How do I know if a HEPA filter needs to be replaced?

First, judge by the change of resistance
Principle of differential pressure monitoring
When the clean workbench is in normal operation, the air passing through the HEPA filter will produce a certain resistance. With the increase in filter use time and the accumulation of particles on the filter material, this resistance will gradually increase. A differential pressure meter is installed on the general equipment to monitor the pressure difference inside and outside the filter.
When the HEPA filter is just installed, it has an initial resistance, which is usually around 100-250Pa (the initial resistance of different models and specifications of filters may vary). In the process of use, if the pressure difference shown by the pressure differential meter reaches 2-3 times the initial resistance, it indicates that the resistance of the filter has increased significantly and may need to be replaced.
For example, a HEPA filter with an initial resistance of 150Pa should be replaced when the differential pressure gauge shows a pressure difference of 300-450Pa.
Monitoring frequency and precautions
The monitoring of differential pressure should be carried out regularly, and it is recommended to check at least once a week. When checking, ensure the accuracy of the differential pressure gauge, which can be verified by calibration. At the same time, the differential pressure data of each inspection should be recorded in order to observe the changing trend of resistance.
If the pressure difference is found to rise sharply in a short period of time (such as a few days), it may be due to a large amount of dust pollution or foreign body blockage and other abnormal conditions, which need to be checked and handled in time.
Second, according to the change in wind speed
Wind speed detection method
Use a professional anemometer to measure the wind speed in the operating area of the clean table. For vertical unidirectional flow clean workbench, the average wind speed in the operating area is generally 0.3-0.5m /s; For horizontal unidirectional flow clean workbench, the average wind speed in the operating area is usually 0.4-0.6m /s.
When detecting the wind speed, the probe of the anemometer should be placed in different positions of the operating area for measurement, such as the four corners of the operating area, the central position, and the edge position, to obtain comprehensive and accurate data. If the wind speed is significantly lower than the standard wind speed at multiple locations, and after excluding other factors such as fan failure, it is likely to be caused by the blockage of the high-efficiency filter, at this time you need to consider replacing the filter.
For example, if the horizontal unidirectional flow clean table with an average wind speed of 0.4m/s in the original operating area is tested the wind speed in multiple locations is reduced to below 0.3m/s, and the fan and other components are operating normally, then the HEPA filter may need to be replaced.
Relation between air velocity and filter performance
The main function of the HEPA filter is to filter the particles in the air to ensure that clean air enters the operating area at the appropriate wind speed and direction. When the filter material of the filter is blocked by a large number of particles, the resistance of the air increases, and the wind speed is reduced. Therefore, the change in wind speed is an important indicator of the filter performance.
Third, judging from the perspective of appearance and performance
Visual inspection
Check the appearance of HEPA filters regularly (once a month is recommended). To ensure safety, open the access port of the device (if permitted) and observe the surface of the filter. If the filter material is found to have obvious damage, cracks, holes, or deformation, the air will bypass the filter layer directly into the operating area, thus affecting the cleanliness, in this case, the filter must be replaced.
At the same time, check that the filter border seal is good. If the sealing rubber strip ages, falls off or has gaps, unfiltered air will leak into the operation area, and it needs to be replaced or repaired in time.
Performance detection
Some simple performance tests can be performed to determine if the filter is failing. For example, some test materials that are sensitive to air quality are placed on the clean workbench, such as high-precision electronic chips or Petri dishes that are sensitive to microorganisms. If in the normal course of operation, these test materials have quality problems (such as short circuits of electronic chips, abnormal growth of microorganisms in Petri dishes, etc.), and other possible pollution factors are ruled out, then it is likely that the performance of the high-efficiency filter is reduced, and it needs to be replaced.
In addition, particle counters can be used to detect the air quality in the operating area of the clean table. If the number of particles detected in the air is significantly increased, exceeding the cleanliness grade standard specified by the equipment, and other equipment such as fans are operating normally, then the HEPA filter may have failed and needs to be replaced.