A cleanroom is defined as a room that maintains an extremely low level of airborne particles through carefully controlled airflow to support quality assurance and prevent contamination (https://www.cleanairtechnology.com/cleanroom-classifications-class.php). These environments are critical for industries like pharmaceutical manufacturing, medical device production, and microprocessor manufacturing where even the smallest contaminants could ruin products or processes. Cleanrooms are classified by the maximum number of permitted particles per volume of air. For example, a Class 10,000 cleanroom indicates there are fewer than 10,000 particles of 0.5 microns or larger per cubic foot of air (https://www.tme.com/us/en-us/news/library-articles/page/55922/cleanroom-definition-purposes-and-equipment-of-controlled-environments/).
The number of air changes per hour is a key factor in maintaining cleanroom classifications. Air changes per hour refer to the rate at which the volume of air inside the cleanroom is replaced. More air changes mean more fresh, filtered air circulated to purge contaminants. Recommended air change rates depend on the cleanroom class, but higher rates generally indicate better contamination control.
Regulatory Standards
Cleanrooms are required to meet certain regulatory standards for air quality and contamination control. The primary standards are from the International Organization for Standardization (ISO) and the European Union Good Manufacturing Practices (EU GMP).
The ISO 14644 standard classifies cleanrooms into 9 increasingly stringent ISO classes based on the concentration of airborne particles. For example, an ISO Class 5 cleanroom has less than 3,520 particles of 0.5 μm and larger per cubic meter of air (https://www.americancleanrooms.com/cleanroom-classifications/). The lower the ISO class, the cleaner the air.
EU GMP Annex 1 classifies cleanrooms based on required air changes per hour. For aseptic preparation, it requires Grade A zones to have at least 20 air changes per hour, Grade B zones at least 15 changes, and Grade C zones at least 10 changes (https://www.mecart-cleanrooms.com/learning-center/cleanroom-classifications-iso-8-iso-7-iso-6-iso-5/). The more air changes, the more effectively airborne contamination is removed.
Calculating Air Changes Per Hour
The air changes per hour (ACH) is a measure of how many times the air within a defined space is completely replaced per hour. The basic formula for calculating ACH is:
ACH = Airflow (cfm) / Room Volume (ft3)
Where cfm is cubic feet per minute of air and ft3 is the room volume in cubic feet. This gives the number of complete air changes per hour (Source: https://www.omnicalculator.com/construction/air-changes-per-hour).
The key factors involved in this calculation are:
- Airflow rate into the room in cfm
- Total volume of the room in cubic feet (length x width x height)
By dividing the airflow by the room volume, you get the number of times the complete air volume is replaced in one hour.
Recommended Rates
Typical recommended air change rates for different cleanroom classifications are:
ISO Class 1: 300-500 air changes per hour (ACH)
ISO Class 2: 150-300 ACH
ISO Class 3: 60-150 ACH
ISO Class 4: 30-60 ACH
ISO Class 5: 20-30 ACH
ISO Class 6-8: 10-20 ACH
These recommended rates are based on guidelines from organizations like the Institute of Environmental Sciences and Technology (IEST). The specific rate depends on the cleanroom class and intended use. More critical applications like pharmaceutical manufacturing may require rates at the higher end of each range.
As noted in the IEST Recommended Practice For Cleanroom Design, “The number of air changes affects the time required to purge the cleanroom or clean zone of contaminants.” Higher air change rates quickly dilute and remove contaminants.
https://eta-publications.lbl.gov/sites/default/files/lbnl-50599.pdf
Impact on Contamination Control
The number of air changes per hour directly impacts the level of contamination control in a cleanroom. Faster air velocity from a higher air change rate helps remove particles from the air more quickly. This reduces the particle concentration and lowers the risk of contamination.
According to Angstrom Technology, higher air change rates in the range of 300-500 per hour are recommended for more contamination sensitive processes like filling sterile injectables. Lower air change rates around 30 per hour may be suitable for less sensitive processes like medical device assembly.
The Cleanroom Technology article emphasizes the balance required between energy efficiency and proper contamination control. While higher air change rates remove more particles, the energy costs also increase. They recommend determining the optimal air change rate based on the cleanroom class, its activities, and contamination risks.
Overall, cleanroom designers must consider the air change rate needed to achieve the target cleanliness classifications like ISO 5 or ISO 7. The air changes per hour directly impact the particle counts and contamination levels. Facilities must find the right balance for their specific process requirements.
Measuring Air Changes
There are a few key methods and equipment used to measure cleanroom air changes per hour (AngstromTechnology). The most accurate method is to use a tracer gas test. This involves releasing a harmless gas into the cleanroom and measuring how diluted it becomes over time. The rate of dilution corresponds to the air change rate. Common tracer gases used include nitrous oxide, carbon dioxide, and helium. Tracer gas tests require specialized equipment like infrared gas analyzers or mass spectrometers to detect gas concentrations (AmericanCleanrooms).
