Why is the concentration at the outlet of the chemical filter higher than at the inlet?

The outlet concentration is higher than the inlet. This phenomenon occasionally occurs in the detection of chemical filters. The first reaction is often 'the filter has failed' or 'the filter material is releasing pollutants'. But the actual situation may be more complicated. Sometimes the problem lies in the measurement link, and sometimes it is the change in operating conditions.

Before directly replacing the filter element, it is recommended to check the possible causes first. The treatment methods for different reasons are very different. If you go in the wrong direction and replace the filter element, the same problem may reoccur.

Confirm measurement comparability first

The outlet concentration is higher than the inlet, and the first thing to suspect is the measurement itself.

The sampling times do not match. If inlet concentrations are declining rapidly and outlet sampling lags for some time, the inlet data may already be low and the outlet data reflect slightly earlier higher concentrations. Putting the two sets of data together, there will be an illusion that the exit is higher than the entrance. In situations where the entrance fluctuates significantly, it is recommended to synchronously sample or perform time matching. For specific methods, please refer to Discussion on whether the concentration before and after the chemical filter should be measured at the same time .

Instrument differences. If different instruments are used at the inlet and outlet, or the same instrument has different calibration status at different times, there may be systematic deviations in the readings. When instrument A is used at the inlet and instrument B is used at the outlet, the response characteristics, detection limits and cross-interference of the two instruments may be different.

Sampling line contamination. If the last high-concentration sample remains in the outlet sampling pipeline, or the pipeline material adsorbs/releases the target gas, the outlet reading may also be higher. The inlet and outlet pipes must be in the same clean state.

Background interference. If there are other pollution sources near the outlet sampling point, or the environmental background increases during sampling, the outlet reading may be pulled up.

The inlet concentration was measured low. Improper location of the inlet sampling point, too short sampling time, and insufficient instrument stabilization may lead to low inlet concentration. When the entrance is low, the exit looks high.

Check filter section and downstream sources

If it is confirmed that there is no problem in the measurement process, then go to the downstream and filter section itself to check.

Media desorption. After the chemical filter is adsorbed saturated, if the temperature rises, the humidity changes, or the inlet concentration suddenly decreases, the adsorbed substances may desorb, causing the outlet concentration to increase temporarily. This situation is common in activated carbon media, especially in situations with large temperature fluctuations. For specific life span judgment, please refer to Discussion of how long chemical filters last .

The filter material itself releases. Some chemical filter materials retain trace amounts of volatile components during the production process, which may be released when new filter materials are first put into operation. This situation usually occurs in the initial operation stage and will gradually weaken after a period of operation.

Downstream material release. The outlet sampling point is located downstream of the filter. If the downstream pipes, seals, and box materials release target gases or interfering components at operating temperatures, the outlet concentration may increase. This type of release is not related to the filter itself, but will be reflected in the export data.

Bypass path. The rim bypass allows untreated air to flow around the filter media. If the concentration of the bypass air is higher than that of the filtered air, the outlet concentration may be pulled up. The presence of a bypass can be confirmed by seal inspection and smoke testing. For specific methods, please refer to Discussion on how to check frame bypass after chemical filter installation .

Wrong sampling location. If the outlet sampling point is actually located downstream of the pollution source or where untreated air is mixed, what is measured is not filtered air. The sampling point position should represent the true status of the outlet of the filter section. For specific point selection, please refer to Discussion of sampling points for chemical filter performance verification .

Analyze load and media status

After eliminating measurement and downstream factors, evaluate the condition of the filter media itself.

Changes in inlet composition or concentration. If materials have been changed recently, processes have been adjusted, or external inputs have changed, the composition and concentration of the inlet gas may have changed. The original filter material may not have enough processing capacity for the new components, and the outlet concentration will increase. For specific selection principles, please refer to Discussion on why chemical filters should be selected according to target gases .

Temperature and humidity changes. Increased temperature may accelerate desorption, and increased humidity may compete for adsorption sites. Changes in operating conditions will affect the actual performance of the filter media. For specific environmental impacts, please refer to Discussion of how long chemical filters last .

The consumption of filter material is nearing the end. The outlet concentration gradually increases, which is a common sign that the filter material is approaching the end of its life. If the outlet concentration is consistently higher than the inlet and measurement factors are eliminated, replacement may need to be evaluated. For specific replacement criteria, please refer to Discussion of the difference between initial efficiency and lifetime efficiency of chemical filters .

Disable exception configurations when necessary. If the outlet concentration continues to be abnormal and the cause cannot be determined, it is recommended to deactivate the filter section or isolate the location to prevent untreated air from affecting the protected area. It is not advisable to just increase the air volume to cover up the cause. Please refer to the specific differences. Discussion on whether the chemical filter can be replaced based on the pressure difference .

Project judgment example

When a project tested the concentration at the outlet of a chemical filter, it was found that the outlet was higher than the inlet. The first reaction on site was that the filter material failed and was ready to be replaced.

When troubleshooting, first review the sampling conditions: inlet sampling is in the equipment startup stage, outlet sampling is in the stable operation stage, and the working conditions at the two time points are inconsistent. After re-pairing sampling under stable operating conditions, the outlet concentration returned to normal levels, and the filter material itself was not replaced.

The concentration at the outlet of another project increased occasionally. The inspection found that the sampling pipeline was in a high-temperature area, and the pipeline material released trace amounts of interfering components after being heated. After replacing the sampling pipeline, the data returned to normal.

These two cases illustrate: when the outlet concentration is abnormal, check the measurement conditions first, and then check the filter section and downstream. involving When selecting a chemical filter product series , it is recommended to provide the testing data, sampling conditions and operating records to the supplier for verification.

Key points for selection and acceptance inspection

About Aili Industrial’s testing and troubleshooting packages

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 items with abnormal outlet concentration, the sampling method, system conditions, filter material status and operation records can be combined to assist in checking the investigation direction and treatment plan. When the filter is matched with MAU, FFU, EFU and other equipment, the location and maintenance method of the sampling port can be confirmed uniformly.

Related products:Chemical filter product series、MAU box chemical filter、FFU comes with chemical filter . Providing test data, system conditions and operating records when enquiring, to help confirm the appropriate solution. The specific selection is subject to the technical documents and test results confirmed by the project.