Where Pocket Microfilters Sit in the Filter Train

Most modern systems no longer rely on a single filter. Instead, they use a multi-stage line-up:

Stage 1 – Pre-filter
G2–G4 panels, pleated filters or washable metal mesh at the intake or return, stopping larger dirt.

Stage 2 – Pocket microfilter (this product)
F7–F9 / roughly MERV 11–15, built as multi-pocket bags. This is where most of the fine dust and a good part of the PM10/PM2.5 load is captured.

Stage 3 – High-efficiency or HEPA
For cleanrooms, healthcare or critical zones, HEPA or high-grade box filters finish the job.

In this chain, the pocket microfilter is the balance point:

•The filtration efficiency must be high enough to protect downstream HEPA filters or sensitive processes;

•At the same time, the resistance and dust holding capacity must be kept within a reasonable range to ensure acceptable fan performance and energy consumption.

For a typical AHU, this stage is where F7–F9 pocket microfilters do their work, keeping indoor air quality within spec and helping final filters reach their expected lifetime.

What "Micro" Means Here – Media and Performance Window

In this product, "microfilter" is not a marketing term, but refers to the particle size range and media structure in which it operates.

Efficiency classes

Available in F7, F8 and F9 grades

Corresponding to MERV 11–15 in common HVAC practice

Can be aligned with ISO 16890 classes such as ePM10, ePM2.5 and ePM1 according to your project standard

Target particle range

Designed to handle the 1–10 μm fine dust fraction that pre-filters cannot efficiently stop

F7 pocket microfilters focus more on PM10 and the coarser part of PM2.5

F9 versions push efficiency further into the ePM1 range for systems with stricter air quality requirements

Filter media

Synthetic microfibre non-woven, formed and layered for graded density

Stable performance in typical AHU humidity and temperature ranges

Low fibre shedding and good compatibility with downstream HEPA or coil surfaces

In practical terms, this means your pocket microfilter takes over where the pre-filter leaves off: it quietly removes suspended fine dust day in, day out, so that occupants and downstream equipment see a much cleaner airstream.

Example F8 Configuration – Numbers You Can Plan Around

Below is an example configuration of a commonly used F8 pocket microfilter, provided to give you a sense of scale when designing or modifying projects. For specific projects, we will still provide detailed parameters based on the operating conditions.

Example model (F8, 595 × 595 × 600 mm):

Nominal size: 595 × 595 × 600 mm

Typical airflow: approx. 3200–3500 m³/h in standard AHU duty

Initial pressure drop: around 55–60 Pa at the recommended face velocity

Average efficiency (colourimetric): 90–95%, corresponding to F8 under EN 779 style testing

Frame options:

Galvanised steel frame for general HVAC use

Aluminium frame where corrosion resistance and lower weight are preferred

Pocket construction: multi-pocket design with internal spacers to keep pockets open and minimise rubbing or collapse.

This set of data will not replace a formal technical solution, but it can help you and the engineers and clients quickly align on expectations:

"At this airflow and efficiency level, we are using an F8-grade pocket microfilter, with approximately these airflow velocity, resistance, and frame structure specifications."

If you require F7 or F9, or other sizes/bag depths, we will provide the corresponding configurations and test values based on the same principles.

Engineering Considerations When Selecting a Pocket Microfilter

a) Face velocity and filter area

The face velocity through the pockets should stay in a sensible window-too high and pressure drop climbs too quickly, too low and the filter volume is under-used.

Pocket depth and number of pockets directly affect total media area; we can adjust these to match your fan and coil selection.

b) Pressure drop vs. fan and energy

Initial resistance (for example in the 55–60 Pa range for the F8 example above) is only part of the story.

The important curve is how pressure drop grows over the dust loading period. A well-sized pocket microfilter offers a long, smooth operating window so you are not forced into early change-out.

c) Frame and sealing details

Choice between galvanised and aluminium frames will depend on your environment, corrosion concerns and handling culture.

Correct sealing to the filter frame (gaskets, clips or tracks) is just as important as the media grade-bypass air can easily undermine a good filter.

d) Change-out strategy and downstream filters

If there is a HEPA or very fine filter downstream, it is usually worth selecting a higher class (F8/F9) pocket microfilter to extend the life of the final stage.

If the pocket microfilter is the last stage before supply, the choice between F7, F8 and F9 should be matched to the building's air quality targets and energy budget.

By considering these points along with your existing AHU fans, coils, airflow velocity, and maintenance schedule, you can essentially determine whether to use F7, F8, or F9 pocket microfilters, and select the most suitable size and number of filter bags.

What We Need from You to Specify the Right Pocket Microfilter

To ensure you select the right product the first time, please prepare the following information before requesting a quote or engaging in technical discussions:

AHU or equipment name and location (supply air / return air / fresh air)

Design airflow rate and acceptable face velocity range for each unit

Target filtration level: F7, F8, or F9, or the corresponding MERV / ISO 16890 requirements

Configuration of pre-filtration and post-filtration (with or without HEPA / high-efficiency filters)

Desired replacement cycle and acceptable initial/final pressure drop range

Any specific preferences or limitations regarding frame material, number of pockets, and pocket depth

With this information, we can quickly provide a pocket microfilter solution that matches your system, instead of having you randomly select a model from a catalog.

exhibition of aaf bag filter