Another common approach is to use an anemometer, a device that measures air velocity. By taking readings around the cleanroom and factoring in the room size, the cumulative air flow rate and air changes per hour can be calculated. Anemometers are relatively inexpensive but can be less precise. Thermal anemometers and rotating vane anemometers are two types used for cleanroom testing (Gotopac).
Pitot tubes can also measure cleanroom air velocity and flow rates. These tubes contain pressure sensors to determine the dynamic and static air pressure for a given point. By combining pitot tube measurements around the cleanroom, the overall air change rate can be determined (Gotopac).
Smoke visualizations are another simple technique. By releasing smoke into the cleanroom and observing its movement and dilution, technicians can gain a qualitative sense of the airflow patterns and air change effectiveness. However, this method does not provide quantitative measurements of the exact air change rate.
Airflow Design
Cleanroom airflow design is critical for controlling contamination. There are two main types of airflow patterns: unidirectional and non-unidirectional. Unidirectional airflow provides a consistent, one-way airflow in a single pass across the entire cleanroom. This is considered the optimal design for the highest cleanliness levels like ISO Class 5 (class 100) cleanrooms. Non-unidirectional designs recirculate air within the cleanroom and can be adequate for cleanliness levels above ISO Class 7 (class 10,000).
There are several factors to consider in cleanroom airflow design:
- Room dimensions – The size and shape of the cleanroom impacts patterns for effective airflow.
- Internal obstructions – Workstations, equipment, and other obstacles need to be designed around to minimize turbulence.
- Airflow velocity – Faster airflow improves contamination control but increases energy usage.
- Uniformity – Consistent airflow from inlet to outlet ensures proper particulate removal.
- Filtration efficiency – Higher MERV rated filters capture smaller particles.
Expert cleanroom design optimizes these parameters for the required cleanliness level. CFD analysis and modeling ensures laminar, uniform airflow across the entire cleanspace. Proper airflow design is critical for operational efficiency, contamination control, and safety.
Sources:
How Cleanroom Airflow Patterns Are Designed
Importance of Cleanroom Airflow Uniformity
Heating, Ventilation and Air Conditioning
Cleanrooms require specialized HVAC systems to maintain the strict environmental controls necessary. The HVAC system must provide heating, cooling, humidity control, and sufficient airflow to achieve the target air changes per hour (ACH). High-efficiency particulate air (HEPA) filters are typically used to remove contaminants and particles from the supply air (Cleanroom HVAC – Heating, Ventilation, Air Conditioning).
Cleanroom HVAC systems should be designed to provide a uniform airflow and positive pressure differential relative to surrounding areas. This helps prevent contaminants from entering the cleanroom. Fully ducted returns are recommended to control airflow paths (Essential Cleanroom HVAC Design Principles – Airlogix).
Separate air handling units are often used for the cleanroom and surrounding support areas. The cleanroom AHU should provide the target air changes, while the support area AHU can operate at a lower rate. Redundant capacity should be considered in critical cleanroom applications (Air Handling Concepts for controlled environments). Careful placement of diffusers is important for achieving uniform, non-turbulent airflow.
Energy Efficiency
Cleanrooms require large amounts of energy to maintain the high air change rates needed for contamination control. However, optimizing air change rates can lead to significant energy savings without compromising cleanliness.
According to research, the number of air changes per hour is not actually a reliable indicator of cleanroom performance. Focusing solely on air change rates can lead to over-ventilation and energy waste. The key is ensuring proper airflow patterns through intelligent HVAC design.
Strategies like ventilated ceilings, laminar airflow, computational fluid dynamics modeling, and point-of-use filters can maintain cleanliness while allowing lower air change rates. Variable speed drives and occupancy sensors can also optimize ventilation on demand. Implementing these best practices can reduce cleanroom energy usage by 30-50% without any increased risk of contamination.
In summary, by moving beyond arbitrary air change rate targets and taking a scientific approach to cleanroom airflow design, substantial energy savings can be achieved while maintaining the highest standards of contamination control.
Conclusion
In summary, air changes per hour is a critical factor in maintaining proper cleanroom conditions. The number of recommended air changes per hour depends on the cleanroom class, but higher air change rates are generally better for reducing contamination. Proper HVAC design, air balancing, and routine testing is necessary to achieve the desired air changes. While more air changes improve cleanliness, it also increases energy usage – so striking the right balance is important.
Maintaining the recommended air changes per hour is vital for contamination control. Insufficient air changes allow particle buildup and increase the risk of product defects or failures. Frequent air exchanges purge contaminants and preserve the cleanroom environment. Proper air changes also help moderate temperature, humidity, and pressure differentials. By understanding the principles behind air changes per hour, cleanroom operators can optimize their facilities for peak performance.
In conclusion, air changes per hour directly impacts the cleanliness and control of critical environments. Careful monitoring and maintenance is essential, along with designing HVAC systems to provide sufficient fresh airflow. With diligent air change control, cleanrooms can maintain the ultra-clean conditions necessary for manufacturing sensitive products.