When the air filter starts to accumulate dust during use, the resistance of the air filter increases more slowly, and the increase of resistance increases with the increase of dust accumulation. Therefore, after the dust accumulation reaches a certain level, the resistance increase corresponding to the unit dust accumulation increase will double. If the cleanroom continues to be used without replacing the new air filter, even if the system has enough backup air pressure, it can ensure that the air volume of the normal operation of the system will not be affected due to the excessive resistance of the filter, but the excessive resistance also enables the system to operate. The consumption is too large.
In most cases, due to the limited air pressure of the system, failure to replace the dusty filter in time will affect the air volume of the system. In addition, there is a situation that some coarse-efficiency filters, such as glass filaments, have the same filtration efficiency as other filters at the beginning of dust accumulation, and increase with the increase of dust accumulation. But when the dust accumulates to a certain level, a "rebound" phenomenon will occur, that is, the dust particles that have been blocked will fall off the fibers and penetrate through. That is to say after the dust accumulation reaches a certain level, the resistance will continue to increase, but the filtration efficiency will decrease, so it is not suitable for continued use. According to the regulations of the American Heating, Refrigeration and Air-Conditioning Association, the filter should be replaced when the dust accumulates to the point where the corresponding filtration efficiency drops to 85% of the highest efficiency.”
At this time, the resistance of the cleanroom filter under the rated air volume is called the final resistance. In principle, when the filter is dusty and should be replaced, its resistance at the rated flow rate is the final resistance of the filter. The final resistance of coarse and medium-efficiency air filters is generally 1～higher than its initial resistance. 2 times and the final resistance of the HEPA filter is generally twice as high as its initial resistance. For example, the initial resistance of a standard high-efficiency filter under the rated air volume is 250Pa, and the final resistance is set to 500Pa.
It should be pointed out that in engineering applications, the concept of final resistance may be decoupled from the rated air volume. That is, no matter it is operated at a value higher or lower than the rated air volume value, it will be updated when the resistance of the filter increases to the final resistance value given by the product sample after the filter has accumulated dust. The final resistance now becomes a filter replacement mark value that is not directly related to the running air volume.