What is the division of labor between chemical filters and primary filters?

In the filtration chain of an air treatment system, primary filters and chemical filters often appear at the same time. Some people may ask: Can the primary efficiency process gas by the way? Can chemical filters replace primary filters?

The answer to both questions is no. The primary filter tube is for particles, and the chemical filter tube is for gases. The objects they deal with are completely different. They have a cooperative relationship in a system, not a substitution relationship. For specific division of labor logic, please refer to Discussion of the difference between chemical filters and HEPA filters .

Each layer carries different loads

Pollutants in the air are often not single, dust and gas may exist at the same time.

The task of the primary filter is to intercept coarser particles and fibers. Its location is usually at the front of the system and bears the largest dust load. Dust, fibers, and larger particles in the air are first stopped by it. Maintenance frequency at this level is relatively high and replacement costs are relatively controllable.

The task of chemical filters is to deal with gaseous pollutants. It relies on an adapted medium to adsorb or react the target gas, and the position may be after the initial effect, before the medium effect or the high efficiency. It deals with pollution at the molecular level, not particles.

The logic of the cooperation between the two is: the front end does rough filtration, and the back end does gas treatment. The primary effect blocks dust, and the chemical filter section can work under relatively clean air flow conditions. If the chemical filter section is placed before the primary effect, a large amount of dust will cover the media surface, reducing the adsorption efficiency and increasing the replacement frequency.

Pre-filtration protects chemical media

The media of chemical filters usually has a certain pore structure, and dust will occupy the surface or block the channel after entering.

Dust coverage reduces the effective capacity of the media. After the medium surface is covered with dust, the contact area between the target gas and the medium is reduced, and the processing capacity is reduced. In severe cases, dust may block the air flow channel and increase system resistance.

Front-end maintenance affects back-end performance. If the primary filter has not been replaced for a long time, or if a bypass is installed, the dust will directly enter the chemical filtration section. In this case, the life of the chemical filter may be significantly shortened and the outlet concentration may rise prematurely.

When checking the effectiveness of chemical filtration, front-end status should not be ignored. When it is found that the gas processing capacity has decreased, first confirm whether the primary filter is normal, and then check whether the chemical media is consumed. For specific troubleshooting ideas, please refer to Discussion of what to check if the pressure difference before and after the chemical filter suddenly rises .

involving In the fresh air system of the MAU box-type chemical filter , the positional relationship between the primary effect and the chemical filter section is usually determined at the unit design stage, and should be checked in the corresponding order during maintenance.

Replacement basis cannot be mixed

The maintenance judgment methods of the two filters are different and cannot be mixed and managed together.

The primary filter mainly depends on dust accumulation and pressure difference. As the running time increases, dust accumulates on the surface of the filter material and the pressure difference gradually increases. When the final resistance agreed upon by the project is reached, replacement will be arranged.

Chemical filters mainly look at gas processing capabilities. After the medium is consumed, the outlet concentration may gradually increase, but the change in pressure difference may not be obvious. The resistance of some chemical filters does not change much during their lifespan, and it is impossible to judge whether they need to be replaced just by looking at the pressure difference.

The replacement cycles of the two layers may be out of sync. The primary filter is usually replaced more frequently than the chemical filter section. The maintenance ledger should be recorded in layers, and inspection cycles and replacement basis should be set respectively. You cannot replace the chemical section just because the primary effect is changed, or conversely, wait until the outlet of the chemical section exceeds the standard before you think of checking the primary effect.

When replacing the chemical filter, it is recommended to check the initial effect status simultaneously. If the initial effect is close to the end of its life, you can consider replacing it together to avoid repeated shutdowns in a short period of time.

Positioning in a combined system

In a complete filtration chain, the positions of the primary effect and chemical filtration sections need to be confirmed according to system conditions.

The common sequence is: primary effect → medium effect → chemical filtration section → high efficiency/ULPA. The primary and intermediate effects process particles first, the chemical filtration section processes gases, and the terminal efficiency is responsible for final particle control. This sequence is not fixed and must be adjusted according to dust load, medium characteristics, temperature and humidity conditions and system resistance.

Whether the chemical filtration section is placed before or after the medium efficiency depends on the dust concentration. If the dust load is high, it is recommended to place it after the medium efficiency to allow more particles to be intercepted at the front end. If the dust concentration is very low, it is acceptable to place it before the medium efficiency, but the maintenance cycle should be adjusted accordingly.

The resistance of the chemical filtration section should be included in the total resistance of the system. After adding this section, the fan operating point will change. When selecting, check the initial resistance, final resistance and fan margin to avoid insufficient terminal air volume. Supporting For FFU systems equipped with chemical filters , the unit capacity also needs to be checked.

Temperature and humidity conditions are also considered. If the chemical filter section is placed after the surface cooler and the air humidity is high, you need to confirm whether the media is suitable. Some media may degrade in high humidity environments or require additional dehumidification measures. For specific layout, please refer to Discussion on whether the MAU chemical filter should be installed before or after the surface cooler .

Project judgment example

A certain fresh air system was equipped with a primary filter and a chemical filtration section. After running for a period of time, it was found that the outlet concentration of the chemical filtration section increased in advance.

Troubleshooting process: First check the status of the chemical media and find that there is obvious dust deposition on the surface of some media. Further inspection of the primary filter revealed that the filter material was heavily dusted and the pressure difference exceeded the replacement standard, but it was not replaced in time.

Treatment method: Replace the primary filter, clean the dust on the surface of the chemical filter section, and re-measure the outlet concentration. After adjustment, the outlet concentration returned to normal levels and the remaining life of the chemical filtration section was extended.

This case shows that the maintenance status of the primary filter directly affects the performance of the chemical filtration section. If the front-end protection is not in place, the burden on the back-end will increase. involving For the configuration of the chemical filter product series , it is recommended that the maintenance requirements for filters at all levels be included in the operating procedures.

Key points for selection and acceptance inspection

About Aili Industrial’s filter chain matching

The core of the cooperation between primary filter and chemical filter is to let each layer bear the load that they are good at, and then uniformly check the resistance and maintenance rhythm at the system level.

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 particle filtration products. For projects that require multi-stage filtration, dust load, gas composition, air volume and installation space can be combined to help check the type, sequence and maintenance methods of filters at each stage. When the filter is matched with MAU, FFU, EFU and other equipment, the resistance, interface and maintenance methods can be confirmed uniformly.

Related products:Chemical filter product series、MAU box chemical filter、FFU comes with chemical filter . Providing dust load, target gas and system conditions at the time of inquiry will help confirm the appropriate filter chain configuration. The specific selection is subject to the technical documents and test results confirmed by the project.