What are the main reasons for the unqualified detection of HEPA filters?

18/08/2020

    High-efficiency filters must be tested one by one before they leave the factory due to strict requirements on the place of use. They can be put into use only after being qualified. The filter test includes factory inspection and on-site inspection. During these two tests, the reasons for the unqualified filter may be as follows. Visual inspection and simple test can find out the reason that the surface of the filter material is broken or slightly damaged. It is easy to observe the filter material damage with the naked eye, and it is easier to repair a small amount of damage. This repair is best performed in the filter manufacturer. Some minor damages are not easy to observe with the naked eye, and can only be detected by the test bench. In the filter manufacturing process, the filter material is too heavy or some human factors can damage the filter material.

    If it is only damaged in a few places and is not serious, it may be repaired into a qualified product, but the repair quality must meet the standard requirements. Sealing defects, air leakage at the junction of the filter material and the filter frame. In the production inspection, most efficiency failures are caused by this kind of air leakage. Most of this situation can be repaired, but the repaired appearance should not have obvious defects. Air leakage at the joints of the filter sealing tape. Many HEPA filters have sealing strips on the frame. Sometimes, if the interface is not handled well, it will cause air leakage. Some manufacturers use on-site foamed polyurethane sealing strips, which have no seams and no air leakage problems.

    If you use a rubber strip with joints, the joints are best made into a labyrinth form. The raw material itself is due to the low efficiency of the filter material itself. If the efficiency of the filter material itself does not meet the requirements (the definition of high-efficiency filter material: testing at a wind speed of 5.3cm/s, the filtration efficiency for 0.3μm dust is ≥99.97%), it is impossible to manufacture a qualified filter. Everyone understands this, but there are indeed some labels on the market that are highly efficient, but they cannot actually achieve high efficiency. Material is dusty. There is a traditional partition high-efficiency filter. If the partition is made of paper, and the raw materials are not strictly controlled, the paper partition may generate dust. The production environment of the filter material is poor, and the filter material itself carries a large amount of dust that may fall off, and the filter made may also generate dust.

    It is worth noting that some inspection methods cannot detect this kind of dust. The problem of the detection itself During the factory inspection, if the scanning detection method is used, there may be eddy currents on the air outlet surface of the filter, and the surrounding dust will enter the scanning area with the eddy currents. If the vortex is present, the dust must exceed the standard during scanning. At this time, you cannot determine whether the dust is from a filter leak or a vortex. Inexperienced operators may judge a good filter as a waste. Some foreign manufacturers, in order to eliminate the misjudgment caused by eddy current, put the filter test bench in the cleanroom. Domestic manufacturers are not so extravagant, so we have to find a way to eliminate eddy currents. In the early summer of 2004, I encountered a case in Beijing.

    The end customer is a well-known key laboratory, and that laboratory requires the filter to be inspected one by one before the project is contracted and installed. The contractor made a simple scanning test bench, and the filter failed to pass the test. After our explanation, they also suspected the vortex. They covered the air outlet surface of the filter with a long pipe. The pipe was increased to 1m long, and the vortex still existed. After the filter is installed, sometimes it is necessary to scan and inspect it on the spot. That is the responsibility of the installation company or the user, but the filter manufacturer is often required to participate. At this time, eddy current is also a headache. If you are not careful, a good filter may be rejected.

    In many cases, after the filter is installed, people use the downstream dust concentration to judge whether the filter is qualified. At this time, if the upstream concentration is too large, even if the filter efficiency is qualified, it may be misjudged as unqualified due to the high downstream concentration... The sealing problem of high-efficiency filter Sometimes, the contact part of the HEPA filter and the external structure is not tightly sealed, causing leakage. This is not a quality problem of the filter itself, but an external structure or installation problem. On-site installers sometimes block the glue around the high-efficiency filter. I don't like the method of blocking the glue, because practice shows that once the contact part is not well sealed, the more the glue is blocked, the more it leaks, and you may never plug it tightly.

    The sealing of the filter should be guaranteed by the sealing structure itself. If the structure is not good, the structural defects should be dealt with. If the defective structure is provided by the filter factory, such as a high-efficiency filter tuyere, it is also a structural problem rather than a filter element problem.