Cleanroom Air Change Rates Explained
Cleanroom air change rates are crucial for maintaining the cleanliness and compliance of a controlled environment. Cleanrooms are specialised environments where the concentration of airborne particles is continuously and meticulously monitored, with air change rates being a key part of cleanroom compliance with the relevant classification.
What are Air Changes per Hour?
Air changes per hour, or the air change rate, is the number of times a cleanroom can completely replace its air volume within an hour.
The higher the air changes per hour, the more effective it will be at removing contamination, therefore achieving a higher class. Air changes per hour are often abbreviated to ACPH or ACR and is calculated by dividing the volume of air supplied into the cleanroom in a unit of time (usually an hour) by the total volume of the cleanroom.
It is crucial to note that the quality of the air being supplied at the correct velocity, is key to the cleanliness of the environment. For example, if an end user was to simply turn up the supply volumes and take the HEPA/ULPA outside its rated efficiency, air quality will drop.
Factors Affecting Air Change Rates in Cleanrooms
There are many factors that can affect the required air change rate within a cleanroom:
Cleanroom Classifications
Room Size and Layout
Type of Work Conducted
Occupancy Levels
Equipment and Processes
Air Changes per Hour or Air Velocity?
As explained above, cleanrooms require regular air changes to ensure the environment is compliant with its relevant classification. As the environment becomes cleaner with lower ISO Classifications, more air changes are needed to ensure that compliance.
ISO standards express air changes per hour differently for cleanrooms with unidirectional and non-unidirectional airflow.
Air Changes per Hour: Non-Unidirectional Airflow
Non-unidirectional flow refers to the airflow pattern in a cleanroom. Typically, cleanrooms that fall within ISO Classes 5 and above use this type of airflow as it is sufficient for removing the designated number and size of particles at a regular rate, as per the classification standard.
Airflow Velocity: Unidirectional Airflow
As cleanroom classifications become more stringent, the air changes per hour must increase to remove particles and keep the air cleaner. Cleanrooms with unidirectional airflow, which are typically ISO Class 5 and below, have the strictest cleanroom classifications. To maintain this air quality, the air changes per hour is so rapid that it is expressed as airflow velocity instead, measured either in metres per second or feet per minute.
The constant flow of contaminant-free air, either vertically or horizontally, is vital within a cleanroom environment to keep particles from settling on surfaces and maintaining classification standards.
Cleanroom Classifications
In the UK, cleanroom air change rates are guided by several standards and guidelines:
- ISO 14644-1: Provides the standard for air cleanliness classification based on particle concentration.
- British Standards (BS EN 1822): Specifies the efficiency requirements for HEPA (High-Efficiency Particulate Air) and ULPA (Ultra-Low Penetration Air) filters, which are crucial for achieving the desired air change rates and maintaining cleanroom standards.
The below table explains the ISO Cleanroom Classifications:
| ISO Classification Number | Maximum allowable concentrations (particles/m3) for particles equal to and greater than the considered sizes, shown below | |||||
|---|---|---|---|---|---|---|
| ≥0.1µm | ;≥0.2µm | ≥0.3µm | ≥0.5µm | ≥1µm | ≥5.0µm | |
| ISO Class 1 | 10b | d | d | d | d | e |
| ISO Class 2 | 100 | 24b | 10b | d | d | e |
| ISO Class 3 | 1,000 | 237 | 102 | 35b | d | e |
| ISO Class 4 | 10,000 | 2,370 | 1,020 | 352 | 83b | e |
| ISO Class 5 | 100,000 | 23,700 | 10,200 | 3,520 | 832 | d,e,f |
| ISO Class 6 | 1,000,000 | 237,000 | 102,000 | 35,200 | 8,320 | 293 |
| ISO Class 7 | c | c | c | 352,000 | 83,200 | 2,930 |
| ISO Class 8 | c | c | c | 3,520,000 | 832,000 | 29,300 |
| ISO Class 9g | c | c | c | 35,200,000 | 8,320,000 | 293,000 |
The achieved ISO class can be defined by removing the number of particles per m3 to acceptable limits for the classification. This can be achieved by increasing or decreasing the air changes per hour and increasing or decreasing the number of fan filter units or terminal filters, if an air handling unit is utilised, allowing you to achieve the desired particle count within the room. These parameters will need to be decided upon after assessing the above factors affecting air change rates and air changes per hour.
Designing Cleanrooms to Meet Air Changes per Hour
Your cleanroom is always designed to meet the relevant classification, with the systems that support air quality and cleanliness working together to achieve continual compliance.
To ensure a cleanroom that meets your classification with adequate air changes per hour, it must contain a uniform airflow pattern and enough fan filter units, with a greater percentage ceiling coverage of fan filter units required to reach higher air changes.
Summary
Cleanroom air change rates are designed to maintain specific cleanliness levels depending on the cleanroom class, the nature of the work, and the equipment used. Adhering to proper air change rates is essential for minimising contamination risks and ensuring the safety and quality of processes and products produced in cleanrooms.
The Angstrom Technology team understand cleanroom classifications implicitly and will work with you and your objectives to ensure best practices and a compliant cleanroom, achieving the relevant air changes per hour. We can also evaluate your existing space and systems to ensure they’re working effectively and support you in upgrading them where required.
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From running particle counts and cleanroom air change rates testing on a small cleanroom to coordinating a full scope of testing on suites of cleanrooms within a planned maintenance period — we have the experience and expertise to manage your cleanroom validation effectively.
You can choose from one of our testing packages, or we can build a custom schedule based on your requirements.
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