Maintaining a certain positive pressure in the clean room is one of the essential conditions to ensure that the clean room is not or less contaminated to maintain the design cleanliness level. Even in a vacuum chamber, it must have a positive pressure in an adjacent room or suite that is not below its cleanliness level, and the cleanliness of the vacuum chamber can be maintained.
The positive pressure value of the clean room refers to the value of the indoor static pressure greater than the outdoor static pressure when the doors and windows are all closed. It is achieved by the method that the air supply volume of the purification system
is greater than the amount of return air and the amount of exhaust air. In order to ensure the positive pressure value of the clean room, the air supply, return air and exhaust fan are preferably interlocked. When the system is started, the blower is started first, then the return fan and the exhaust fan are started. When the system is closed, the exhaust fan is closed first, then the fan and blower are turned off. To prevent the clean room from being contaminated when the system is turned on and off.
The amount of air required to maintain a positive pressure in a clean room is determined primarily by the tightness of the maintenance structure. In the early stage of clean room construction in China, due to the poor airtightness of the retaining structure, it is necessary to maintain the positive air pressure of ≥5Pa for 2~6 times/h. The current airtightness of the maintenance structure has been greatly improved, and the same positive pressure is maintained as long as 1 ~2 times / h air supply; maintain ≥ 1OPa as long as 2 ~ 3 times / h air supply.
China's design code  stipulates: the static pressure difference between clean rooms and clean areas and non-clean areas of different grades should be no less than 0.5mmH2O (~5Pa), and the static pressure difference between clean area and outdoor should be not less than 1.OmmH2O (~l0Pa). The author believes that the value seems to be low for three reasons: 1. Positive pressure is the ability of the clean room to suppress indoor air pollution through the gap between doors and windows, or to reduce the amount of pollutants in the room to a minimum when opening doors and windows for a short time. The size of the pressure indicates the strength of the ability to suppress pollution. Of course, the greater the positive pressure, the better (more on this later). 2. The air volume required for positive pressure is limited. The air volume required for positive pressure of 5Pa and positive pressure of 10Pa is only about 1 time/h. Why not do it? Obviously, the positive pressure value is lower than l0Pa. 3. US Federal Standard (FS209A~B) stipulates that when all inlets and outlets are closed, the minimum positive pressure difference between the clean room and any adjacent low-cleanness areas is 0.05 inches of water (12.5Pa), which has been Adopted by the state.
However, the positive pressure value of the clean room is not as large as possible. According to the actual engineering test of the unit for more than 30 years, when the positive pressure value is ≥30Pa, it is more difficult to open the door. Close the door and pay attention to it, slamming! Scary! When positive pressure When the value is ≥50~70Pa, the gap between the door and window will be whistling, and the infirm or the person with some incompatibility will feel uncomfortable. However, the relevant norms or standards of many countries in China and abroad do not specify the upper limit of positive pressure. As a result, many units only ask the lower limit to meet the requirements, regardless of the upper limit, which is the actual cleanliness that I have encountered. The positive pressure value in the chamber is as high as 100 Pa or more, which causes very bad results. In fact, adjusting positive pressure is not a difficult thing. It is entirely possible to control it within a certain range. There have been reports that a country in Eastern Europe has a positive pressure value of 1-3mmH20 (about 10~30Pa). I think this range is more suitable.
For a clean room, the above four necessary conditions must be met at the same time to ensure that the cleanliness meets the requirements and has the ability to resist pollution.