Optimal cleanroom airflow is achieved by HEPA fan filter units mounted in ceiling, pushing particles to floor in smooth laminar action. Low wall air returns capture the dirty air at floor level and return it to the plenum to be cleaned by HEPA fan filter units again. The downward laminar air flow prevents particles from swirling upward back into the cleanroom work zone.
Figure 1. Cleanroom air flow simulation – recirculating modular cleanroom
HEPA fan filter units need to be evenly spread in cleanroom to ensure good clean air coverage in all areas. Location of HEPA fan filter units should be coordinated with location of return air walls to ensure there are no dead spots in cleanroom. Distance between HEPA fan filter units and return air walls should be no more than 10’ to ensure good filtered air flow.
Figure 2. Return air grills in ISO-5 cleanroom
Recirculating cleanrooms HEPA filter the air over and over again, to achieve the lowest particle counts. ISO-7 cleanrooms clean the air 60X per hour. ISO-6 cleanrooms clean the air 180X per hour. The more times per hour the air is HEPA filtered the less particles will be present.
In larger ISO-5 cleanrooms raised floors are used to improve air flow. Raised floors use perforated cleanroom floor tiles to allow the air to be returned underneath the floor to return air walls. This provides HEPA filtered air the shortest path (ceiling to floor) and does the best job removing particles from the air. In these type of cleanrooms, it is common to have 100% HEPA filter ceiling coverage to ensure that every square foot of the room receives HEPA filtered air. Tear drop lights mounted on cleanroom ceiling grid are typically used to achieve 100% HEPA filter ceiling coverage.
Figure 3. Raised flooring for class 100 cleanroom Figure 4. Raised flooring installation in class 100 cleanroom Figure 5. Cleanroom tear drop lights
Making sure not to block HEPA filter air flow is critical to cleanroom performance. Placing equipment in front of low wall air returns will reduce air flow. Walls of equipment that block air flow will can create dead spots.
Figure 6. Row of equipment blocking air flow
Summary: Optimal cleanroom airflow is achieved by HEPA fan filter units mounted in ceiling, pushing particles to floor in smooth laminar action. Low wall air returns capture the dirty air at floor level and return it to the plenum to be cleaned by HEPA fan filter units again. HEPA filter and return air placement are critical to ensure no dead areas and good cleanroom air flow. For larger class 100 cleanrooms 100% HEPA filter ceiling coverage and raised floors are often used to get even better air flow and eliminate all dead areas.
It is important to not block return air walls with equipment in cleanrooms – otherwise user will reduce air flow and create dead areas in cleanroom.
Author: Anthony Chien has worked at American Cleanroom Systems for past 11 years. He has more than 40 years of cleanroom experience. Anthony has a BS and MS in Electrical Engineering specializing in semiconductor manufacturing from the University of Illinois. American Cleanroom Systems is a design build modular cleanroom manufacturer based in Rancho Santa Margarita CA. It is expert in ISO-5 through ISO-8 (class 100 to class 100k) modular cleanrooms for pharmaceutical cleanrooms, medical device cleanrooms, and industrial cleanrooms.
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