When the final resistance reaches 2-4 times the initial resistance, the differential pressure gauge will give an alarm and the air filter should be cleaned or replaced.
Air resistance introduction:
The moving air stream hits the filter material, which causes the air stream to deflect and create little resistance. The multiple fibers of the filter are the total resistance of the entire filter device and the air resistance of the filter device at specific wind speed. When the filter device reaches a certain resistance value (final resistance value), the filter device must be replaced to ensure the filtering effect.
In most cases, the final resistance value is set to 2-4 times that of the ancestor. The dirtier the filter, the higher the resistance value, and the lower the corresponding wind speed. Maintaining a positive pressure in the air filter is one of the essential conditions to ensure that the air filter is not contaminated or meets the planned cleanliness level. Even for a negative pressure air filter, it must have a positive pressure not lower than the cleanliness level in its adjacent room or suite and maintain the cleanliness of the negative pressure air filter.
The positive pressure value of the air filter refers to the value at which the internal static pressure is greater than the external static pressure when all doors and windows are closed. This is achieved by a method in which the air supplied to the purification system is greater than the return air volume and the exhaust air volume. In order to ensure the positive pressure value of the air filter, air supply, return air and exhaust fans are preferably interlocked. When opening the system, it is recommended to use a blower first, and then declare a backup.
Fan and exhaust fan: When the system is turned off, first turn off the fan, then turn off the fan and fan to prevent the air filter from being contaminated when the system is turned on and off. The amount of air required to adhere to the positive pressure of the air filter is mainly determined by the tightness of the maintenance structure. In the initial stage of air filter construction in China, due to the poor sealing of the housing structure, it is necessary to maintain a positive pressure of ≥5Pa and a supply of 2~6 times/h. Now the tightness of the maintenance structure is very reluctant. The same positive pressure only needs 1~2 times/hour of air supply; if ≥1OPa, 2~3 times/hour of air is required.
China’s planning standard  stipulates that the static pressure difference between the air filter and the clean area and the unclean area: should not be less than 0.5mmH2O (~5Pa). The difference is that the static pressure between the clean area and the outside should not Less than 1.0mmH2O (~10Pa). The author thinks that the value seems very low. There are three reasons for this.
1. Positive pressure is the adjustment of the air filter, which pollutes the indoor air through the gap between doors and windows or minimizes the entry of pollutants into the room within a short time when the door is opened, and the positive pressure of the windows is represented by the pressure. Of course, to adjust the pollution intensity, the higher the positive pressure, the better (later).
2. The amount of air required for positive pressure is limited. The positive pressure is 5 Pa, and the amount of air required for a positive pressure of 10 Pa is only about 1 time/h. Why not? The positive pressure value is below the 10Pa limit.
3. US Federal Standard UU. (FS209A~B) Rule: When all air inlets and outlets are closed, the small positive pressure difference between the air filter and any adjacent low clean area is 0.05 inches of water column (12.5Pa), which is a lot Country selected.
Summary: The positive pressure value of the air filter is not very large. According to more than 30 years of practical verification, when the positive pressure value is greater than 30, it is more difficult to open the door; when the positive pressure value is greater than or equal to 50~70Pa, the gap between the door and the window will be a whistle, discomfort or any discomfort announcement. People will feel uncomfortable. However, many countries have not yet provided positive pressure restrictions on relevant domestic and foreign standards or standards. The effect of many of these units is that the lower limit meets the requirements. No matter how much, the air filter pressure exceeds 100PaALU. They constitute an adverse effect. In fact, adjusting positive pressure is not difficult. According to reports, the positive pressure established by Eastern European countries is 1-3 mm H2O (about 10~30Pa), which I think is more appropriate.