Overpressure in the cleanroom
What is cleanroom overpressure and why is it important?
Cleanroom overpressure stops the flow of external air. When a door or sliding door opens, the overpressure pushes the air flow out, so that polluted air has no chance to enter. Typically, the air pressure outside the cleanroom is lower or comes from a contaminated area. That is why cleanroom differential pressure measurements are crucial to manage the cleanliness of the space.
Overpressure and underpressure in cleanrooms
Overpressure (positive pressure) and underpressure (negative pressure) create controlled air flows that function as airlocks. These separations prevent air mixing and the associated risk of contamination. This way, areas producing different products, such as heart tablets and antacids, remain strictly and safely separated from each other.
Differential pressure measurement and pressure cascade (Pa)
Effectively protecting a cleanroom with overpressure (a positive air pressure clean room) requires a precise differential pressure measurement. This difference is expressed in Pascal (Pa) and compared with what is called the outer box, the building in which the cleanroom is located. The inner box refers to all cleanroom areas together.
What is a pressure cascade?
The pressure in the outer box serves as reference point for the cleanrooms in the inner box. Mutual differences in differential pressure measurements arise in these areas, whereby the outer box is always taken as the starting point. This series of pressure levels is called a pressure cascade and ensures a controlled distribution of air flows between zones with different classes and risks of contamination.
How is differential pressure measurement applied?
Adjusting the pressure difference between cleaner and less clean zones requires care. Too large a difference can lead to unwanted turbulence, while too small a difference can disrupt the direction of the airflow.
Therefore, the pressure difference must be stable and well balanced and meet clean room positive pressure requirements. In practice, the correct differential pressure measurement usually ranges between 5 and 20 Pa. Within this range, doors can still be opened with ease and the risk of cross currents is limited.
In some situations, only a difference in pressure is used, but often it is combined with other contamination control techniques and concepts.
Setting and controlling overpressure in the cleanroom
The static pressure between cleanrooms of different classes, or between a cleanroom and an unclassified area, can be controlled using airflow balancing techniques. These techniques consist of both active and automated systems, as well as passive and manual systems. They compensate for differences by precisely adjusting the amount of air supplied or extracted through air ducts and transfer systems, taking heat losses into account in the process.
If the differential pressure measurement is at the lower limit of the target value of the desired range, additional measures are necessary. A subsequent precise differential pressure measurement of the air flow or pressure must then indicate that the installation remains stable and contamination is prevented. In some cases this can be achieved with a box-in-box cleanroom, which forms an additional barrier against unwanted air currents.
In addition, the overpressure can be set by limiting the return air in relation to the supply in the cleanroom positive pressure environment. Optimal air distribution ensures even circulation throughout the room, with supply and return carefully coordinated. Sufficient air changes are essential to keep the cleanroom within the established ISO classification.
Locks, doors and passages
The way doors and passages are used is crucial for maintaining a stable overpressure condition. Without clear rules, the accumulated overpressure can quickly be lost. To prevent this, hierarchies and interlock systems have been developed to manage access to the different zones.
Hierarchy between passages
Every time a door is opened, the airflow in the cleanroom changes. The circulation rate and the magnitude of the pressure difference compensate for this temporarily, but the recovery time remains dependent on a strict hierarchy between sliding doors and other passages. Without this hierarchy, the overpressure would be lost too quickly causing the risk of contamination to increase. That is why the differential pressure must always be protected with supporting systems such as an interlock.
Interlock system for controlled access
An interlock system ensures that only one access point can be opened at a time. The sliding door, pass-through box or revolving door remains closed as long as another passage is still open. Only when this is locked again can the next door be opened.
This system monitors the hierarchy between aisles and assists operators in maintaining the correct pressure. It is intended as an aid and warning, not as a method for completely sealing a space.
Preventing air loss through seams and cracks
The preservation of overpressure in a cleanroom depends on the airtightness of the room. Even small openings can cause the built-up overpressure to dissipate quickly. Everyday details, such as a keyhole or wall socket, can unintentionally form a leak. Complete airtightness is practically impossible, as only a balloon is truly completely sealed, but in a cleanroom it is essential to minimize leakage as much as possible.
Do you want to maintain stable overpressure in your cleanroom and have the assurance of correct differential pressure measurements? Contact us.