Medium Filter (F6-F9): The Indispensable Intermediate Stage in AHU Systems

In a cleanroom filtration system, the three "filter stages" — Pre, Medium, and HEPA — always work as a three-layer defensive line. Of these, the Medium Filter plays the pivotal role: catching the fine dust that the Pre Filter lets through and shielding HEPA — the most expensive filter — from early clogging. Understanding Medium Filters properly lets you run your AHU efficiently and extend HEPA life by 3-5×.

1. What is a Medium Filter?

The name Medium already says it all — it sits in the middle. The Medium Filter sits between the Pre Filter (coarse) and HEPA/ULPA (fine). It is not the first layer to contact dirty air, nor is it the final guarantor of absolute cleanliness.

Medium Filters are used most commonly in:

• Central air-conditioning (AHU - Air Handling Unit) systems for office towers, shopping centres, five-star hotels, and hospitals.
• GMP / ISO 14644 cleanroom supply-air systems — always between Pre and HEPA.
• Paint booths, sand-blasting booths, welding booths — moderate fine-dust loads.
• Large industrial air purifiers.

2. Efficiency and classification

A Medium Filter can remove up to 95% of fine and intermediate dust with a minimum penetrating size of around 0.5 µm (500 nanometres). Compared with HEPA (capturing 0.3 µm at 99.95%), Medium is far lighter, but this is by design: Medium needs enough "breathing room" to handle real-world dust loads without choking.

Under the old European standard EN 779 (now replaced by ISO 16890), Medium Filters fall into the F6 → F9 group, where higher numbers mean higher efficiency:

Grade	Average efficiency @ 0.4 µm	Representative application
F6	60-80%	Pre-filter for low-grade HEPA, premium office AHU
F7	80-90%	The most common intermediate filter for hospital and hotel AHUs
F8	90-95%	Upstream of HEPA H13 in pharma and food plants
F9	>95%	Upstream of HEPA H14 in electronics cleanrooms and high-grade pharma

Equivalence in the new ISO 16890: F7 ≈ ePM1 50-65%, F8 ≈ ePM1 65-80%, F9 ≈ ePM1 80-90%. The conversion between the two standards should be checked before ordering.

3. Three basic Medium Filter constructions

Media-wise, Medium cores are typically F8-grade filter paper, Puchikpo paper, or synthetic non-woven cloth, shaped in three main ways:

a. Separator type (flat panel with aluminium dividers)

• Aluminium or powder-coated steel frame with aluminium foils between each paper pleat.
• Pros: high mechanical stability, vibration-resistant, easy to clean the frame.
• Cons: smaller surface area than mini-pleat, heavier.
• Use: older industrial AHUs, heavy-duty plants.

b. Mini-pleat type (dense pleats with no separator)

• Dense paper pleats spaced with string or hot-melt beads.
• Pros: largest surface area, lowest initial pressure drop, lightweight, energy-saving for AHUs.
• Use: premium office AHUs, modern pharma and electronics cleanrooms.

c. Bag type (multi-pocket bag filter)

• Filter media sewn into 4-10 pockets suspended in a metal or plastic frame.
• Pros: massive surface area (8-15× that of a flat panel of the same envelope size), high dust capacity, long life.
• Cons: large installation depth (600-900 mm).
• Use: large-capacity AHUs for commercial towers, hospitals, hotels, airports.

4. The Medium's role in the 3-stage filter system

The standard design for cleanrooms and high-end AHUs:

1. Pre Filter (G3 / G4 / EU3-EU4) — blocks large dust, hair, fibres. Removes ~65-85% of coarse dust.
2. Medium Filter (F6 → F9) — blocks fine dust at 0.5 µm and above with 60-95% efficiency.
3. HEPA H13/H14 or ULPA U15-U17 — captures the rest of the ultrafine dust, bacteria, and viruses with 99.95% - 99.999995% efficiency.

Placing the Medium correctly:

• Cuts HEPA replacement costs by 3-5×: HEPA is the most expensive filter, costing millions to tens of millions of VND per unit.
• Stabilises airflow: pressure drop rises more slowly, the fan does not have to accelerate suddenly.
• Saves electricity: the AHU operates at optimum pressure, avoiding overload due to clogging.
• Protects the product: bacteria and mould are caught early.

5. When to replace a Medium Filter

Three common signs:

• Filter ΔP exceeds 2× the initial value — the golden rule for every filter stage.
• Particle counts exceed the limit downstream of Medium but upstream of HEPA.
• Visible brown/black discolouration of the filter face, with an odour.

Real-world replacement cycles: 3-6 months in heavy industrial environments, 6-12 months in office or hospital AHUs. Keep a log to spot trends and plan annual budget.

6. Tips for picking the right Medium Filter

• Match the downstream HEPA: H13 → minimum F8; H14 → F9.
• Choose bag type if AHU depth allows — 2-3× longer life.
• Order to the actual AHU dimensions — avoid gasket gaps that cause bypass.
• Insist on EN 779 or ISO 16890 certificates from your supplier — never buy "no-name".
• Synchronise Pre + Medium change-out: typically two Pre changes per one Medium change.

Conclusion

The Medium Filter is not a "star" like HEPA or ULPA, but it is the intermediate layer that decides the service life and operating cost of the entire system. Picking the right F6-F9 grade and the right construction (separator - mini-pleat - bag) and placing it correctly in the AHU saves tens of millions of VND per year.

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About Green Filter

Green Filter supplies the full range of Medium Filters F6, F7, F8, F9 in flat-panel, mini-pleat, and bag configurations, made to the customer's AHU dimensions, with EN 779 / ISO 16890 certificates. Green Filter's technical team is ready to survey and advise on the optimum filter chain for your HVAC / cleanroom system.

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

See also: Pre Filter — the first line of defence · AHU/MAU and the 3-stage Pre-Medium-HEPA filter train · What is a HEPA Filter?.