AHU/MAU and the 3-Stage Pre-Medium-HEPA Filter Train: Optimised Operation

In every cleanroom system — whether a pharmaceutical plant, an electronics factory, or a hospital — the AHU (Air Handling Unit) or MAU (Make-up Air Unit) is the "heart" that supplies treated air to the entire area. Understanding the correct structure, the role of each filter stage, and the optimal layout will determine both filtration efficiency and electricity costs over a 10-15 year lifecycle.

1. How do AHU and MAU differ?

AHU — Air Handling Unit

An integrated air-treatment unit: it takes in air (partly fresh + partly recirculated) → filters → conditions temperature and humidity → supplies the room.

Characteristics:

• Recirculation ratio 70-90%, fresh air 10-30%.
• Energy-efficient because recirculated air has already been cooled/heated.
• Used for cleanrooms without high cross-contamination requirements (Class C, D; premium offices; hospital outpatient areas).

MAU — Make-up Air Unit

A unit that processes 100% fresh air from outside. After treatment, the air goes to an RCU (Recirculation Unit) or directly to ceiling FFUs.

Characteristics:

• 100% fresh air — no recirculation.
• Higher energy use than an AHU (must cool all incoming air from 35°C to 18°C with deep dehumidification).
• Used for cleanrooms where cross-contamination must be avoided: sterile pharmaceuticals, semiconductors, vaccine rooms, BSL-3.

When should you use which?

Application	Type
Grade D pharma cleanroom, cosmetics, food care	Recirculating AHU
Grade A, B pharma cleanroom, vaccines	MAU + RCU
Semiconductor cleanroom	MAU + RCU
Standard operating room	AHU
Ultra-clean operating room, isolation room	MAU
Offices, hotels	AHU

2. Filter-stage structure inside an AHU/MAU

A standard AHU/MAU for a cleanroom has 3 filter stages arranged along the airflow:

Stage 1 — Pre Filter (G3/G4 or Coarse 65-85%)

• Position: right after the intake, before the cooling coil.
• Role: blocks coarse dust >10 µm, protects the Medium filter and coil.
• Form: polyester fibre pad in a wire frame or panel.

Stage 2 — Medium Filter (F6-F9)

• Position: after the cooling coil and fan, before Stage 3.
• Role: traps fine dust at 0.5 µm and above with 60-95% efficiency.
• Form: flat-panel separator, mini-pleat, or bag filter.

Stage 3 — HEPA H13/H14 (depending on the room)

• Position: inside the AHU, or at a terminal box on the cleanroom ceiling (terminal HEPA).
• Role: traps ultrafine dust at 0.3 µm with ≥99.95% efficiency.
• Form: deep-pleat HEPA box or mini-pleat V-bank.

Note: if the HEPA sits at a ceiling terminal box (common in pharma cleanrooms), the AHU only needs Pre + Medium. Terminal-box HEPAs are easier to test, easier to replace, and there is no need to open the entire AHU.

3. Layout by airflow path

Standard order along the airflow:

Intake → Pre Filter → Mixing chamber → Cooling coil → Heating coil → Humidifier → Medium Filter → Fan → HEPA Filter (if in the AHU) → Supply duct → Terminal HEPA → Room

Some premium AHUs also include:

• UV-C lamp before the cooling coil to sterilise the coil surface.
• Carbon filter after the Medium to remove VOCs and odours.
• Ioniser to neutralise odours and some toxic gases.

4. Calculating airflow and pressure drop

Required airflow

Q = V × N Q = airflow (m³/h), V = room volume (m³), N = ACH (changes per hour)

Example: a 200 m³ room at Grade C with ACH = 30: Q = 200 × 30 = 6,000 m³/h.

Total Static Pressure

Total static pressure = filter pressure drop + coil pressure drop + duct pressure drop + terminal pressure drop.

