When the chemical filter is located in the negative pressure section, you should focus on checking the external air intake of the access door, seams and interfaces. External air leakage may bypass the intended treatment path and affect pollution control. You cannot just check the filter element frame. The trouble with the negative pressure section is that the leaked air has not passed through the filter, but the detection point may not be able to see it at all.

The direction of air leakage in the negative pressure section is opposite to that of positive pressure

When the chemical filter section is installed on the suction side of the fan, the pressure inside the box is lower than the surrounding environment. Once there is a gap in the box, the outside air will be 'sucked' in instead of the internal air leaking out. This difference in direction determines the focus of inspection. The positive pressure section checks 'whether the treated air is leaked', and the negative pressure section checks 'whether the untreated air is mixed in'. The mixing point may be downstream of the filter element, in which case the outlet test may fail even if the filter element itself is intact. Air leaks in the negative pressure section are often more hidden, because the outside air looks 'clean', but its gas composition may not meet the process requirements.

Access doors and flanges are the first inspection objects

The most common air leakage points in negative pressure boxes are not at the filter element frame, but at the access doors, flange interfaces and seams. The aging of the sealing strip of the access door, the loosening of the pressing lock, and the deformation of the door frame will all cause continuous suction under negative pressure. Uneven compression of flange gaskets, missing bolts, or misaligned pipe connections will also allow outside air to enter through gaps. The amount of air leakage at these locations may not be large, but if it occurs downstream of the filter section, the impact on the outlet concentration will be amplified. During inspection, the pressure difference status under normal operating air volume should be confirmed one by one, rather than visual judgment during shutdown. For general principles of sealing edge protection, please refer toHow to avoid damaging the sealing edge when installing a chemical filter?。

Filter element sealing and frame sealing should be judged separately

The sealing problem of the negative pressure section is divided into two levels: the sealing between the filter element and the frame, and the sealing between the frame and the box. If the former fails, the air will bypass the filter but remains inside the box; if the latter fails, external air will directly enter the box. The two have different investigation methods and scope of influence. The filter element seal can be disassembled to check the status and tightness of the sealing gasket. The sealing of the frame and box involves sheet metal splicing, welding seams and insulation layer penetration. When Aili Industrial is supporting chemical filtration in the negative pressure section, it can check the interface mode between the frame and the box according to the box structure. The sheet metal and complete machine assembly capabilities can make adjustments to the joints and flange forms.

Leak detection methods should be suitable for negative pressure conditions

For leak detection in the negative pressure section, the conventional positive pressure smoke or soapy water method is in the wrong direction. The smoke generation method is easy to observe under positive pressure. Under negative pressure, the smoke will be inhaled instead of blown out, and visibility will be reduced. More practical methods include: using a portable gas detector to scan closely at suspicious joints to see if the readings increase as you get closer; or performing a pressure holding test on the box after shutting down to confirm whether the pressure drop rate is within the allowable range. Method selection should be based on site conditions and safety requirements, and reagents that will contaminate the filter material or affect subsequent operations cannot be used. For the overall verification ideas after installation, please refer toDo I need to clean the original filter section before installing the chemical filter?。

After correction, review in running state

After the seal correction is completed, it should be reviewed under normal operating air volume and pressure difference. The position that does not leak under static conditions may cause gaps due to changes in the force on the box after the fan is running. The review includes: whether the pressure difference before and after the filter section is stable, whether the mixed supply air or outlet gas returns to the expected level, and whether there is any abnormal air flow sound around the access door and flange. If air leakage occurs downstream of the filter section, you may not be able to detect the problem by measuring only the outlet of the filter element. You need to also measure the gas after mixing or in the protected position. Aili Industry can provide selection packages for box-type, flat-plate and V-type chemical filtration products. For box-type MAU packages, please refer toMAU box type chemical filter , product series can be viewedChemical filter product range . Device-side applications can useChemical filter FFU type, integrated chemical adsorption layer withHEPA/ULPA filter particle filtration. When installing the negative pressure section, the sealing interface and box structure can be checked together during the matching stage. The specific gas effect, sealing performance and replacement cycle still need to be verified based on the corresponding configuration data and on-site working conditions. The actual results are subject to on-site leak detection and gas detection results.

Related questions and answers

How to distinguish between air leakage in the negative pressure section and failure of the filter element seal?
When the filter element seal fails, the air circulates around the filter element but still circulates in the box; air leakage in the box is caused by direct entry of external air. Point detection can be set up and downstream of the filter element respectively, and combined with box seam scanning and judgment.

There is no leakage during shutdown, but the outlet exceeds the standard after operation. What may be the reason?
The force on the box under negative pressure is different during operation and shutdown, and static leak-proof gaps may appear after the fan is started. Seams and door seals should be rechecked under normal operating pressure differentials.

Can Aili Industrial help check the negative pressure section sealing plan?
The matching method of frame seal and box seal can be checked based on the box structure, flange interface and filter section position. The specific plan needs to be confirmed based on product data and on-site working conditions.

Related articles

• How to prevent the leakage of contaminated air when a chemical filter is installed in the positive pressure section?

• Can the chemical filter still be installed after the frame is deformed?

• Do I need to clean the original filter section before installing the chemical filter?

• What should I do if the chemical filter cannot fit into the existing card slot?

Key points for selection and acceptance inspection