Guardtech Nordic AB GMP specialist Thomas Loof gives his take on the pros and cons of utilising a recirc plenum in your cleanroom project
A plenum can be a wise design choice when considering your airflow management within your Cleanroom Design. It is a sealed space that manages the transition of air from one path to another and can cut the need for complicated and messy infrastructure as well as reduce overall project cost.
It is widely adopted in a range of high-tech classified environments but typically remains industry specific, with Semiconductor being the widest industry adopter. It can be applied across industry, but it presents challenges that need careful consideration when deciding if this should be a tool to be adopted within your airflow management design.
Benefits of a ducted plenum system:
Simplified Airflow Distribution – Rather than a network of ducts, dampers and valves terminally connected to individual filter boxes, a plenum provides uniform air distribution across a vast area, covering multiple rooms, typically utilising fan filter units to draw air from the shared void space. This can save significant cost by eliminating ductwork materials and the significant labour resources associated to install it. It can also simplify and expedite the balancing exercise, allowing fan speeds to be adjusted at a central point of control to achieve desired volumes and pressures.
Improved Temperature Control – The return air from the cleanroom exhaust and the conditioned fresh air supply air from the air handling unit can mix and circulate within the plenum space, providing the opportunity for a mixed uniform condition prior to delivery back into the controlled environment via terminal filtration. This ensure that a common condition is delivered at point of delivery, eliminating the issue of hot and cold spots within specific rooms.
Energy Efficiency – Properly designed plenums optimise airflow by reducing air transit time, eliminating duct networks and minimising return air back to the AHU, whilst reducing energy consumption by decreasing fan size and power consumption at the air handling unit. With this design philosophy the AHU (and coils) only needs to be sized to consider the percentage of returned air to the AHU and the conditioned fresh air percentage, rather than supplying the full demand to meet the supply volumes required to meet high air change rates. This can be significantly more cost effective for ISO5 environments especially, where the volume of air movement is significant to meet the high air change rate values required for compliance.
Challenges of designing and installing a ducted plenum:
Segregation – This design is not always the most suitable approach. The Pharmaceutical industry, for example, will use a plenum design more sparingly and will rarely consider a plenum that broaches between two rooms. A mixed airstream that can be shared between two rooms introduces the risk of cross-contamination from one room (and process) to another, this can present challenges and often will return in Pharma adopting a closed loop ducted system.
Void Management – The plenum will require cleaning at some point, so appropriate access must be considered. The same is true of maintenance access for installed services within the space, therefore consideration may need to be given for loading and walkability as well as appropriate lighting and escape routes. Smoke detection within the void will need to be considered if the plenum is greater than 800mm in depth, this can add cost and complexity, maintenance challenges and risk is aspiration is also required.
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Cost Comparison – On a larger scale project with high air supply volumes a plenum will often present significant cost savings that are worth the compromises that may be required to implement it. However, for smaller scale jobs or lower grade classification projects, when the supply volumes are lower, a more traditional approach may be the better cost option, much of this decision will come down to the temperature and humidity requirements of the environment and be impacted by the AHU selection.
Conclusion
The largest benefits offered by adopting a shared space recirculating plenum is a lower cost in energy and air distribution materials, typically a quicker install, a more aesthetically pleasing installation, a uniform and well mixed air supply and a reduced volumetric burden on the AHU for passing air stream. Industries that do not have to worry about cross process contamination, especially those operating at ISO5 & ISO6, should be looking to this solution at design stage to see if its implementation is suitable for their specific process. A cost vs. risk analysis should be conducted and if the balance is right the client will benefit from lower installation costs and lower ongoing operating costs.
For more information on the Guardtech Group’s wide range of cleanroom construction and contamination control services, call 0330 113 0303, email sales@guardtech.com or visit www.guardtechgroup.com.
