ACR for Cleanrooms

For decades, HVAC & cleanroom engineers traditionally have used these rules-of-thumb values for designs. This approach, however, uses only the required room cleanliness class to determine an ACR value and ignores many variables that could significantly impact the room particle concentration and the required air change rate, such as Particle Generation (PG) inside the room, efficiencies of HEPA filters and filters installed in AHU and RFUs, Outdoor Air (OA) intake particle concentration, particle surface deposition, particle entry through filtered Supply Air (SA), particle exit through Return Air (RA), Exhaust Air (EA), and leakage air (particle loss or gain) under pressurisation or depressurisation, etc.

Strategies for reduction of ACRs for energy consumption optimisation

ACRs are one of the many factors involved in contamination control in cleanrooms. However, even though ACR µ contamination control, other factors can affect and reduce the effectiveness of ACR upon contamination control. The following measures can be used to lower the ACR requirement when possible:

• Selection of cleanroom equipment, machinery, furniture, and construction materials with lower particle generation levels: Various ranges of particles can be generated by different items inside a cleanroom, including people, furniture, machinery, process equipment, and room enclosure construction materials, among others. A higher Particle Generation Rate (PGR) results in the requirement of higher ACR to dilute particle concentration. Design professionals should select cleanroom furniture, machinery, equipment, and room enclosure construction materials at the lowest PGRs possible.
• Isolation and removal of high-concentration particles generated in the cleanroom: After the above step is taken, local exhaust could be added to isolate and remove particles from the equipment, machinery, or processes identified as the sources of high-concentration particles. The goal is to minimise the particles from major sources being dispersed into the remaining cleanroom spaces.
• Designation of critical zones within the cleanroom: Not every spot in a cleanroom requires the same cleanliness. Only the areas with critical processes or operations need better cleanliness, while the surrounding areas inside the cleanroom can be designed and maintained at a lower cleanliness level. The areas with critical processes or operations should be designated as critical clean zones and mini environments should be installed & operated in these zones to give a higher ACR for the critical clean zones.
• Implementation of proper cleanroom gowning procedures & personnel conduct: For personnel PG, one cost-effective way of reducing PG is enhanced cleanroom gowning with better body coverage and gowning protocols. Fast motions such as walking fast, running, or horseplay are prohibited in the cleanroom.

No exposed paper materials of any type shall be allowed in cleanrooms Class 8 or cleaner unless approved by the Custodian. Cleanroom paper may be permitted when fully enclosed within a clean zipper storage bag.

Pencils and erasers shall not be used. non-retractable ballpoint pens (without pocket clips) are permitted in the cleanroom.

Use of paper or fabric towels is prohibited. Instead, hand dryers equipped with HEPA filters are suggested.

Appropriate cleanroom gowning procedures and personnel conduct lead to lower PGR that in turn reduces the number of ACR required.

• Designing of RA & EA systems for effective particle exit: While SA, which is a mixture of OA & RA, is given to a cleanroom, the RA is drawn through low sidewall grilles or a raised perforated floor and then recirculated and filtered to remove particles generated from processes and process equipment. Exhaust devices such as direct connections to process equipment, canopies, and hoods can significantly remove highly concentrated particles. Proper placement of return and exhaust points close to low & high-concentration areas respectively can remove particles more efficiently. Effective return and exhaust pattern designs should achieve a higher contaminant removal effectiveness value to ensure fewer particles accumulate on exposed surfaces and become surface particle contamination in cleanrooms.
• Enhancement of surface cleaning protocols to prevent surface particles from turning into airborne particles: High velocities of SA and high ACRs are intended to dilute and wash down the particles, yet no return and exhaust system can completely remove all particles from a cleanroom. Some particles accumulate on equipment, furniture, and hard-to-reach floor areas underneath the equipment and furniture. Over a time, these particles build up and could reenter the air and become airborne particles – if there is a disturbance on the exposed surface, which increases the airborne concentration. Therefore, scheduled cleaning and vacuuming is necessary for the removal of most of the residuals.
• Controlling the entry of particles through the air supply: SA, which is a mixture of RA and OA, should be filtered by ceiling HEPA or ULPA filters before it enters a cleanroom. These filters’ efficiencies determine how many particles could be released into cleanroom spaces. Higher filter efficiencies reduce particle entry through the SA; however, replacing HEPA filters with ULPA filters at the room level may not be very cost-effective as using higher-efficiency filters inside the AHU in terms of air change rate reduction. HVAC design engineers also need to consider the extra operating costs from higher pressure drops across higher-efficiency filter media.

Predictive analytics can be applied in cleanrooms to optimise HEPA and ULPA filtration systems. AI algorithms can dynamically adjust airflow rates based on real-time contamination levels, which are in turn measured by particle counters, ensuring that cleanroom air quality remains within specified ISO classification standards. Whenever the contamination levels are low, the airflow rates get decreased. Since cross sectional area of the filters is constant, air velocities are lowered, leading to lower pressure drops across the filter media, reducing the operating costs. This adaptive approach reduces energy consumption while maintaining optimal filtration efficiency.

• Air filter testing and maintenance: Pressure differential across all filters should be periodically monitored to check for clogging or air leakage (Pressure differential will be 0 or ∞ for air leakage or clogging respectively). Depending on whether filters under consideration are reusable or non-reusable, they should be cleaned or replaced respectively, if they are clogged. After periodic leakage testing, if there are leaks or defects within the filters, they should be repaired or replaced depending on the recommendation of the filter manufacturer. Periodic air filter testing and maintenance is useful for reduction of ACR due to the working of filters at the appropriate efficiency.
• Appropriate pressurisation and no depressurization of cleanrooms for avoidance of particle gain through air leakage: If a pressure differential exists, then whenever the door to a cleanroom is opened, it is inevitable that air leakage will occur between the room enclosure and its surroundings. Particle migrations following the leakage air paths should be considered, especially under depressurisation and when the air in the surrounding area is dirtier than the cleanroom. To prevent or minimise particle migration from surrounding areas, these rooms should be kept under relative positive pressure. Usually, the higher the pressure differential, the more effective it is in preventing particle migration.

However, this strategy might not work completely in cases in which a cleanroom under consideration is under negative pressure or less positive pressure than the adjacent room in which more critical activities are underway.

• Computational Fluid Dynamics (CFD): CFD, an emerging technology, can be explored in cleanroom design analysis to predict the impact of room pressurisation (relative SA & RA flow rates) and PGR on the cleanliness in a room. CFD analysis can be performed to optimise the location of SA diffusers and RA grilles & operating parameters such as the velocity of air through the supply diffusers and ACR required for the cleanroom.

Conclusion

Each cleanroom is unique. Its room cleanliness requirements, space configurations, production or process activities, HVAC systems, building construction, location, etc., can impact the ACR requirement for each room. Using the table approach without considering all these variables could result in under designed HVAC systems or cause energy waste.

After assigning ACR values using the table approach to the cleanrooms in the design stage, strategies for reduction of ACRs should be implemented in the operation stage for energy optimisation in cleanrooms.

Rushikesh Jog is currently working as a Jr. HVAC design engineer and business development assistant at Proficient, a pharmaceutical consultancy firm. Proficient provides various services such as pharmaceutical facility design, validation, Good Manufacturing Practices (GMPs), compliance, etc. He possesses a bachelor’s degree in mechanical engineering and also a post-graduate diploma in HVAC. He writes technical posts and articles on LinkedIn related to HVAC & interrelated topics like pharmaceutical manufacturing, energy efficiency, cleanrooms, etc.