Pressure Drop & Maintenance in FAHU

1. What is Pressure Drop?

• Definition: Pressure drop is the resistance to airflow caused by FAHU components such as filters, coils, dampers, and duct connections.
• Units: Pascals (Pa) or Inches of Water Gauge (in. w.g.).
• Key Point: As components get dirty (filters clog, coils foul), pressure drop increases, airflow reduces, and the fan consumes more energy.

2. Sources of Pressure Drop in FAHU

• Filters:
  • Clean pre-filter: 50–100 Pa
  • Fine filter: 100–200 Pa
  • HEPA filter: 250–400 Pa
• Cooling/Heating Coils: 50–100 Pa (depends on coil rows & fin density).
• Dampers & Silencers: 20–60 Pa.
• Ductwork & Accessories: Adds extra resistance to be considered in total static pressure.

3. Monitoring Pressure Drop

• Differential Pressure Gauges (Magnehelic): Installed across filter banks and coils.
• Differential Pressure Transmitters (DPTs): Provide real-time data to the BMS (Building Management System).
• Setpoints for Action:
  • Pre-filter: Replace/clean at 150–200 Pa.
  • Fine filter: Replace at 250–300 Pa.
  • HEPA: Replace at >400 Pa.
• Warning: Operating with clogged filters → reduced ventilation, IAQ failure, and excessive fan energy.

4. Effects of High Pressure Drop

• Increased fan power consumption (higher kW demand).
• Reduced air supply → occupants feel discomfort, IAQ worsens.
• Risk of condensation & microbial growth on coils due to low airflow.
• Overheating of fan motors if prolonged.
• Shorter lifespan of filters, fans, and motors.

5. Maintenance Practices

🔹 Filters

• Pre-filters: Wash/replace every 1–3 months.
• Fine filters: Replace every 6–12 months.
• HEPA filters: Replace every 12–24 months (not washable).
• Always check ΔP across filters before replacement.

🔹 Coils

• Clean cooling/heating coils every 6–12 months to remove dust, biofilm, and scale.
• Maintain clean drain pans to avoid bacterial growth (Legionella risk).

🔹 Fans & Dampers

• Inspect fan belts, bearings, and VFDs every 3 months.
• Check damper operation and actuator calibration.

🔹 General

• Inspect casing and access doors for air leakage.
• Verify differential pressure sensors regularly.

6. Example: Office Building FAHU

• Design airflow: 10,000 CFM.
• Initial filter ΔP: 200 Pa.
• After 6 months, ΔP rises to 400 Pa → airflow drops to 7,000 CFM.
• Result: Occupants complain of “stuffy” rooms, energy bill rises due to fan overwork.
• Solution: Filter replacement restores airflow and reduces fan power.

7. Recommended Maintenance Schedule

✅ Summary:

• Pressure drop is a key performance indicator in FAHUs.
• Monitoring with gauges/sensors ensures timely action before airflow/IAQ problems occur.
• Preventive maintenance (filters, coils, fans) reduces energy cost, maintains IAQ, and extends system life.