How to Ensure Proper Ventilation in a Cleanroom with CFD
For many recent manufacturing, pharmaceutical, and scientific research applications, a cleanroom is required for activities performed under a controlled environment and containing a low level of pollutants. It has a controlled level of contamination which is measured by the number of particles per cubic meter at a specified particle size. The ambient air outside in a typical urban environment contains 35,000,000 particles per cubic meter in the size range 0.5 µm or larger in diameter, which according to standards, is an ISO 9 cleanroom.
An ISO 1 room only allows 12 particles per cubic meter of diameter 0.3 μm or smaller . It is absolutely necessary to maintain a high level of cleanliness in order to obtain desired outputs from equipment and processes that are sensitive to environmental conditions.
The crucial decisions made during the design of a cleanroom affects the cost while building, and also for its operation in the long run. There are several configurations available to get the desired airflow and recirculation pattern in order to remove the internally generated contaminants. The International Society for Pharmaceutical Engineering prescribes a training course on current Good Manufacturing Practice (cGMP), where one of the important course modules is HVAC solutions.
It focuses on the understanding of contamination control, properties of air and HVAC principles in critical environments, like the cleanroom . The American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE), also conducts courses on cleanroom design to highlight key topics such as air flow management, room layout etc. .
Hence it might be important to understand the role of HVAC principles in cleanroom applications. One of the most optimal ways to recognize this is by performing simulations. Simulation technology is a cost-effective and time-efficient tool which helps during design.
Role of Engineering Simulation
How does engineering simulation help? For instance, we consider the following 3 cases in which a possible contaminant leakage is assumed.
The cases have different ventilation configurations and we can compare them to find the best possible solution to eliminate the contaminant effect. In the simulation study, the flow is assumed to have a single pass, i.e. freshly treated air is allowed into the cleanroom every time.
- Configuration 1 has central duct inlets and the exhaust along the floor.
- Configuration 2 has similar outlets like the previous case but the inlet is distributed throughout the top ceiling with multiple ducts.
- Configuration 3 has a raised tiled floor where the outlets are placed below the floor. The inlet is distributed like in configuration 2 and the tiles have several holes to ventilate air below the floor. It is very important to understand what is actually expected out of the simulations, both qualitatively and quantitatively.
Velocity plot comparison
All three cases are analysed assuming the same conditions of inlet and outlet in order to make the results comparable. It can be seen that case 1 has large recirculation in many regions, as marked in the following image. These are the zones where we try to avoid recirculation in order to avert the contaminant from spreading. The configuration 2 has comparatively less recirculation in critical zones. On the contrary, the last configuration has very less circulation, and mostly in corners of the domain.
Handling laminar and turbulent air flow principles might also be required in specific cases of cleanrooms, which can be analysed using simulation. Laminar flow usually directs air downward with a constant velocity. This is usually applied over the entire ceiling to maintain a unidirectional flow. Laminar flow criterium is generally stated in portable workstations and is made compulsory in ISO-1 through ISO-4 classified cleanrooms . Another important aspect to be noted is the ‘volume of air’ which is circulated. Certain equipment operation and some processes take place under strict pressure and temperature conditions, which may be very high or extremely low (even vacuum pressure). Any uncontrolled conditions can give rise to an adverse and dangerous outcome.
Another important aspect to be noted is the ‘volume of air’ which is circulated. Certain equipment operation and some processes take place under strict pressure and temperature conditions, which may be very high or extremely low (even vacuum pressure). Any uncontrolled conditions can give rise to dangerous situations. Maintaining the right pressure in the room can be achieved by controlling the air volume. A separate simulation study of air handlers can be done to study the volume of air discharged into the clean room. In some cases, these clean rooms are operational 24 hours a day, all throughout the year. Hence ‘pressure drop’ is a factor which can be controlled in the design in order to maintain the conditions in a functioning cleanroom .
Contaminant spread comparison in a cleanroom
People working in cleanrooms face the risk of exposure to chemical vapours if there is a mishap or mishandling. Hence it might of concern to assume a particular source of contamination and analyse how it spreads in the room. The ultimate aim is to obtain a flow to get rid of the contaminant by regulating it directly to the outlets. To analyse the results, a particular plane is considered along the cross-section where the contaminant source is present and study its path.
When a contour plot is created, it is seen that the contaminant spreads near side walls and reaches the room ceiling in configuration 1.
This can be explained due to the re-circulation of air as seen during velocity comparison. In the next case, the contaminant tries to approach the exhaust, since the source is placed near the side outlet. The consequence might not be the same if a different point of contaminant source is assumed far away from the outlet. There is even a possibility of recirculation.
In configuration 3, the contaminant is exhausted down through the floor tiles and approaches the outlet under the flooring. Hence a similar result can be expected for placement of contaminant source anywhere in the room. To conclude from this comparison, configuration 3 is the winner!
It could be seen that HVAC solutions play an important role in designing cleanrooms and in principal achieving an energy efficient model. This is incredibly important for lowering operating costs. Some of the energy efficient concepts are to reduce exhaust, room size, room air leakage, pressure drop and simultaneous heating and cooling. All of these can be analysed using engineering simulation and in particular CFD software.
These are some basic ideas indicating the need to perform a simulation to achieve the best possible design. In this context, the term best design depends on various circumstances and in reality there are many tiny factors which could adversely affect the system. Hence it is our responsibility to make a safe environment for our fellow researchers and workers in a cleanroom, who end up saving lives!
 Environmental Systems Corporation, www.e-s-c.com
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