What should I do after ULPA reaches the final resistance?

When resistance finally arrives, the first reaction of many people on site is 'it's time to change the filter element.' This judgment is correct, but the final resistance is only a maintenance limit, not the only basis for replacement.

First figure out how the final resistance is determined: it is usually the operation and maintenance limit specified by the project or product. At this point, the resistance of the filter element has risen to a level that needs to be dealt with. But the approach isn’t necessarily replacement—it might be inspection, adjustment, or scheduled replacement first. Moreover, if the pressure difference has not reached the final resistance, but there is leakage, damage or insufficient air volume, it also needs to be dealt with.

So the question is not 'what to do when the resistance reaches the end', but 'what to check next when we reach this point'.

How is the final resistance itself set?

Final resistance is not an arbitrary number. It usually comes from the balance of several conditions:

Fan capacity. After the filter element resistance increases, can the fan still maintain the target air volume? If the final resistance is set too high, the fan may not be able to provide enough air before it reaches this line.

other parts of the system. The capabilities of pre-filtration, air ducts, terminal adjustment devices, and the total pressure loss of the air conditioning system will all affect the reasonable range of final resistance.

Cleanliness requirements. If the project has strict particle control requirements, it may be necessary to arrange processing during the rising resistance stage instead of waiting for a fixed value.

Maintenance strategy. Replacement cycle, spare parts inventory, and shutdown schedule will also affect the final resistance setting. Some projects prefer to replace them in advance to reduce operational risks; some projects hope to use up the life of the filter element and reduce the frequency of purchases.

Therefore, the final resistance of different projects may be different and cannot be directly applied. Specific settings should be written into maintenance documents or contracts and kept consistent during operation.

When you reach the final resistance, check these few items first

When the pressure difference reaches the agreed final resistance, do not disassemble the filter element directly. Confirm a few things first:

Whether the air volume can still meet the requirements? Measure the actual air supply volume under the final resistance condition. If the air volume is already lower than the design value, even if the pressure difference has just reached the final resistance, arrangements need to be made. involving For the FFU fan filter unit system, the unit gear and single table wind speed must be checked; the equipment-side application is as follows: For the fan filter unit at the EFU equipment end , it is necessary to confirm whether the air supply reaches the standard under the actual operating status of the equipment.

Whether the on-site leak detection is qualified. Rising resistance does not necessarily mean there is a problem with the integrity, but at the final resistance stage, it is recommended to conduct a leak test according to project requirements. If a leak is found, the treatment method is different from simply replacing the filter element. Leak detection methods and criteria can refer to relevant standards.

Is the rising trend of pressure difference normal? If the pressure difference gradually rises to the final resistance, it is normal dust accumulation. If it is accompanied by a sudden jump or abnormal fluctuation, you need to investigate the cause first and then decide whether to replace it. For specific troubleshooting ideas, please refer to the relevant instructions for sudden rise in pressure difference.

Filter element appearance and operating records. Are there any moisture, deformation, oil stains or other abnormalities? Are there any over-working conditions, abnormal events or maintenance history in the operation records? This information can help determine the reasons for replacement and subsequent improvement directions.

Replacement is not just about replacing a filter element

After confirming that it needs to be replaced, there are still a few things that need to be done simultaneously.

Check new filter specifications. The model, grade, size, and sealing method must be consistent with the original filter element, or be confirmed according to the project change process. involving For HEPA/ULPA air filter products of different grades, the test data must be stored accordingly.

Check the mounting frame and seals. When replacing the filter element, check the flatness of the installation frame, the condition of the sealing gasket, and the matching dimensions of the liquid tank. If the seal has aged or deformed, it is recommended to deal with it together, otherwise the new filter element may not be tightly sealed when installed.

Record the data before and after replacement. The final resistance, running time, and reason for replacement of the old filter element, and the initial resistance, installation date, and leak detection results of the new filter element must be recorded accordingly. This set of data is the basis for subsequent judgments on whether the replacement cycle is reasonable.

Re-establish the operating baseline. After the new filter element is installed, measure the air volume and pressure difference under the same working conditions as the starting point for subsequent comparisons. Don't take the dust-collected data from the old filter element as the standard for the new filter element.

If you don’t want to wait until the final resistance

Some projects will choose to replace them in advance without waiting for the pressure difference to reach the limit value. Whether this is appropriate depends on operational risks and maintenance strategy.

Reasons for early replacement: If it is difficult to replace the filter element position, the cost of downtime is high, or if a problem occurs during operation and the impact is large, early replacement can reduce the risk. Some projects arrange replacement in the planned shutdown window instead of equalizing the pressure difference alarm.

Reasons to wait for the final resistance: If the system is running stably, the monitoring means are perfect, and spare parts are supplied in a timely manner, replacing it according to the final resistance can make fuller use of the filter element life and reduce the frequency of purchases.

There is no absolute advantage or disadvantage between the two strategies. The key is to write them clearly in the maintenance document and keep them consistent. You cannot change it according to the final resistance today, change it in advance tomorrow, and wait for the alarm the day after tomorrow. Recording and analyzing the reasons for replacement can help optimize subsequent maintenance plans.

In addition, terminal resistance is not the only triggering condition. Damage, leakage, moisture, and process changes may trigger replacement when the pressure difference does not reach the limit. The results of the integrity check sometimes illustrate the actual status of the filter element better than the pressure difference.

Project judgment example

The ULPA pressure difference in a certain clean room reached the agreed final resistance of 600 Pa. On-site air volume measurement found that the actual air supply volume was about 15% lower than the design value.

In this case, even if the pressure difference has just reached the final resistance, replacement or adjustment needs to be arranged. Because insufficient air volume will affect the particle control effect in the clean area, continued operation may bring risks.

During the replacement, the installation frame and gasket were simultaneously checked and it was found that the gasket was slightly deformed, so they were replaced together. After the new filter element is installed, measure the air volume at the same gear, restore it to the design range, and complete on-site leak detection. The final resistance and air volume data of the old filter element are marked in the record as a reference for subsequent replacement cycles.

This case illustrates: The final resistance is the reference point, but the actual decision-making must take into account air volume, leak detection and operational risks. involving ULPA ultra high efficiency filter or HEPA high-efficiency filter has the same judgment logic. The replacement of

Selection and Acceptance Checklist

About Aili Industrial’s final resistance and replacement support

The final resistance setting and replacement judgment need to be combined with product parameters, fan capacity and operating data.

Aili Industrial's manufacturing system covers filters, motors, fans, control systems and complete machine assembly, and can provide ULPA ultra high efficiency filter、HEPA high-efficiency filter and supporting air supply equipment. filter with FFU fan filter unit、When the EFU equipment-end fan filter unit is matched in the same system, the filter element resistance curve and the fan operating point can be calculated together during the planning stage to facilitate the setting of a reasonable final resistance range.

For projects in operation, the original filter element model, operating data and replacement records can be provided to assist in checking spare parts plans. The specific maintenance limits and replacement cycles are subject to the actual operation data and maintenance documents of the project.