All air entering a rated cleanroom should be HEPA filtered. The point of entry should be evaluated relative to the equipment, process, or personnel that the air will encounter and the point at which the air leaves the room should located to insure that the air can readily exit the room without depositing any entrained particles in critical areas.
High entry, low return is still the preferred air flow scheme in most cleanrooms. Increasingly the use of computer software is being used to permit this flow pattern to be visualized. As a practical matter unidirectional cleanrooms with 100% HEPA filter coverage and raised floor plenum returns tend not to be affected much by airflow pattern issues unless process equipment within the room creates eddy currents.
However, non-unidirectional cleanrooms may very well not meet expectations if airflow patterns are ignored.
This cleanroom tip was taken from “Why Do Cleanrooms Fail to Meet Owners’ Expectations?,” which appeared in the April 2012 issue of Controlled Environments.