slightly oversupplies air compared to return/exhaust systems.
• Maintains healthy, filtered air inside, preventing infiltration of dust, smoke, or pollutants.

Applications:

• Healthcare:
  • Operating Rooms (ORs): FAHU supplies sterile filtered air to maintain +15 Pa relative to adjacent spaces.
  • ICUs and clean wards.
• Commercial High-Rise:
  • Lift lobbies and stairwells are pressurized via FAHU during fire events to prevent smoke entry.
• Data Centers & Cleanrooms:
  • Keeps contaminants out by pressurizing equipment spaces.

3. FAHU in Negative Pressure Control

Definition:

Negative pressure exists when exhaust airflow > supply airflow, causing air to flow inward when doors or leaks occur.

FAHU Role:

• Supplies controlled fresh air, but the exhaust system is designed to exceed supply.
• FAHU ensures fresh air enters, but contaminated air is pulled out and safely discharged.
• Helps contain hazards by preventing air from leaking out.

Applications:

• Healthcare:
  • Isolation Rooms (TB, COVID wards): FAHU supplies filtered air, while exhaust fans maintain –15 Pa relative to corridors.
• Industrial & Labs:
  • Chemical laboratories, fume hood areas.
  • Pharmaceutical production with hazardous by-products.
• Kitchens & Toilets:
  • FAHU provides make-up air while exhaust maintains negative pressure.

4. How FAHUs Achieve Pressure Control

• Airflow Balancing: FAHU supply is coordinated with exhaust/return air fans.
• Differential Pressure Sensors: Continuously monitor room pressure; signals sent to FAHU dampers/fans.
• BMS Integration: Building Management System adjusts FAHU fan speed (VFDs) to maintain setpoint pressure.
• Airlocks & Cascade Design: FAHUs ensure correct airflow direction by maintaining cascaded pressures (e.g., OR → corridor → utility zone).

5. Practical Example

Hospital Wing with FAHU:

• Operating Theater (OT):
  • FAHU supplies 100% HEPA-filtered air at +15 Pa positive pressure.
  • Prevents bacteria from corridors entering OT.
• Isolation Room:
  • FAHU supplies air, but exhaust > supply → –15 Pa negative pressure.
  • Prevents infectious air from leaking into adjacent areas.

6. Typical Pressure Differentials Maintained by FAHU