How much is the positive and negative pressure difference in the clean room?
What is the positive and negative pressure difference in a cleanroom?
The issue of positive and negative pressure differences in cleanrooms has long been a challenge for engineers in the field of air purification. How much should this pressure difference be? Here's a summary from the author based on some key points:
Maintaining a certain level of positive pressure inside a cleanroom is essential to ensure that the environment remains as free from contamination as possible, helping to maintain the desired cleanliness level. Even in non-vacuum environments, adjacent rooms or zones must have a positive pressure that is not lower than the cleanroom itself, which helps preserve the cleanroom’s conditions.
The positive pressure in a cleanroom refers to the static pressure inside being higher than the outside when all doors and windows are closed. This is achieved by ensuring that the supply air volume from the purification system exceeds the return air and exhaust air volumes. To maintain this positive 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 return fan, and finally the blower. 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 2–6 air changes per hour to maintain a positive pressure of at least 5 Pa. Today, with improved airtightness, maintaining a positive pressure of 5 Pa only requires about 1–2 air changes per hour, and for 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 cleanrooms and between clean and non-clean areas should be no less than 0.5 mmH2O (~5 Pa), while the difference between a clean area and the outside should be no less than 1.0 mmH2O (~10 Pa). However, the author believes these values may be too low for several reasons:
1. Positive pressure helps suppress contaminants entering through gaps in doors and windows, or reduce infiltration when doors are briefly opened. A higher positive pressure generally means better contamination control, though there are limits.
2. The air volume required for positive pressure is relatively small. The difference between 5 Pa and 10 Pa is only about one air change per hour. So why not aim higher? It seems that many systems are designed with lower values than necessary.
3. The U.S. Federal Standard (FS209A~B) specifies a minimum positive pressure difference of 0.05 inches of water (12.5 Pa) between a cleanroom and adjacent less-clean areas when all entrances are closed. Many countries have adopted this standard.
While higher positive pressure can offer better protection, it’s not always better. From over 30 years of real-world engineering experience, it’s clear that when the pressure reaches 30 Pa or more, it becomes difficult to open doors. Closing them can even cause loud noises, which can be uncomfortable or even scary. At 50–70 Pa, you might hear whistling from door and window gaps, causing discomfort for sensitive individuals. However, most standards don’t set an upper limit for positive pressure. As a result, many projects focus only on meeting the minimum requirements without considering the upper limits, leading to excessively high pressure values—sometimes exceeding 100 Pa—which can cause serious issues. In reality, adjusting the positive pressure is simple and can be controlled within a safe range.
Some related links:
Air shower: http://
High efficiency air filter: http://
High efficiency filter leak detection: http://
Operating room purification project: http://
Clean room level standard: http://
Cleanroom testing standards: http://
Cleanroom design specification: http://
Cleanroom management system: http://
Clean room ventilation frequency standard: http://
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