In general, the final level of air filter determines the cleanliness of the air supply, while the upstream level of air filter plays a protective role. It protects the lower end air filter to extend its service life, or protects the air conditioning system to ensure its normal operation.

When designing, the efficiency of the final air filter should be determined first based on the cleanliness requirements of the air supply, and then the air filter (pre filter) that plays a protective role should be selected. If this level of air filter also needs protection, a filter should be added at its upwind end. The efficiency of each level of air filter should be properly matched. If the efficiency difference between adjacent levels is too large, the first level will not be able to protect the second level.

When using the European "G-F-H-U" efficiency specification classification, a primary filter can be set every 2-4 levels. For example: G3 → F5 → F8 → H12, where the end H12 high-efficiency filter determines the cleanliness level of the air supply, F8 protects H12, F5 protects F8, and G3 protects F5.

When selecting a pre air filter, factors such as usage environment, spare parts costs, operating energy consumption, maintenance, and supply should be comprehensively considered before making a decision. Specifically, the high-efficiency (HEPA) air filter at the end of the cleanroom should be protected by a medium efficiency filter with an efficiency specification not lower than F8; Ultra high efficiency (ULPA) air filters can be equipped with F9 medium efficiency filters to H11 air filters. The air conditioning system itself should have an efficiency specification not lower than F5 medium efficiency filter for protection. In areas with no wind, sand, and low pollution, a pre filter may not be installed before the F7 medium efficiency filter; The commonly used primary filters in urban air conditioning systems are G3-F6.

The key points to determine the efficiency of air filters are: the performance of the final air filter should be reliable, the efficiency specifications of the pre filter should be reasonable, and the maintenance of the initial and pre filters should be convenient.