How to judge the difference in resistance of the same batch of ULPA?

When a batch of filter elements arrived, I took out a few to measure the resistance and found that the values ​​were not exactly the same. This in itself is not surprising - small fluctuations in resistance are normal for products from the same batch. The question is: how much fluctuation is considered normal and how much indicates something is wrong.

To judge this, you cannot just look at the resistance number itself. It is necessary to unify the test conditions first, then compare the manufacturing tolerances of the product structure, and finally evaluate it based on the actual performance after installation. If one of the three links is missing, the judgment may go astray.

Unify the test conditions first

If the resistance data of the same batch of products are not measured under the same conditions, comparison will be meaningless.

The air volume should be consistent. Resistance changes with flow, and readings at different air volumes cannot be directly compared. Record the actual air volume during measurement, convert it to the same air volume and then compare. For specific conversion methods, please refer to the relevant instructions on the resistance curve.

The measuring point locations must be consistent. Whether the pressure tapping port is before and after the filter element or at the inlet and outlet of the box, the resistance measured is different. When comparing the same batch, all data must be defined by the same measurement point.

Instruments and measurement methods should be consistent. Different differential pressure gauges, different anemometers, and different sampling times may cause differences in readings. When measuring one unit at a time, try to use the same set of instruments, the same operator, and the same set of processes.

Environmental conditions are also recorded. Temperature and humidity have a certain impact on the resistance of the filter material. If the measurement time span is large, environmental changes also need to be taken into account.

It only makes sense to compare the resistance values ​​after unifying these items. If you find that the reading of a certain unit deviates significantly, first review the measurement process to confirm that it is not caused by measurement error.

Manufacturing tolerances determine the tolerance range

Intra-batch fluctuations in resistance come from the accumulation of several manufacturing links.

Batch-to-batch consistency of the filter media itself. Filter materials from the same supplier and batch may still have slight fluctuations in fiber diameter, weight, and porosity. These fluctuations are passed on to the finished resistance.

Stability of pleating process. The pleat height, pleat distance, and pleat uniformity will all affect the effective filtration area and airflow channel. The status of the folding equipment and the control of operating parameters determine the consistency of this link.

The amount and distribution of edge sealing glue. The colloid occupies part of the filter area and also affects the air flow channel. If the amount of glue is not accurately controlled, the resistance will vary.

Frame assembly accuracy. The flatness of the outer frame and the position of the sealing surface affect the matching state of the filter element and the installation frame. The actual resistance may be different depending on the fit status.

Therefore, there are differences in resistance within batches, which is a normal phenomenon in the manufacturing process. The key is to have an agreed tolerance range. This range is usually confirmed in a technical agreement between the supplier and the buyer, or specified in the product standard. Single products that exceed the tolerance require further inspection. involving ULPA ultra high efficiency filter or HEPA high-efficiency filter is the same. The acceptance method of

Similar resistance does not mean that other indicators are consistent

This is an easily overlooked point.

Resistance is just one performance parameter of a filter. The resistance is similar, indicating that the difficulty of airflow through the filter material is similar, but it cannot be inferred that the efficiency, leak detection results, and dust holding capacity are also the same.

The intra-batch consistency of efficiency needs to be confirmed by efficiency testing. If the project has requirements for efficiency consistency, it is necessary to check the efficiency data of each unit or batch, and not just look at the resistance.

Integrity and intra-batch consistency need to be confirmed by leak detection. Leak detection records one by one are the basis for judging the sealing status of each product, and resistance data cannot replace it.

Air volume characteristics should also be checked separately. Under the same resistance, the air volume-resistance curves of different products may be different. Whether the actual air supply volume is consistent after installation depends on the system matching.

Therefore, when judging the consistency of products from the same batch, resistance is only one of them. Which indicators the project requires are consistent, and which data must be provided accordingly. involving For HEPA/ULPA air filter products of different grades, testing items are carried out according to corresponding standards.

How to evaluate the consistency after installation

The factory data is consistent and may not be consistent after installation. System-level factors can also contribute to performance differences.

Branch resistance is different. When multiple filters are connected in parallel, the air duct length, number of elbows, and air valve positions of each branch may be different. Even if the resistance of the filter element itself is consistent, the actual air volume distribution may be uneven. For specific balancing methods, please refer to the instructions related to parallel air volume.

The installation status is different. The pressing force, gasket compression amount, and installation direction may all affect the actual passing wind speed and resistance. Checking the installation status one by one is a necessary step to evaluate consistency.

Supporting FFU fan filter unit system, the performance difference of the unit itself will also be superimposed. The fan curves and control parameters of the same batch of FFU may be slightly different. The resistance of the filter element is the same, but the performance of the whole machine may still be different. Device-side applications such as The EFU equipment-end fan filter unit has a compact internal structure, and the installation status has a more obvious impact on the actual resistance.

Therefore, the evaluation after installation must look at the filter element, installation and system conditions together. Only by measuring the surface wind speed or pressure difference one by one and recording the operating gear can the actual consistency of the entire batch of products be judged.

Project judgment example

For a certain project, 20 ULPA units arrived, and 5 units were randomly tested. The resistance readings were 210 Pa, 215 Pa, 208 Pa, 212 Pa, and 230 Pa respectively.

The values ​​of the first four stations fluctuate around 215 Pa, with a deviation of about ±3%, which is within the normal range. The fifth station is 230 Pa, which is about 7% higher than the average and beyond the fluctuation range of the other stations.

Solution: First review the measurement process of the fifth station - confirm that the air volume, measuring points, instruments and operation methods are consistent with the other four stations. If the measurement is correct, conduct a more detailed inspection of the unit separately: appearance, installation status, and resistance curve review. If it is confirmed that the resistance is indeed high, determine whether it needs to be replaced or adjusted according to the tolerance specified in the technical agreement.

The key to this case is: eliminate measurement errors first, and then determine product differences. Directly judging based on average values ​​may accidentally damage normal products, while ignoring abnormal values ​​may miss problems.

Selection and Acceptance Checklist

About Aili Industrial’s intra-batch consistency guarantee

The control of intra-batch resistance differences relies on the consistency of the manufacturing process and the integrity of the testing process.

Aili Industrial has independent manufacturing capabilities in filters, motors, fans, control systems and sheet metal processing. Filter material incoming materials, pleating process, edge sealing and assembly processes are all controlled within the same manufacturing system. For projects with consistency requirements, unit-by-unit testing data can be provided as agreed, and the corresponding relationship between test conditions and numbers can be noted in the delivery materials. When the filter is delivered together with FFU, EFU and other equipment, the detection range of the filter element and the entire machine can be confirmed uniformly.

Clarifying tolerance requirements, test items and reporting forms when enquiring, will help the smooth implementation of subsequent batch acceptance. Specific indicators are subject to the technical documents and testing data confirmed by the project.