Should the FFU chemical filter be placed before or after the HEPA?

When configuring both the particle filtration section and the chemical filtration section in the FFU, a common question is: who is in front and who is in the back.

This order is not arbitrary. It affects the risk of powder loss from chemical media, HEPA burden, airflow path and maintenance methods. The order is different, and the tasks they undertake and the working conditions they face are also different. For specific differences, please refer to Discussion of the difference between chemical filters and HEPA filters .

Identify filter objects individually

Before discussing the sequence, first confirm the division of labor between the two filters.

HEPA is responsible for particle control. Its efficiency level, sealing method and leak detection requirements are implemented in accordance with corresponding standards. Particle filters focus on particle size range, filtration efficiency, installation sealing and on-site leak detection.

The chemical filtration section is responsible for target gas treatment. Its media type, capacity and replacement cycle are determined based on gas composition, concentration and air volume.

The two cannot be substituted for each other. The chemical filter section has limited ability to intercept particles, and HEPA has no ability to process gas molecules. Sequential discussion is not a substitute for checking the respective functions. For specific principles, please refer to Discussion on why chemical filters should be selected according to the target gas .

The situation when the chemical filtration section is placed in front of HEPA

This is a relatively common configuration method.

Protect HEPA. The chemical filtration section handles the gas first, and HEPA is responsible for final particle control at the back. If there is a trace amount of powder falling out of the chemical medium, HEPA can intercept these particles and prevent them from entering the clean area. For specific verification methods, please refer to Discussion on how to verify the low outgassing requirements of ULPA .

HEPA burdens may vary. If the chemical filtration section can effectively reduce gaseous pollution, HEPA mainly faces particles. However, the chemical filter section itself may also produce resistance, and it is necessary to confirm whether the fan can overcome the resistance of the two filters at the same time. For specific accounting methods, please refer to Discussion of how much the added resistance of the chemical filter will increase the fan's electricity usage .

Maintenance method. The replacement cycle of the chemical filter section is usually different from that of HEPA. If both sections are installed inside the FFU, it may be necessary to disassemble the HEPA or bypass the HEPA when replacing the chemical filter section. Maintenance access and operating sequences should be confirmed during the design stage.

Impact on HEPA sealing. After adding a chemical filtration section, the internal structure of the FFU may change. HEPA installation sealing needs to be reaffirmed. Frame bypass allows untreated air to bypass the particle filter, affecting air supply quality. For specific inspection methods, please refer to Discussion on how to check frame bypass after chemical filter installation .

The chemical filtration section is placed after HEPA

This configuration is relatively rare, but it also has applicable scenarios.

Media cleanliness and compatibility requirements are higher. If the chemical filter section is placed after the HEPA, the air it treats will be sent directly to the clean area. At this time, the powder loss and gas release of the chemical medium must be strictly controlled, otherwise it will directly affect the air supply quality. When selecting, confirm whether the medium meets the clean compatibility requirements. For specific detection methods, please refer to Discussion of why chemical filter performance reports list specific target gases .

HEPA protection disappears. Chemical filtration section In the previous configuration, HEPA can intercept the powder loss of chemical media. But in this configuration, HEPA is in the front and the chemical filter section is in the back. HEPA cannot provide particle protection for the chemical filter section. If the chemical media is sensitive to dust, additional pre-filtration may be required.

Maintenance spaces may be more flexible. If the HEPA is at the top and the chemical filter section is at the bottom, you may not need to move the HEPA when replacing the chemical filter section. But this depends on the specific structure of the FFU.

Airflow path needs to be confirmed. The fan position and internal structure of some FFU determine the airflow direction. If the chemical filter section is placed after the HEPA, you need to confirm that the airflow actually passes through the two-stage filter without bypass or short circuit. For specific configuration, please refer to Discussion of what localized contamination can be treated by FFU plus chemical filter .

Pay attention to media cleanliness and compatibility

Regardless of the sequence, cleanliness compatibility of the chemical media needs to be confirmed.

Risk of falling powder. Certain chemical filter materials may generate dust during operation. If these dusts enter clean areas, they may affect particle control. For specific detection methods, please refer to Discussion on how to deal with dust falling on the surface of chemical filters .

Risk of outgassing. The chemical media themselves may release trace amounts of organic matter. If the filter section is located before the end of the air supply, this part of the release will directly enter the protected area. When selecting, confirm whether the medium meets the project's requirements for gas release. For specific verification methods, please refer to Discussion of the difference between initial efficiency and lifetime efficiency of chemical filters .

Structurally sealed. The frame seals of both filters must be reliable. Any bypass in any section will affect the overall effect. It is recommended to check the sealing status section by section after installation. For specific methods, please refer to Discussion on how to check frame bypass after chemical filter installation .

Verify overall airflow

After the sequence is determined, the running status of the entire FFU needs to be verified.

Wind speed distribution. After installing a two-stage filter, the wind speed distribution on the air outlet surface may change. Multi-point measurements confirm wind speed uniformity. For specific methods, please refer to Discussion on how to check the target wind speed after FFU is equipped with a chemical filter .

Resistance and fan margin. The resistance of the two filters must be calculated together. Whether the fan can maintain the target air volume needs to be confirmed based on the actual working point. For specific methods, please refer to Discussion of how much the added resistance of the chemical filter will increase the fan's electricity usage .

Outlet concentration detection. Detect the target gas outlet concentration according to project requirements to confirm the effect of the chemical filtration section. The particle cleanliness must also be confirmed according to the original requirements, and the effects of the two-stage filters must be verified separately.

Maintenance operation confirmation. Is the operation feasible when replacing the chemical filter section or HEPA? The internal space of the equipment is limited, and the replacement sequence and disassembly and assembly methods must be confirmed during the planning stage. involving The system, interfaces and maintenance methods of the FFU supporting chemical filter need to be checked together.

Project judgment example

A certain FFU needs to install a chemical filtration section before the original HEPA to process organic gases in the circulating air flow.

The preliminary plan puts the chemical filtration section in front of the HEPA. However, the evaluation found that the internal space of the FFU is limited, and the distance between the two filters after installation is too small, and the airflow distribution may be uneven. At the same time, the resistance of the chemical filter section brings the fan operating point close to the upper limit.

Adjustment plan: Change the chemical filtration section to a thin structure to reduce the space occupied; keep a sufficient flow equalization distance in front of the HEPA; after calculating the fan working point, adjust the operating gear and confirm that the air volume meets the requirements. After installation, measure the wind speed distribution and outlet concentration to confirm that the design goals are met.

This case illustrates: The sequence before and after should be comprehensively judged based on space, resistance and maintenance conditions. involving For the configuration of the chemical filter product series , it is recommended that the FFU parameters, gas requirements and space conditions be provided to the supplier for verification.

Key points for selection and acceptance inspection

About Aili Industrial’s FFU chemical filtration package

The core of the arrangement of the FFU chemical filtration section and HEPA is to match the filter objects, media characteristics, space conditions and maintenance methods.

Aili Industrial has independent manufacturing capabilities in filters, motors, fans, control systems and sheet metal processing, and can provide chemical filter product series and supporting air supply equipment. For FFU projects, we can assist in checking the sequence and configuration of the filter sections based on the original unit parameters, gas requirements, installation space and operation requirements. When the filter is matched with FFU, EFU and other equipment, the resistance, interface and maintenance methods can be confirmed uniformly.

Related products:Chemical filter product series、FFU matching chemical filter、MAU box chemical filter . Providing FFU parameters, gas requirements and system conditions when enquiring, will help confirm the appropriate filter section layout plan. The specific selection is subject to the technical documents and test results confirmed by the project.