Cleanroom velocity not subject to outdated standards
MYTH: “90 fpm _ It`s the cardinal rule.” “Who says?” “Why, the Federal Standard 209B says, that`s who.” REALITY: Any velocity is acceptable. The governing factor is the process for which the clean space is being used. Also, Federal Standard 209B was made obsolete in 1987.
by Richard A. Matthews
One still hears that cleanroom air has to flow at 90 fpm (feet per minute). Otherwise you are not in compliance.
Compliance with what? According to some regulatory people, “compliance with Federal Standard 209B.”
Let us examine the two major reasons to debunk this myth.
Federal Standard 209B was issued April 24, 1973. It represents 1960s and early 1970s cleanroom technology. It was a seminal document for later revisions of itself and for a whole family of contamination control recommended practices. It was in effect for 15 years until Federal Standard 209C was issued on October 27, 1987. Unfortunately, Federal Standard 209C was editorially flawed, had to be rewritten, and was made obsolete and replaced eight months later with Federal Standard 209D on June 15, 1988.
The normative (mandatory) section of Federal Standard 209B specified only three classes of air cleanliness (Class 100, Class 10,000, and Class 100,000) as measured at only two particle sizes (0.5 and 5.0 micron) and only covered cleanrooms and clean work stations. In 1976, Class 1,000 was added by Amendment -1. This normative section also referred to concern for “other environmental air conditions.”
Its lengthy informative (voluntary) appendix section included rather rigid references to airflow, temperature, relative humidity, lighting levels, etc. Examples:
Temperature was recommended at 72 degrees Fahrenheit.
Relative humidity was actually specified in the normative (mandatory) section as 40 percent ± 5 percent — this is unnecessarily expensive.
Room-to-room pressurization was shown only as 0.5-inch of water positive only. There was no provision for negative pressure areas.
Lighting intensity was specified at 100 to 150 foot candles — also unnecessarily expensive.
Airflow velocity was defined as 90 fpm (feet per minute) plus or minus 20 percent.
This appendix went on to cover comments on cleanroom and clean work station testing, monitoring, and operations. It was a good all-inclusive document, but it had flaws.
Meanwhile, starting in the early 1980s, the Institute of Environmental Sciences (IES) [now called the Institute of Environmental Sciences and Technology (IEST)], which had earlier (1973) absorbed the American Association for Contamination Control, and its standards and practices activity had begun an aggressive campaign to address the expanding needs of the contamination control community.
Working groups were formed to address specific contamination control needs. These working groups consisted of vendors, users and government authorities with a common goal of creating broad recommended practices that addressed the fast growing and broadening cleanroom community. There are currently more than 30 active IEST working groups.
Other factors
Industry experience was determining that even though 90 fpm seemed a good baseline airflow, other factors, both negative and positive, were equally important. Animals in cleanrooms could not live properly in 90 fpm air. They dehydrated, so 15 to 20 fpm air was used and it worked to maintain the necessary cleanliness levels. In some pharmaceutical filling operations, 90 fpm was not enough flow, so velocities of 100 to 300 FPM were utilized successfully.
It became apparent that the airflow pattern was as valuable as the velocity. It mattered how and where the clean air was introduced into the clean space, how it affected the process being kept clean, and how and where this air was removed from the clean space.
For proper contamination control, velocity was only one of many airflow factors of importance to an end-user.
These issues were addressed in the IES Contamination Control Recommended Practices, which expanded contamination control criteria. On a parallel path, the IES was also revising 209B into 209C to overcome its rigidity and flaws.
Federal Standard 209C which quickly became 209D created six classes of air cleanliness at five different micron sizes at three stages of occupancy, “as built,” “at rest,” and “operational.” Version 209D reflected the changing cleanroom community reality. It also added criteria for sampling locations, time, and air volume. It dropped all specific ties to environmental conditions by commenting on them by reference only.
It also was a good document particularly when used in combination with the new IES Contamination Control Recommended Practices. Its statistical analysis was superior to 209B. With the publication of 209C, in effect, 209B was no longer a viable U.S. Government document. Yet some regulatory personnel refused to accept this reality. Since 209B does not officially exist, the suggested criteria of 90 fpm is not an official government standard criteria and has not been so since October 1987.
Meanwhile, the cleanroom community had become much more sophisticated in its design and application of contamination control technology. Clean space built around end-user process needs had a variety of velocity and airflow patterns, whether in minienvironments and isolators (now called enhanced clean devices) or entire buildings housing large spacecraft.
Federal Standard 209E, published on September 11, 1992, included classes of air cleanliness by metric measurements as well as newly defined criteria for ultrafine particles. It is a comprehensive document.
Today, Federal Standard 209B no longer exists. It should not be used as a reference. It is flawed. Its flaws have been corrected by three further generations of Federal Standard 209 documents.
The reality of today`s more sophisticated cleanroom design and operations make the absolute rule of 90 fpm airflow archaic. Suffice it to say that 90 fpm is only one of the many criteria of airflow choice. It is certainly not an absolute regulatory necessity.
Richard A. Matthews is founder of Filtration Technology Inc. (Greensboro, NC) a licensed cleanroom general contractor and a manufacturer and distributor of industrial filter equipment. He is chairman of the International Organization for Standardization Technical Committee ISO/TC 209 “Cleanrooms and associated clean environments.” He is past president of the Institute of Environmental Sciences and Technology, and is vice chairman of the standing committee of the International Confederation of Contamination Control Societies. He is also president of Micron Video International. He is on the CleanRooms Editorial Advisory Board.
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A smoke demonstration of airflow in a cleanroom.