Reference values:

Component	Initial pressure drop (Pa)	End-of-life pressure drop (Pa)
Pre Filter G4	30-80	250
Medium Filter F8	80-150	450
Cooling coil	100-200	250
Heating coil	30-80	100
Humidifier	30-50	80
Ductwork	80-200	200
HEPA H14 terminal	200-250	500
Total (initial / end-of-life)	550-1,110	1,830

→ The AHU fan must cover a pressure range from 550 to 1,830 Pa to maintain airflow even as the filters load up. That is why an EC inverter fan — one that adjusts speed automatically — is the right choice.

5. HEPA placement: inside the AHU or at the terminal?

Option A — HEPA inside the AHU

• Pros: centralised; one large HEPA is cheaper than many small terminal HEPAs.
• Cons: if the HEPA leaks after testing, the whole cleanroom is contaminated. On-site integrity testing is difficult.

Option B — HEPA at the terminal (ceiling HEPA box)

• Pros: easy leak testing on the spot, easy replacement when loaded, good isolation between rooms.
• Cons: many small HEPAs → higher total initial cost.

Recommendation

• Pharma cleanrooms, hospitals, cosmetics: terminal HEPA (Option B).
• AHU supplying corridors, offices, Zone D: HEPA in the AHU (Option A), or no HEPA at all depending on requirements.

6. Fan choice — AC, EC, or inverter?

Traditional AC fan

• Fixed speed, adjusted by transformer.
• Cheap, robust, but power-hungry.

EC fan (Electronically Commutated)

• Stepless speed control via 0-10V signal or Modbus.
• Saves 30-50% electricity vs. AC.
• Suitable for FFUs and small AHUs.

AC fan + inverter

• Common for large AHUs (>10 kW).
• Saves 20-40% under variable-load operation.

Recommendation: for projects with a 10+ year operating life, EC or inverter pays back in 2-3 years.

7. Energy optimisation for AHU/MAU

a. Heat recovery

• Rotary wheel or plate heat exchanger between exhaust and fresh air.
• Saves 30-60% cooling energy in summer and heating energy in winter.

b. VAV (Variable Air Volume)

• Adjusts airflow to actual demand instead of a fixed flow rate.
• Saves 20-40% of electricity and cooling load.

c. Demand-controlled ventilation

• CO₂ / PM2.5 / VOC sensors set the fresh-air flow.
• Avoids over-ventilating sparsely occupied rooms.

d. High-efficiency chillers

• COP > 5.5; inverter screw chiller; high-efficiency cooling tower.

8. Disciplined AHU/MAU maintenance

Weekly

• Clean washable Pre Filters.
• Check ΔP across each filter stage.

Monthly

• Replace disposable Pre Filters.
• Clean coil exterior and condensate tray.
• Check fan motor, belts, and shaft temperature.

Every 3-6 months

• Replace Medium Filters.
• Calibrate temperature, humidity, and pressure sensors.

Every 12 months

• Replace Pre Filters at year-end (after the peak season).
• DOP test on terminal HEPAs.
• Deep-clean coils with chemicals.

Every 3-5 years

• Replace HEPAs.
• Overhaul fans, coils, humidifiers.

Conclusion

The AHU/MAU and the Pre + Medium + HEPA filter train form the backbone trio of every cleanroom system. Designing the right fresh-air ratio, picking the right filter grade for each stage, and investing in EC fans + heat recovery can save billions of VND in electricity over a 10-year lifecycle while ensuring stable air quality for the product.

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About Green Filter

Green Filter supplies the full range of filters for AHU and MAU at every scale: Pre, Medium F6-F9, HEPA H13/H14, and ULPA U15/U17. Products are manufactured to actual AHU dimensions, come with EN 779 / EN 1822 certificates, and are available for fast delivery.

📞 Contact Green Filter for filter consulting for your AHU/MAU system: [insert hotline / email / website]

See also: Medium Filter F6-F9 · What is a Pre Filter? · FFU and the formula for sizing quantity.