Influence of Airflow (Face velocity) on Filtration Efficiency in Filter Testing

A series of experiments triggered by a phenomenon that doesn't comply with the theory

Theoretically, for the same filter, the higher the airflow, the lower the filtration efficiency.

However, our engineers found in the testing process, when the velocity of the wind is high to a certain value, the filtration efficiency no longer follows the rule of decreasing with the increase in air volume, but increases with the increase in air volume.

1. Test equipments

The test equipments used for these tests are SC-7099 medium and high efficiency filter test rig and SC-13011 small filter tester, both branded as SCINCE PURGE.

Except for the panel filter 3 is tested with SC-13011, all other filter elements are tested with SC-7099.

2. Filter tested

A total of 5 filters were tested in this test, including 3 panel filters and 2 cylinder filters.

Panel filter element 1 is an unbranded cabin air conditioning filter element purchased from JD, panel filter element 2 is a MANN+HUMMEL brand car air conditioning filter element, and panel filter element 3 is the filter element sent by our customer for testing.

Cylinder filter element 1 is the filter element produced by our customer, cylinder filter element 2 is purchased from JD, no brand.

The specific parameters of the 3 panel filters are shown in Table 1, and the specific parameters of the 2 cylinder filters are shown in Table 2. 

3.      Test results

3.1 Panel filter 1 test results

The results of the filtration efficiency of 0.5 μm at different velocities were analyzed and the results are shown in Fig. 1.

3.2 Panel filter2 test results

The results of the filtration efficiency of 0.5 μm at different velocities were analyzed and the results are shown in Fig. 2.

3.3 Panel filter3 test results

The results of the filtration efficiency of 0.5 μm at different velocities were analyzed and the results are shown in Fig. 3.

3.4 Cylinder filter 1 test results

The results of the filtration efficiency of 0.5 μm at different velocities were analyzed and the results are shown in Fig. 4.

3.5 Cylinder filter 2 test results

The results of the filtration efficiency of 0.5 μm at different velocities were analyzed and the results are shown in Fig. 5.

4. Analysis of test results

The test result curves for all the filters show a trend of decline and then increase in filtration efficiency as the airflow increases, with a minimum point of efficiency. The first half of the curve, where efficiency is decreasing as airflow increases, is consistent with existing theory.

The minimum efficiency point almost always occurs near the surface air velocity recommended by the standard. This indicates that the setting of face velocity in the standards related to filter testing takes into account the risk of using filters, and the use of the minimum efficiency as the evaluation standard for filters makes it possible to achieve good results when filters are used at other air velocities.

In the second half of the curve, the efficiency turns up as the airflow increases. This change is not in accordance with the existing theory. What are the possible reasons for the above phenomenon?

5. Summary

Is it the influence of the test equipment itself?

We sent the same batch of samples to a third-party testing organization for testing, and found the same phenomenon, which could exclude the cause of the testing equipment.

Later on, after we checked the related information from many sources, as well as communicated with the professionals in the industry and analyzed them, we considered the following possible reasons:

1) When tested in higher air volume, the filter element strength is not enough, the structure is damaged.

2) Filter materials with high filtration efficiency under high air volume do exist.