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The cleanroom classification will influence the layout of a facility. For example, you can enter an ISO 8 cleanroom directly from an uncontrolled environment. On the other hand, you will need at least one airlock (my recommendation is two) before entering an ISO 6 environment. It is important to keep this in mind when planning your space.
A cleanroom isn’t simply walls and ceilings. The heart of the cleanroom is the air treatment system (HVAC). Cleanroom HVACs differ from conventional systems with their increased air supply, airflow patterns, use of high efficiency filters, and room pressurization. In order to meet the required air cleanliness, the air must pass through HEPA filters. The lower the ISO class, the more often you will need to pass the air through the HEPA filter. This is what we call air change per hour (ACH). Thus, the HVAC’s required air flow (cubic feet per minute (CFM)) is calculated using the ACH and room volume.
In order to reach a desired cleanliness level inside the cleanroom, the HVAC system combines air changes per hour (ACH) with HEPA filtered air circulating into the cleanroom many times per hour. The required ACH is determined by the ISO class to be met, as illustrated below. In comparison, a conventional HVAC system usually makes two to four air changes per hour, whereas in a cleanroom it can range anywhere from 10 to 250 or even more.