By Robert P. Donovan

Last month's column touted computational modeling as a useful tool for predicting airflow patterns in a cleanroom. The complex geometries and boundary conditions of a real cleanroom, however, render this approach impractical.

Airflow visualization is an alternate, more direct-and ultimately more convincing method for identifying flow patterns in an operational cleanroom filled with numerous odd-shaped objects and hustling people.

There are a number of methods by which airflow trajectories can be made visible in an operating cleanroom:

• Neutrally buoyant helium bubbles;
• Smoke generators/foggers;
• Party balloons filled with helium and weighted to become neutrally buoyant;
• 3-D ultrasonic anemometers.

Neutrally buoyant helium bubbles

In this technique, an SAI Bubble Generator produces 300 to 400 “neutrally bouyant” bubbles per second, which are released or “seeded” into the area of interest.

The bubbles are about one-eighth-inch in diameter and have an average life of one to two minutes. The wall of each bubble is made of a special soap bubble film solution. Filled with just enough helium to offset the weight of the bubble wall, the bubbles instantly “lock in” to the local air movement and exactly follow the flow. Any kind of flow may be easily seen, whether it is laminar or turbulent, steady or non-steady.

Airflow paths traced by neutrally buoyant helium bubbles.1 (Note the neat recirculation loop under the tabletop.)Click here to enlarge image

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With suitable lighting in a darkened room, the bubbles appear as well-defined streaks, which may be followed visually, photographed or videotaped to document the airflow patterns. The photograph below shows a short time exposure of airflow around a tabletop located in a vertical unidirectional flow cleanroom. It clearly confirms a recirculation zone under the table similar to those predicted by the flow-modeling analyses of a similar geometry presented last month—including a vortical center.

Smoke generators/foggers

Smoke bombs, cigarette smoke, carbon dioxide vapor and many other sources have all been used to trace airflow patterns in the past, but not typically in cleanrooms. A commercially available fogger that is cleanroom-compatible generates ultrapure water droplets, which the manufacturer claims are “neutrally buoyant, non-contaminating and highly visible.” 2

Helium-filled party balloons

This is my favorite because it's cheap. Phil Lawless of the Research Triangle Institute (RTI) suggested filling a common party balloon with helium and attaching a tail of string to it.3 (We used a length of magnetic tape.) The assembly could be rendered neutrally buoyant by clipping off short, incremental lengths of the tape until the balloon remained motionless in still air. We showed that the airflow in one particular cleanroom (~70 percent of the ceiling covered with HEPA filters and a side wall return) was far from unidirectional. We could release the balloon near the floor in certain spots and watch it be carried up to the ceiling rather than to an exit duct.

3-D ultrasonic anemometer

Compared to the party balloon method, the 3-D ultrasonic anemometer approach represents the opposite end of the sophistication spectrum, and is a flow-mapping technique rather than a direct visualization method. The operating principle is the measurement of the transit time of an ultrasonic acoustic wave between a transmitter and a receiver probe.4 The transit time depends upon the speed of sound plus or minus any component of convective airflow parallel to the direction between the two probes. Since the same probe can serve as a transmitter and a receiver, transit times can be measured in both directions—the probe that acts as the transmitter for the first measurement becomes the receiver for the second measurement.

Knowing the distance between the two probes and the two transit times provides sufficient information for calculating the component of convective airflow in the direction between the two probes, as well as the speed of sound (an independent measure of temperature). By arranging three mutually perpendicular sets of two probes each, three-dimensional mapping of airflow can be carried out.

This method, while quantitative and detailed, is also time-consuming and expensive. But some National Environmental Balancing Bureau (NEBB) cleanroom certifiers (see CleanRooms, March 2003, pg. 12) own such equipment and offer this service.

Click here to enlarge image

ROBERT P. DONOVAN is a process engineer assigned to the Sandia National Laboratories and a monthly columnist for CleanRooms magazine. He can be reached at: [email protected]

References

1. Ordway, D. E. “Flow Visualization with Neutrally Buoyant Bubbles Is an Extremely Powerful Tool for Gaining Insight into Complex Airflow Phenomena” [http://www.sageaction.com]
2. “Ultrapure Cleanroom Foggers” [http://www.mspcorp.com/cleanroom_fogger.htm]
3. Lawless, P. A. and R. P. Donovan, “Visualizing Airflow Patterns with Helium-Filled Balloons”, Microcontamination 6(9), September 1988, pp. 72-75
4. Hope, D. and D. Milholland, “The Use of a Three-Dimensional Ultrasonic Anemometer to Measure the Performance of Clean Zone Air Delivery Systems,” 1993 Proceedings of the IES, pp. 516-528