How much is the positive and negative pressure difference in the clean room?
What is the positive and negative pressure difference in a clean room?
The issue of positive and negative pressure differences in clean rooms has long been a challenge for engineers working in purification systems. How much should the pressure difference be? Here's a summary of some key points:
Maintaining a certain level of positive pressure inside a clean room is essential to ensure that it remains free from contamination and maintains its designed cleanliness level. Even in vacuum-free areas, adjacent rooms or zones must have a positive pressure that is not lower than the cleanliness level of the main area, which helps maintain the desired air quality.
The positive pressure value in a clean room refers to the indoor static pressure being higher than the outdoor static pressure when all doors and windows are closed. This is achieved by ensuring that the air supply from the purification system exceeds the return air and exhaust air volume. To maintain this pressure, it's recommended that the air supply, return, and exhaust fans be interlocked. When starting the system, the blower should be turned on first, followed by the return and exhaust fans. When shutting down, the exhaust fan should be turned off first, then the others. This setup helps prevent contamination during system start-up and shutdown.
The amount of air needed to maintain positive pressure depends largely on how airtight the building envelope is. In the early stages of cleanroom construction in China, due to poor airtightness, it was common to use an air supply rate of 2–6 times per hour to maintain a positive pressure of at least 5 Pa. Today, with improved airtightness, the same positive pressure can be maintained with just 1–2 air changes per hour, and for pressures above 10 Pa, around 2–3 air changes per hour are sufficient.
According to China’s design code [6], the static pressure difference between different levels of clean rooms and between clean and non-clean areas should be no less than 0.5 mmH2O (~5 Pa), and the difference between clean areas and the outside should be at least 1.0 mmH2O (~10 Pa). However, the author believes these values might be too low for several reasons:
1. Positive pressure helps suppress contaminants entering through gaps around doors and windows, or limits short-term pollution when doors are opened. A higher positive pressure means better contamination control. Naturally, the higher the pressure, the better (more details later).
2. The air volume required to maintain a positive pressure is limited. The difference between 5 Pa and 10 Pa is only about one air change per hour. Why not go higher? It seems the current standard sets a lower limit rather than an optimal one.
3. The U.S. Federal Standard (FS209A~B) requires a minimum positive pressure difference of 0.05 inches of water (12.5 Pa) between a clean room and adjacent areas with lower cleanliness. This standard is widely adopted globally.
However, a higher positive pressure isn’t always better. Based on over 30 years of engineering experience, when the pressure reaches 30 Pa or more, it becomes very difficult to open doors. Closing them can even cause loud noises—scary! At 50–70 Pa, you might hear whistling from door and window gaps, causing discomfort for sensitive individuals. Yet, many countries’ regulations don’t specify an upper limit for positive pressure. As a result, some facilities only meet the minimum requirements without considering the upper limit. In reality, the author has encountered cleanrooms with positive pressures as high as 100 Pa or more, leading to serious issues. Adjusting pressure is not complicated—it’s entirely possible to keep it within a safe range.
Some useful links:
Air shower: http://
High-efficiency air filter: http://
High-efficiency filter leak detection: http://
Operating room purification project: http://
Clean room classification standards: http://
Cleanroom testing standards: http://
Cleanroom design specifications: http://
Cleanroom management system: http://
Cleanroom ventilation frequency standards: http://
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