Pressure Drop & Maintenance in FAHU

1. What is Pressure Drop?

• Definition: Pressure drop is the resistance to airflow caused by components inside the FAHU — mainly filters, coils, and dampers.
• Unit of Measurement: Usually in Pascals (Pa) or inches of water gauge (in. w.g.).
• Impact:
  • Higher pressure drop → higher fan power required.
  • Increases energy consumption.
  • Indicates filter clogging or dirty coils.

2. Pressure Drop Across Filters

Filters are the main contributors to FAHU pressure drop.

• Pre-Filters: 50–100 Pa (0.2–0.4 in. w.g.)
• Fine Filters (F7–F9): 100–200 Pa (0.4–0.8 in. w.g.)
• HEPA Filters: 250–400 Pa (1.0–1.6 in. w.g.)

🔹 As filters get dirty, their pressure drop increases significantly. This is why monitoring is essential.

3. Monitoring Pressure Drop

• Differential Pressure Gauges (Magnehelic gauges):
  Installed across filter banks, showing clean vs. dirty filter resistance.
• Differential Pressure Sensors (DPTs):
  Provide signals to BMS for real-time monitoring and alarms.
• Setpoints:
  • Pre-filter replacement: 150–200 Pa
  • Fine filter replacement: 250–300 Pa
  • HEPA replacement: >400 Pa

4. Maintenance Practices

🔹 Filters

• Pre-filters: Washable, cleaned every 1–3 months, replaced every 6–12 months.
• Fine filters: Replaced every 6–12 months.
• HEPA filters: Replaced every 1–2 years (cannot be washed).

🔹 Coils

• Need periodic cleaning to avoid fouling and biofilm growth (which increases pressure drop and reduces heat transfer).
• Cleaning frequency: 6–12 months depending on application.

🔹 Fans & Ducts

• Fans should be checked for balance and bearing wear.
• Ducts inspected for leakage and dust accumulation.

5. Effects of High Pressure Drop

• Increased fan energy consumption.
• Reduced airflow → poor IAQ, discomfort, and code violations.
• Shortened filter life and increased maintenance cost.
• Risk of bypass air if filters are not sealed properly.

6. Example Calculation

If a FAHU is designed for 300 Pa pressure drop across filters but readings show 500 Pa, this means:

• Filters are dirty.
• Fan motor consumes more power.
• Airflow may drop below required ventilation rates.

Action: Replace or clean filters immediately.

7. Maintenance Schedule (Typical)