Whether a cleanroom can maintain its cleanliness level depends not on a single number, but on three parameters working together: differential pressure prevents uncontrolled infiltration from outside; air changes per hour (ACH) dilute internally generated particles in non-unidirectional flow rooms; face velocity directionally removes particles in unidirectional flow rooms. These three quantities are often cited in the same table, and are also frequently confused—especially treating "≥10 Pa" as a uniform lower limit between clean zones and non-clean zones, or reversing the ACH values for Class 10,000 and Class 100,000.
Below, the basis for values, mutual conversion, and the most common failure points during acceptance inspection are clarified at once according to current standards. First, the reference table:
| Parameter | Function | Typical Value | Basis |
|---|---|---|---|
| Differential Pressure | Prevent uncontrolled infiltration and maintain directional airflow | Between cleanrooms of different classes: should not be less than 5 Pa; between clean zone and non-clean zone: shall not be less than 5 Pa; between clean zone and outdoors: shall not be less than 10 Pa | GB 50073-2013 Clause 6.2.2. For pharmaceutical cleanrooms, refer to GB 50457-2019, where all three locations shall not be less than 10 Pa |
| Air Changes per Hour | Dilute internally generated particles in non-unidirectional flow rooms | ISO Class 8 (Class 100,000) 10–15 ACH; ISO Class 7 (Class 10,000) 15–25 ACH; ISO Class 6 (Class 1,000) 50–60 ACH | GB 50073-2013 Table 6.3.3. Applicable for rooms with floor-to-ceiling height less than 4.0 m; values also depend on personnel, equipment, and materials; table values are static references |
| Face Velocity | Directional particle removal in unidirectional flow rooms | ISO Class 1–3: 0.3–0.5 m/s; ISO Class 4–5: 0.2–0.4 m/s |