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Cleanroom Ionization in Laminar VS Non Laminar

flow-illustration

Many variables will influence the efficiency of air ion delivery to the target work level to discharge insulative materials and isolated conductors. The most important factor to consider is the uniformity of airflow in the cleanroom. In both images above we could imagine that the airflow at work height of 42 inches off the floor is approximately 70fpm as illustrated by the blue downward arrows.

Laminar Air Flow Settings.

In the case of the full laminar cleanroom image on the right, the airflow is consistently 70fpm across the entire area. For this application we would set the IRIS system output for shorter, more powerful bursts of ions using a voltage of 8-9kV and a pulse of approximately 4 seconds positive and 3 seconds negative with a short delay between pulses of .7 seconds (7.7 seconds total pulse cycle) to produce a relatively low and consistent offset voltage reading of 80-100V +/-. This would provide excellent charge decay rates of approximately 25-40 seconds at 5ft distance from emitter tip to test point.

Non-Laminar Air Flow Settings.

In the scenario of a non-laminar airflow as shown in the illustration above left, the setting would need to be quite different to manage the various conditions we would find and the constant changes in the area to direction and efficiency of airflow. In this case we may have 70fpm airflow at work level, but this location would shift due to changing high pressure areas and when we did get a true reading of 70fpm that lasted for a duration to provide a reading we would see that the offset voltage would be higher and the decay rate would be somewhat longer for this reason:

To overcome the less efficient areas of swirling air, a much longer setting would be used to achieve the best acceptable performance in the entire area as an average – not set to just the few transitory areas of good airflow. The output of the power supply would be lower – 7.5kV and the pulse times would be set much longer (7 seconds positive, 4 seconds positive and a 3 second delay between pulses (14 seconds total) making the total cycle nearly twice as long) to allow the ions to repel from each other and flood the air with thicker blankets of like polarity ions, then rest and continue to spread during the delay period. The lower output of 7.5kV will help control the offset voltage from climbing too high, and the longer pulse times will cause the offset voltage to slowly become higher over the entire pulse cycle. This will cause higher offset voltage readings in areas of higher downward airflow while still producing the best possible decay times in the non-laminar areas. In these areas the ions are not traveling 5ft to the CPM meter from the emitter tip but more often 10-20ft in a swirling pattern of eddies created by low and high air pressure in the area. A closed ceiling tile next to a positive air pressure FFU will produce a turbulent air flow.

Using an air fogger or similar visual tool you can see the effects of non-laminar airflow.

fogger

dye-airflow-demo

Laminar compared to Non-Laminar in stages. As you can see, there may be 70fpm in the various illustrations below, but the readings would begin to become more irregular and without the correct meter, it is be difficult to tell when air flow readings are travelling downward, sideways or upwards. The actual distance and interference of swirling air patterns would increasingly change the actual performance and the air ion delivery. The illustration below is a dye in fluid but the principle is the same as in cleanroom airflow.

Because the non-laminar airflow holds the ions for longer and longer periods of time from ceiling level to work level, greater and greater numbers of free ions are recombined with air molecules and ion concentration numbers drop exponentially. In the most unstable airflow patterns, air ions become so depleted by the time they reach the test level that the decay times may be measured in minutes to dissipate an isolated charge from 1000V to 100V.

All IRIS system settings and expected outcome are based on best practice and personal experience over many years. We cannot promise a specific outcome without a complete understanding of the conditions in the area. Even with a full layout of the FFU ceiling units and return air locations and size, the actual performance can vary based on many other factors. Based on the limitations of air ions to overcome changing airflow patterns, the system would be fine tuned and adjusted from time to time based on data tracking of performance and emerging patterns that suggest a different setting would be more efficient. With the correct settings room ionization can be effective in non-laminar air flow situations and careful system set up is the key to optimal static charge control.