by Mike Fitzpatrick and Ken Goldstein, Ph.D.
Owners, designers and builders banter constantly about 90 feet-per-minute velocity, 600 air changes per hour, and 100 percent filter coverage. What does this all mean, and why can't we settle on a single method of describing airflow?
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When we speak of airflow velocity of a room we are referring to the average velocity throughout the horizontal plane of the room. This is significantly different from filter face velocity. Velocities are expressed in units of length divided by units of time. Examples would include meters per second and feet per minute.
To understand airflow rates we first need to consider the type of airflow we are using. Clean room airflow is described as being either unidirectional (laminar) or non-unidirectional (turbulent). We have stopped using the terms “laminar” and “turbulent” because they were too easily understood and because the purists amongst us insisted that the velocities we were considering were neither laminar nor turbulent when looked at from the perspective of their Reynolds numbers.
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So we're now stuck with “unidirectional” and “non-unidirectional” which, although technically correct, certainly lack the pizzazz of laminar and turbulent. We typically see full filter coverage in unidirectional cleanrooms and partial filter coverage in non-unidirectional cleanrooms.
Unidirectional cleanrooms have generally parallel streamlines and approximately uniform velocity throughout. They “clean” the air by transporting particles in the direction of the airflow out of the cleanroom. This has been likened to a “piston effect” where incoming clean air “pushes” contaminated air from the room. The advantage of unidirectional flow is that airborne particles are carried out of the room in minimum time and the shortest path, thereby giving them less chance to start any trouble.
Non-unidirectional rooms have non-parallel flow streams and non-uniform velocities. These rooms operate on the principle of the “dilution effect,” with clean air entering the room and diluting the contaminated air. This mechanism is similar to placing a garden hose into a bucket of muddy water and stirring the water while letting the excess drain out over the top. Eventually the clean water entering the bucket will dilute and displace the dirty water.
Rather than trying to achieve laminarity we want to encourage turbulence in this type of room, much like stirring the muddy water. Without this mixing, areas of increased contamination and temperature and humidity gradients will form.
We have stopped using the terms “laminar” and “turbulent” because they were too easily understood and because the purists amongst us insisted that the velocities we were considering were neither laminar nor turbulent when looked at from the perspective of their Reynolds numbers.
Describing airflow in terms of air changes per hour is common for non-unidirectional flow rooms (ISO classes 6 through 9) and high-bay installations. Since the airflow in these rooms is non-uniform, attempting to directly measure the average air velocity is not feasible. The average velocity may be calculated, however, using volumetric measurements from the terminal filters. This velocity is then converted into an equivalent room air changes per hour (AC/H).
Leaving it in terms of average velocity would be confusing and misleading and may result in someone attempting to use a velometer to directly measure this variable. Use of the AC/H, however, presents a problem-the calculated quantity is dependent on the height of the room.
For a given volumetric airflow, the corresponding AC/H increases when the vertical dimension of the room decreases. If we wish to compare one room to another we need to know the height of each. By informal convention most velocity tables calculate AC/H based upon a 3 m (10 ft.) ceiling height.
For the unidirectional flow rooms (ISO classes 1 through 5) we typically specify airflow rate by velocity rather than the air change rate. Since the airflow is presumed to be uniform throughout, the concept of average room velocity conveys real information and can be determined by direct measurement with a velometer.
Expressing airflow in terms of velocity allows us to compare the airflow rates of one cleanroom to the next and to quantify any changes in a room airflow rate. The measured velocity is totally independent of the room height.
A third method of describing cleanroom airflow has been to specify the percent filter coverage. Using this approach, a room with 25 percent filter coverage translates into an airflow velocity of 25 fpm and a room with 100 percent coverage yields a velocity of 100 fpm. This method works well as long as the filter face velocity is maintained at 100 fpm. Deviating from 100 fpm introduces an additional variable. One of the advantages of the percent filter coverage method is that one can merely look at a cleanroom and determine the approximate airflow rate.
Two standards that offer cleanroom airflow tables are ISO 14644-4 and IEST Recommended Practice (RP) CC-012. ISO 14644-4 contains Annex B, an informative (non-mandatory) appendix, which defines airflows for ISO Class 1 through 5 rooms in units of meters/sec and airflows for ISO Class 6 through 9 in AC/H. IEST RP CC-012 Considerations in Cleanroom Design offers a table of recommended airflows expressed in both velocities and AC/H.
When describing airflow for non-unidirectional rooms or high-bay facilities it makes sense to use air changes per hour since we can't directly measure the velocity. For unidirectional rooms with uniform airflow specifying average velocity is appropriate. The percent filter coverage method works with either type of room.
Some people favor using velocity because velocity is velocity regardless of room dimensions. Some favor AC/H because it is familiar to them, while others opt for percent filter coverage due to its simplicity.
Then there are those of us that lean towards AC/H or percent filter coverage because they can be converted to metric units with little chance of error.
Michael A. Fitzpatrick is program director of microelectronics for Lockwood Greene Engineers. Ken Goldstein is principal of Cleanroom Consultants, Inc. in Phoenix, AZ, and is a member of the CleanRooms Editorial Advisory Board.
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