Finding the Needle in the Haystack
Athier Alani and Sarah Bocas of Flomerics discuss airflow modelling software
Cleanroom operators spend excessive amounts of time trying to locate sources of contamination. This is particularly important in the semiconductor industry where wafer contamination can lead to a dramatic drop in yield. Clearly, a tool that will give those involved in contamination control an indication of the location of possible particle or gaseous sources could be extremely valuable. In addition to saving many man-hours of trial and error analysis using particle counters as well as past experience it will also make a financial contribution to the bottom line profit. Since 1988, our company has played a central role in the use of airflow modelling in built environments by delivering analysis software for designers. FLOVENT, Flomerics airflow modelling software, has become widely used throughout the building industry and is used extensively in the design and modification of cleanrooms. FLOVENT uses Computational Fluid Dynamics (CFD) as this provides the only method for accurately simulating the ventilation design prior to build. The company has pioneered the use of CFD in design environments, otherwise known as Design-Class Analysis.
Why use Airflow Modelling Software? How can you be sure whether your cleanroom design is able to adequately satisfy your contamination control requirements before it is built? One way is to run a computer simulation of the ventilation and contamination spread in the room. Using these techniques it is possible to model not only the ventilation systems, but also any equipment or even people occupying the room. If the system does not meet specifications then a redesign simply involves a few geometrical changes to the model and rerunning the simulation. In this way it becomes a quick and easy exercise to optimise your design, therefore reducing design and running costs. Most cleanroom designers would agree that the ability to locate the source of any contamination would be invaluable. Imagine a contaminant spreading through your cleanroom, how do you effectively determine the source of this contamination? Our company has recently introduced the Reverse Passive Tracer (RPT). This feature significantly aids in the determination of the location of the contamination source by reducing the area in which you need to search. The basis for the Reverse Passive Tracer (RPT) methodology is that the contamination is airborne either as very fine particles or as another gas, and for this the contamination can be tracked back a long the flow lines. This assumption is valid provided that convection dominates diffusion by a significant amount. This method is the first of its kind in the world of Computational Fluid Dynamics. RPT's main strength is to narrow the area where the search can be conducted to find the contamination source. Rather than having to search all over a cleanroom to find the source, RPT will actually reduce the area by a significant percentage, as shown in the worked example later.
So how does the RPT work? Take a source of concentration, representing the leak in the facility as illustrated in Fig. 1. From this contamination will disperse to other parts of the room. Three locations are identified as C1, C2 and C3 respectively. These locations are where contamination measurements have been made in the facility. For the purposes of this demonstration the measured values will be taken from a prediction of the distribution of the contamination leak. These 'measured values' are then used as RPT source points for a reverse flow calculation. The method relies on the principle that the contaminant dilution is the same whichever way you travel along an air path. This is not strictly true since the effect of turbulence is not reversible.
Worked Example The room measures 6m by 3m by 4m and consists of two air terminal devices, ceiling, floor and a desk. A cleanroom worker is going about his or her daily job and is being exposed to contamination from a leak of chemical, but where is it coming from? Four measurement points are chosen (indicated in brown) as an indication of contamination dispersion at different points in the room. The flow will be reversed by the RPT functionality and then upon the end of the solution the minimum error (Concentration error) is plotted. This is close to the actual leak source (indicated in yellow), demonstrating a significant reduction in the search area, time and money. It is important to note here that often the minimum error does not pin point where the leak source is because diffusion is impossible to reverse with 100% accuracy due to its random nature. The minimum error only gives an indication of the general area to look for the leak source. As can be seen by the example above, the RPT can significantly reduce the time spent in trying to determine the source of the contamination in your cleanroom and can therefore dramatically reduce the costs involved in the design or redesign. In addition to the RPT, the latest version of the software has introduced a variety of other features that aid in the design of your ventilation systems. The most significant of these is the Command Centre. This is a feature built into the software that allows fast, accurate parametric studies to be conducted on any aspect of your design. It can be used in conjunction with the RPT to assist further in the fast, efficient determination of the source of contamination. With the Command Centre it is possible to adjust volume flow rate from a vent and simulate how this will affect the airflow around the room. If you need to optimise this then it is possible to enter a series of flow rates using the command centre interface. It is not necessary to completely remodel the room. It can then run each simulation either consecutively on the same computer or simultaneously by utilising a number of networked computers. In this way the design can be optimised in a fraction of the time that was previously required.
Summary Airflow modelling software is an incredibly powerful tool to use when designing a new build or modifying the existing design of any building. It significantly reduces the time involved in the design through the ability to optimise without having to do lengthy hand calculations. In this way it is possible to maximise the use of your facility, or to move onto the next project faster, whilst enabling you to keep your clients happy by delivering their facility faster and reducing their running costs
