Cleanroom airflows Part II: The messy details
07/01/2002
by Mike Fitzpatrick and Ken Goldstein, Ph.D.
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In our May column, we discussed some of the basics of cleanroom airflow. Remember that unidirectional-flow rooms depend on a "piston effect" for their cleaning action since that design strives to minimize turbulence and the mixing of parallel air streams. In contrast, non-unidirectional flow rooms actually require turbulence and good mixing because that design operates on the principle of dilution.
We discussed that when we refer to cleanroom airflow rates, we are not referring to the face velocity of the individual filters but the average velocity of the filtered airflow through the entire room.
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We also noted two common methods for quantifying the airflow: average velocity, calculated as distance divided by time; and the air change rate, which uses air changes per hour. Another uncommon approach, percentage of filter coverage, can present some difficult issues; however, it is intuitively appealing.
It has been common to relate various average airflow velocities to specific cleanroom cleanliness classes, such as, "An ISO Class X cleanroom requires an average velocity of Y m/sec."
But the messy and inconvenient details invariably intrude into our neat and elegant methods, and specifying cleanroom airflow velocities is no exception.
The Messy Details
Cleanroom cleanliness depends on much more than the airflow velocity; however we do choose to measure it. For facility designers, builders and operators, these additional factors can be difficult to control. Their effect can only be estimated and taken into account.
Let's look at some of these factors and examine how they influence room classification and our selection of airflow rates.
Particle Generation: Particle generation rates within a cleanroom have a profound affect on room classification. Particles originate from a many sources: building com ponents, people, pro cess equipment, and the manufacturing pro cess itself. Oper ator protocols and gowning procedures are also a major influence on room particle counts.
Particle generation within the cleanroom tends to vary greatly and is not readily quantifiable. Frequently, designers in crease their anticipated airflows based upon an assessment of a client's plant, process and procedures. The bottom line is that if you make a big mess it will take a lot of clean air to clean it up.
Airflow direction: Unidirectional rooms are designed to have uniform and parallel streamlines. Any obstruction of the airflow will create turbulence and limit the ability of the air to remove particles. Consequently, horizontal surfaces should be minimized and the placement of equipment should consider any resulting disruption of the airflow.
Non-unidirectional rooms, designed to have non-uniform (turbulent) airflows, depend upon the mixing of the air to maintain cleanliness, temperature and humidity. Without this mixing, stagnant areas with high particle concentrations may develop and temperature stratification may occur.
Avoid the temptation of trying to make your non-unidirectional room mimic unidirectional flow. It might seem like a neat thing to do but will probably create problems.
Temperature-determined airflow: In addition to controlling airborne particles, cleanroom airflow also determines the room's temperature and humidity. In nearly all cases, cleanrooms require cooling rather than heating. Temperature control is achieved by introducing relatively cold air through the filters into the room-the only acceptable way to deliberately introduce air into a cleanroom is through filters.
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If the cooling-air requirement exceeds the amount of air that can be supplied through the filters, additional filters must be used, even though they are not required for airborne cleanliness.
Room recovery: Airflow rate also determines the ability of a room to recover from upset conditions. These upsets may be the result of mishap-chemical spills, interruption of airflow, incidents that release or introduce contaminants-or may result from normal operation such as intermittent convection currents, personnel and equipment movement.
Economic considerations: It costs money to move air, and the cost of installing equipment and subsequent operational costs need to be considered when selecting an airflow rate. More air may not necessarily be better.
What is the appropriate airflow rate for your cleanroom? It is the rate that best balances the considerations listed above.
Recent developments
During the past year, the members of the Institute of Environmental Science and Technology (IEST) Working Group (WG12) have been moving toward an update and rewrite of Recommended Practice-12, "Considerations in Cleanroom Design."
As part of this effort, the velocity table contained in RP12 will be brought up to date. For the past year we have been discussing this topic informally with Prof. William Whyte of the University of Glasgow who is a member of the working group and well known for his outstanding contributions to the field.
As a result of these discussions, it quickly became apparent that average velocity, length divided by time, was a much better measure than the air change rate, air changes per hour, for unidirectional flow rooms ISO class 1 to ISO class 5. Conversely, the air change rate is a better measure than velocity for non-unidirectional flow rooms (ISO class 6 to ISO class 9).
Prof.Whyte volunteered to write a draft of the velocity section of the document. At the recent IEST annual meeting in Anaheim, the Working Group met to discuss progress to date. True to form, the group spent 95 percent of its time wrangling over the velocity table. A slightly edited excerpt from the new velocity table is offered in Figure 1.
Michael A. Fitzpatrick is program director of microelectronics for Lockwood Greene Engineers. A senior member of the Institute of Environmental Sciences and Technology (IEST), he is Chairman for WG012 (Considerations in Cleanroom Design) and WG028 (Minienvironments). Ken Goldstein is principal of Cleanroom Consultants Inc. in Phoenix, Arizona, and is a member of the CleanRooms Editorial Advisory Board.