How to choose F5, F7, F8 and F9 bag filters?

1. What Is the Difference Between F5, F7, F8, and F9?

F5, F7, F8, and F9 are traditional EN 779 filter classifications that are still commonly used in engineering projects. For current international projects, buyers should also confirm the corresponding ISO 16890 classification, such as ISO ePM1, ePM2.5, or ePM10, together with the relevant efficiency percentage.

It should be noted that an F grade cannot simply be considered equivalent to a fixed ISO 16890 class. The actual ePM efficiency may vary depending on the filter media and structural design. For international procurement projects, suppliers should therefore be required to provide ISO 16890 test data.

2. Why Should Most Projects Consider More Than Filtration Efficiency?

Upgrading directly from F7 to F9 will normally improve particle filtration performance, but this does not necessarily mean that the overall system will perform better.

Higher filtration efficiency usually requires greater attention to airflow resistance. If the existing fan does not have sufficient available static pressure, the following problems may occur:

• Reduced actual supply airflow;

• Increased AHU operating load;

• Higher fan energy consumption;

• Insufficient air changes in clean areas.

Engineering selection should therefore consider:

Filter Grade + Airflow + Pressure Drop + Dust-Holding Capacity + Available System Static Pressure

F5, F7, F8, and F9 should not be compared solely by filtration grade.

3. Which Applications Are Suitable for F5, F7, F8, and F9?

F5: Suitable for General Pre-Filtration

F5 is suitable for ventilation systems without strict fine-particle control requirements. It can be used as an intermediate filtration stage in an AHU or to protect higher-grade downstream filters.

Typical applications include office buildings, industrial plants, fresh-air systems, and general air-handling equipment.

If the primary objective is to reduce the dust load on downstream filters rather than strictly control fine particles such as PM1, F5 can be considered first.

F7: A Common Choice for Most HVAC Projects

F7 offers a good balance among filtration efficiency, airflow resistance, and procurement cost. It is therefore widely used in commercial HVAC, hospitals, pharmaceutical facilities, electronics plants, and industrial air-handling systems.

For projects requiring control of finer particles while maintaining an acceptable system pressure drop, F7 is often a key option.

F8: Suitable for Stricter Air-Quality Requirements

F8 can be considered when F7 cannot meet the required particle-control performance but upgrading directly to F9 may result in excessive resistance.

Typical applications include electronics manufacturing, pharmaceutical workshops, precision production environments, and cleanroom HVAC systems.

F9: Suitable for HEPA Pre-Filtration and High-Cleanliness Applications

F9 is generally used in air-handling systems requiring stricter fine-particle control.

For example:

G4 Primary Filter → F7/F9 Bag Filter → H13/H14 HEPA Filter

Pre-filtration reduces the amount of dust reaching the HEPA filter, slows the increase in HEPA pressure drop, and reduces replacement frequency.

4. What Other Parameters Should Be Considered When Selecting Bag Filters?

1. Rated Airflow

The design airflow for each filter must be provided during procurement.

Although 592 × 592 mm is a common size, filters with the same frame dimensions may have different numbers and depths of pockets. Their rated airflow may therefore differ.

Operating above the rated airflow will increase pressure drop and may shorten service life.

2. Initial Pressure Drop

Initial pressure drop is a critical parameter when purchasing bag filters.

Instead of asking only, “How much does an F7 filter cost?”, purchasers should request:

Filter Grade, Rated Airflow, and Initial Pressure Drop at That Airflow

Even when two filters have similar efficiency, their resistance may differ significantly because of structural and media differences.

3. Number and Depth of Pockets

Common bag filters may be designed with 4, 6, 8, or 10 pockets.

Increasing the number of pockets or selecting an appropriate pocket depth can enlarge the effective filtration area, reduce media velocity, and improve dust-holding capacity. However, the available installation space inside the AHU must also be checked.

More pockets are not always better. The pocket quantity and depth should be selected according to equipment dimensions and design airflow.

4. Filter Media

Common bag-filter media include:

• Synthetic fiber;

• Glass fiber;

• Special composite filter media.

Synthetic-fiber media are suitable for conventional HVAC systems. When a project has special requirements for efficiency stability, operating conditions, or compliance standards, the media should be selected according to the technical specification.

5. Can an F7 Filter Be Directly Upgraded to F9?

Identical external dimensions do not mean that the filters can be directly interchanged.

At least three parameters should be checked before upgrading:

First, whether the rated airflow is the same;
Second, whether the F9 initial pressure drop is within the fan’s available static-pressure capacity;
Third, whether the final pressure-drop setting is suitable for the existing system.

If the system does not have sufficient available static pressure, directly upgrading to F9 may reduce the supply airflow.

6. What Information Should Be Provided When Purchasing Bag Filters?

To avoid repeated confirmation after receiving a quotation, purchasers should provide:

Filter grade/ISO 16890 class, frame dimensions, number of pockets, pocket depth, rated airflow, allowable initial pressure drop, filter-media type, frame material, operating temperature, and humidity.

If the bag filter will be used as a pre-filter for a HEPA filter, the downstream HEPA class and the design airflow of the complete filtration system should also be specified.

7. Frequently Asked Questions

Which Is More Commonly Used, F7 or F9?

It depends on the project requirements. F7 is widely used in general HVAC and medium-efficiency filtration systems. F8 or F9 may be selected for clean environments or HEPA pre-filtration.

Does an F9 Filter Have a Longer Service Life?

Not necessarily. Filter service life mainly depends on airborne dust concentration, filtration area, airflow, operating hours, and upstream filter configuration—not simply on the filtration grade.

When Should a Bag Filter Be Replaced?

Replacement should be determined through differential-pressure monitoring. The filter should be replaced when it reaches the specified final pressure drop or when the pockets become damaged, contaminated, or deformed. Replacement should not be based solely on a fixed number of months.

8. Boyang Bag Filter Solutions

Boyang provides F5, F7, F8, F9, and corresponding ISO 16890 bag air filters for different HVAC and industrial air-handling systems. Pocket quantity, pocket depth, filter media, and frame structure can be configured according to project requirements.

For new projects, purchasers should provide the filter grade, ISO ePM efficiency, dimensions, design airflow, and target pressure drop. The pocket quantity and depth can then be determined according to the AHU structure.

For replacement projects, the original filter nameplate or complete technical specifications should be provided. Purchasing solely according to an “F7” or “F9” designation may result in airflow and pressure-drop mismatches.

The objective is not simply to select the highest grade, but to achieve a practical balance among filtration efficiency, system airflow, operating resistance, dust-holding capacity, and procurement cost.