Cleanroom modelling

The ventilation system employed within a cleanroom is fundamental to its overall performance. A well-designed ventilation system should prevent contamination of products and protect operators from being exposed to hazardous materials. For many years, empirically based techniques have been used when calculating the required airflow rate to achieve the cleanliness standards specified by the regulatory bodies. These 'rule of thumb' techniques give no real indication of how a system will perform before it has been built. The only method of determining performance of the ventilation system prior to construction has been to build a mock up of the facilities. The costs and time involved in conducting tests in a mock up make this method prohibitive except in extraordinary cases.Airflow modelling offers a flexible alternative to physical testing that will fit easily into any design cycle. Airflow modelling using Computational Fluid Dynamics (CFD) dispenses with rule of thumb techniques and solves the fundamental laws of physics for each problem. This allows the 'over design' which has been incorporated into rule of thumb techniques to be removed and provides the opportunity to optimise the ventilation system at the earliest possible stage of the design process.

What is airflow modelling? Airflow modelling within a cleanroom involves the calculation of the velocity, pressure, temperature and concentration fields within the built environment. In order to build a solution within the software, a user defines the problem to be solved by specifying its boundary conditions. In reality this involves specifying the geometric representation of the cleanroom including details of the shape and dimensions of the building fabric, ventilation system and equipment. A calculation grid is then imposed on the model to allow the software to solve the equations necessary (Navier-Stokes, thermal energy, species conservation and continuity equations) to build a picture of the overall flow patterns and temperatures within the model. These can then be displayed in a number of visual formats that allow the engineer to see how the air flows within the cleanroom. Why use airflow modelling? Firstly, there is a significant cost advantage. Anyone who has used airflow modelling will be well aware that the technique offers a unique ability to make informed installation decisions and optimise the design before it is finally commissioned. If the design of a cleanroom has to be changed the cost of implementing these changes increases as the project progresses. By modelling in the initial stages of a design these costs can be kept to a minimum. Therefore airflow modelling is not restricted to a particular design stage; indeed it should be used at the earliest possible stage within the design process. This allows a constant review of the ventilation system's performance and will highlight areas of concern at the earliest opportunity allowing designers to reduce the capital cost of the project. Each new building is essentially a prototype, a one off. Traditional design methods have relied on the engineer's experience and rule of thumb techniques. There is a tendency to add significant margins into the design to compensate for the shortcomings of the rules of thumb. Thus as the design progresses these margins are then multiplied with additional margins resulting in a system over designed for the purpose it was originally intended. This will result in increased running costs as the amount of clean air supplied is in excess of what is actually required. Physical mock-ups have traditionally been the only way to determine the performance of a system prior to commissioning. Unfortunately this method is both expensive and time consuming and often only utilised when the design is almost complete with the aim of verifying that the design will operate as intended. The amount of data that can be obtained from physical testing is far less than would be obtained from a single airflow simulation. Airflow modelling is quick and allows simulations to be carried out to investigate all of the design options at a conceptual stage, thus allowing the design to proceed with more confidence and with a clearer understanding of its final operation. It is well documented that the best way to ensure that a clean environment exists is to supply air directly downwards from ceiling mounted supplies. The air then passes downwards over the processes and operators where it is withdrawn at a low level. Any contamination is picked up in the airflow and is removed so that there is no chance of it contaminating the process. This airflow regime is fundamental to the operation of a cleanroom and it is vital that there is no upward movement, causing areas of recirculation.

Case studies bioMérieux are renowned in the field of vitro infectious disease diagnostics. They design, develop and manufacture reagents and automated instruments for medical analyses, for cosmetics, and product quality control in the agri-food and pharmaceutical industries. As part of their commitment to reducing the risk of contamination within their cleanrooms, bioMérieux decided that airflow modelling would give them a tool with which to simulate their current design and determine its limitations before simulating the effects of modifications. The modelling work was carried out by Flomerics using FLOVENT® airflow modelling software. The first level of modelling was focused towards the operator and the interaction with the vial filling equipment. Two operators work at the vial filling machine, which is located underneath a laminar flow unit and is surrounded by flexible curtains. bioMérieux wished to modify the machine area in order to reduce the risk of contamination which would result in an increased yield. Fig. 2 shows velocity vectors around the vial filling machine. The air can be seen to be flowing directly down from the laminar flow unit where it collides with the vial filling machine and passes out underneath the bottom of the flexible curtains. Two recirculation regions can be seen on either side of the operator, which was considered undesirable as this provides a potential area of contamination. The results also showed the air speed around the filling head of the machine to be undesirably low. Fig. 3 shows a plot of speed at the height of the filling head. The slow moving air shown in purple is around the filling head and is considerably lower than the intended 0.45 ms-1. To trace the spread of contamination, sources were added to the operators' hands and at the pump associated with the vial filling machine. These locations were chosen as bioMérieux had seen contamination problems arising from the pump and the operators. Fig. 4 shows the spread of contamination from the pump, where it is entrained in the recirculation regions shown in Fig. 2. The contamination released from the operators' hands was easily swept away from the vial filling machine. The second level of analysis was carried out on a separate vial filling room where two machines where operating. These machines were intended to operate on their own with the possibility of filling different products on each machine. The primary concern was whether there was a risk of cross contamination of the two products whilst operating in this way. Each filling machine was equipped with its own laminar flow units, plastic curtains, with separate supply and exhaust ducts. Fig. 5 shows the layout of the room with the two filling machines and the interaction of the ventilation systems. The particles show that the airflow passes from one curtained area to the other, thus providing a possibility of cross contamination. The initial models were studied to identify the causes of contamination spread before further simulations were carried out to test whether alternative designs would prevent the contamination spread. In both cases it was found that simplest and cheapest method of preventing contamination spread was to modify the arrangement of the plastic curtains.

Conclusion The FLOVENT® results shown in this article enabled bioMérieux to simulate their design and make modifications to improve its effectiveness. The technique has enabled them to cost-effectively implement a solution to contain and remove contamination in a way that best suits their installed design. These techniques are equally useful when looking at new installations, potentially saving valuable cost and time through a greater understanding of the cleanroom's eventual operation. It has been shown that by using airflow modelling, designers and facilities managers can assess the operation of their facilities and improve the cleanliness of the environment.

This paper was originally presented at Cleanrooms Ireland 2002.

For further information, contact Flomerics on tel: +44 (0)20 8941 8810

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