1. Why Multi-Stage Filtering?
- Outdoor air contains dust, pollen, smoke, microbes, and gases.
- If we install only a high-efficiency filter (like HEPA), it would clog very quickly.
- A progressive arrangement is used:
- Coarse filters catch large particles.
- Fine filters catch smaller particles.
- HEPA filters catch ultra-fine particles (for critical spaces).
This protects downstream filters and coils, reduces maintenance cost, and ensures sustained IAQ.
2. Typical Filter Stages in FAHU
🔹 Stage 1 – Pre-Filter (Coarse Filter)
- Position: First in line, at FAHU air intake.
- Efficiency: MERV 4–8 / G2–G4.
- Function: Captures large dust, sand, insects, and fibers.
- Role: Protects fine and HEPA filters from clogging.
- Maintenance: Washable; replaced every 3–6 months.
🔹 Stage 2 – Fine Filter (Secondary Filter)
- Position: After pre-filter, before coil section.
- Efficiency: MERV 9–13 / F5–F9.
- Function: Removes medium to small particles (smoke, mold spores, bacteria carriers).
- Role: Protects coils and ducts; maintains IAQ in commercial spaces.
- Maintenance: Replace every 6–12 months.
🔹 Stage 3 – HEPA Filter (Final Filter, Optional)
- Position: After fine filter, just before supply fan section.
- Efficiency: ≥99.97% at 0.3 µm (H13/H14).
- Function: Removes bacteria, viruses, and ultra-fine particles.
- Role: Critical for hospitals, pharma cleanrooms, labs.
- Maintenance: Replace every 1–2 years.
3. Typical Arrangements by Application
- Commercial Offices / Malls: Pre-filter + Fine filter (F7/F8).
- Hotels / Residential Towers: Pre-filter + Fine filter (F7/F8).
- Hospitals: Pre-filter + Fine filter (F9) + HEPA filter (H13/H14).
- Industrial / Pharma: Pre-filter + Fine filter + HEPA + (optional activated carbon filter for odors/chemicals).
4. Example Filter Bank Layout in FAHU
Airflow Direction →
1️⃣ Air Intake Louver + Sand Trap Filter (optional in desert climates)
⬇️
2️⃣ Pre-Filter (G4 / MERV 7)
⬇️
3️⃣ Fine Filter (F7–F9 / MERV 13)
⬇️
4️⃣ Cooling/Heating Coils
⬇️
5️⃣ HEPA Filter (if required: hospitals, labs)
⬇️
6️⃣ Fan Section → Supply Ductwork
5. Key Design Considerations
- Filter Housing: Should be airtight to prevent bypass leakage.
- Access Doors: Allow easy filter replacement.
- Differential Pressure Gauges: Monitor pressure drop across each filter stage.
- Bypass Filters: Some FAHUs use bypass arrangements with dampers for emergency operation.
6. Case Example
- Hospital ICU FAHU:
- Pre-filter (MERV 8) → captures dust.
- Fine filter (F9) → captures small particles & bacteria carriers.
- HEPA filter (H14) → ensures sterile supply to patient rooms.
- Office Tower FAHU:
- Pre-filter (MERV 6) + Fine filter (F7).
- No HEPA (not required, saves energy).
