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HEPA filter leak detection
HEPA filter leak detectionI. Principle of the completeness meter methodAt the upstream end of the HEPA filter to be tested, PAO aerosols are sent as a dust source, and when the aerosols are mixed evenly, i.e. when the upstream concentration value is stable, an aerosol finisher is used to test and set the concentration value as a reference value.Once the upstream concentration has been set, do not adjust the PAO aerosol concentration again. The ratio between the downstream concentration and the set upstream concentration is the leakage rate (also called permeability) of the HEPA filter. II. Test conditions To test the leakage rate of a high-efficiency filter, three conditions must be met.1. Aerosols can be introduced into the system.A very good place to introduce aerosols is the negative pressure end of the fan. At this point the dust particles generated by the generator can be sucked into the system by the fan, the aerosol has little interference with the environment; the aerosol is mixed more evenly; the dust generation does not require a positive pressure confined environment, and the dust generation difficulty is low. For example FFU air inlet, clean bench air inlet, etc. In some places where the conditions are not available, dusting can only be carried out at a location close to the high-efficiency filter, where it is necessary to overcome the airflow pressure at the upper end of the high-efficiency filter to press the aerosols into the static pressure box, making dusting more difficult; more aerosols are leaked into the test environment when dusting, and the aerosol concentration in the environment is high, which simply interferes with the test; the aerosol mixture is not uniform, and the upstream concentration fluctuates greatly. We do not recommend dust generation here unless we are forced to do so. Some equipment manufacturers do not have the relevant testing experience and set the aerosol dust outlet at the positive pressure end of the fan, which causes a lot of unnecessary problems in testing. Therefore, when writing the URS for the equipment, we should write this point and ask the equipment manufacturer to set the dust generation port at the negative pressure end of the fan to create favorable conditions for later verification. 2. Stable and uniform aerosol upstream concentrations can be detected. Upstream concentration test ports are set close to the upstream end of the HEPA filter and are connected by a hose to a simple sampling location, such as the static pressure box of the room air supply of a purified air conditioning system. Some older static pressure boxes do not have an upstream concentration test port, so the box must be modified. 3. The ability to perform a point-by-point scan of the air outlet side of the filter at the downstream end of the HEPA filter. If you can scan point by point, you can measure the leakage rate of any position of the high-efficiency filter including the edge frame. A small amount of production equipment with high-efficiency filters located inside the equipment cannot be scanned point by point, so you can only test the overall leakage rate of the filter at the outlet of the equipment. III. Testing steps (taking a purified air conditioning unit as an example)1. Determine the upstream concentration sampling port2. Determine the location of dust generation As the air conditioning unit is far away from the high-efficiency filter, dust generation in the air conditioning unit can be fully mixed, and the aerosol concentration at the upstream end of each filter is particularly stable, so it is generally chosen to generate dust in the air conditioning unit. To avoid the medium efficiency filter blocking off a large number of aerosol particles, the specific location for dust generation is chosen at the back end of the medium efficiency filter. At this point, although not at the negative pressure end of the fan, the aerosol generator can be placed in the air conditioning enclosure as a whole, so dust generation is not difficult as well. If there is an access door at the back end of the medium efficiency filter, the aerosol generator can be placed directly at the back end of the medium efficiency filter; if there is no access door at the back end of the medium efficiency filter, a piece of the medium efficiency filter can be dismantled and the aerosol generator can be placed at the back end of the medium efficiency filter. 3. Dust generation, test upstream concentration Heat the aerosol generator electrically so that the temperature rises above 400°C. Connect the aerosol generator with an inert gas source and adjust the gas pressure to make the aerosol generator dusty. Test the aerosol concentration at the upstream concentration test port and when the upstream concentration is stable, set this concentration as the reference concentration. The change of upstream concentration over time should not exceed ±15% of the average measured value, and the concentration should be 20-80mg/m3. For the regulation of whether the upstream concentration is stable, we understand that: after the upstream concentration is set, check the immediate fluctuation value of the upstream concentration, if the fluctuation value is between 85% and 115% of the set concentration, it is considered that the upstream concentration is stable and can be tested. 4. Scan test The sampling head of the finished meter is used to scan the entire working surface of the high-efficiency filter (including the filter media, the filter frame, between the filter frame and the static pressure box, and the static pressure box itself). When scanning, the sampling head is less than 5cm away from the filter and the speed is less than 5cm/s. The scanning should be carried out back and forth and the lines should overlap. 5. Qualification standard ISO14611-3 stipulates: the standard transmittance rate for confirming the local leakage of the filter by completeness meter leak detection method is 0.01%, that is when the sampling probe is aimed at a certain point of the air outlet surface of the filter under test when the static detection if the measured transmittance rate is higher than 0.01%, it is considered that the point is the leakage point. If the efficiency of the high-efficiency filter is 99.99%, then it is reasonable to stipulate that the permeability is not higher than 0.01%. If the efficiency of the high-efficiency filter is 99.995%, why is it agreed that the permeability is not 0.005% but 0.01%? Forcing a passing value of 0.01% is too harsh for filters with an efficiency of less than 99.99%, and too lenient for filters with an efficiency higher than 99.99%. The British standard BS EN 1822-1 "High-efficiency air filters (HEPA and ULPA) Part 1: Classification, performance testing, marking" is more reasonable in terms of leakage testing of high-efficiency filters. The efficiency of the filter is subtracted from the efficiency of the filter to obtain the agreed high permeability of the filter. The standard stipulates that a filter will pass if its partial permeability does not exceed five times the agreed maximum permeability and the overall average permeability does not exceed the agreed maximum permeability. For example, if the efficiency of a filter is 99.995%, its great permeability should be 0.005%, then as long as its local permeability does not exceed 0.025% and the overall permeability does not exceed 0.005%, it can be judged as qualified. Therefore, for high-efficiency filters with 99.99% efficiency, we take 0.01% as the standard for determining whether they are qualified. For filters with an efficiency of 99.99%, we will discuss with the customer to develop a more reasonable standard for qualification. 6. Filter repair, replacement, and re-testing If any part of the filter is found to be leaking beyond the standard, the leak can be repaired with glass glue. The plugging area of a single leak cannot exceed 1% of the total area of the filter, and the total plugging area cannot exceed 5% of the total area of the filter. If the filter cannot be repaired by plugging, the filter must be replaced. After any form of plugging and replacement of the filter, the filter must be retested until all the requirements of the standard are met. |
