Test the Airflow Uniformity of Your Fan/Filter Units!
Airflow uniformity is critical for applications requiring adherence to ISO, FDA or cGMP cleanliness requirements. However, many Fan/Filter Units (FFUs) that appear to meet nominal cleanliness requirements fail to provide the uniform velocity and flow your processes depend on.
What is airflow uniformity?
Certifiers measure air velocity at multiple (typically six or eight) locations across a filter face. Ideally, air speed measurements taken at these locations vary by no more than 15% from mean values. For example, in an ISO 5 laminar flow system with a specified minimum average air velocity of 90 fpm (feet per minute), the minimum air speed measurements typically should be in the range of 80 – 100 fpm.
Discrepancies greater than this create turbulence as the faster air streams (lamina) are drawn into the localized areas of lower pressure created by the slower ones.
The greater the airspeed differential, the more likely and the more severe this turbulence becomes, resulting in swirling that can suspend minute particles rather than sweeping them downward and out of the laminar flow system. Worse, localized turbulence can create second-pass air that swirls up and re-enters the downward flow, drawing particles upward and redistributing them into other locations inside the classified work space. Under such conditions, a cleanroom or laminar flow cabinet fails to meet ISO laminar flow conditions – and ceases to be a clean!
How does airflow uniformity differ from airflow average?
Some FFU manufacturers specify an average airflow (or average velocity) that can mask serious performance deficiencies.
In the case below, a Fan/Filter Unit produces an average airflow of 1004 CFM (cubic feet per minute) and average air speed of 125 fpm on high speed, perfectly acceptable performance values. However, individual lamina measurements demonstrate substantially non-uniform filter face velocity and flow, with discrepancies of over 60% at various locations across the filter (30% from mean values). FFU performance will create areas of turbulence due to this lack of uniformity.