Cleanroom Air Change Rates

Often when we discuss cleanroom design, we talk about the physical layout of the room, including walls and ceilings, but a cleanroom isn’t a cleanroom if the air isn’t changed out. The room’s air change rate affects its performance, its classification, and its cost to run. 

What Is An Air Change Rate?

An air change rate, or ACR, is the number of times that the air within a cleanroom is replaced with clean, filtered air per hour. For comparison, when the air conditioning in your home runs, it typically changes out the air at a rate of 2 times or less per hour. The air in a cleanroom is changed out many more times per hour, up to 600 times per hour depending on its cleanroom class. 

How do you calculate the air change rate of a cleanroom? You must divide the volume of air sent into the room as a unit of time by the volume of the cleanroom as a whole. ACR is a key measurement in determining ISO and federal cleanliness standards. It can vary between cleanroom to cleanroom because of other factors such as equipment, the number of employees within the cleanroom space, and the activity happening within the room. This is because the room’s filtering system needs to remove contamination generated inside the room as well as contamination entering the room. 

The cleanroom class will determine the air change rate. The lower the ISO class, the more air changes per hour have to occur in order to keep the particulate count low enough and reach cleanliness standards. Determining the correct number of air changes for a cleanroom application is an involved process and must take into account all potential contamination sources. This is why there are ranges for air change rates for each ISO class instead of one set number. 

Cleanroom Airflow 

Cleanrooms that are classified as ISO Class 5 or above use non-unidirectional airflow – or mixed flow – to remove particulate matter. Because their cleanliness standards are less stringent, they rely solely on air changes per hour to maintain cleanliness within the room. Cleanrooms used within the pharmaceutical industry, manufacturing, and the healthcare industry often use mixed flow airflow. 

Cleanrooms within the strictest cleanroom classifications use unidirectional airflow, and often the air is changed so frequently that we do not count changes but instead calculate airflow velocity. This can be measured in either meters per second or feet per minute. This constant flow of air keeps any particles from settling on any surface. Aerospace engineering and microelectronic manufacturing both require extremely stringent cleanroom environments with very high air change rates. 

In the below chart we list the ISO classes, along with air change rate ranges, airflow velocity, and ceiling coverage for each class. 

Class	Air Changes per Hour	Airflow Velocity (ft/min)	Ceiling Coverage
ISO 1	360-600	60-100	90-100%
ISO 2	360-600	60-100	80-100%
ISO 3	360-540	60-90	60-100%
ISO 4	360-540	50-90	50-90%
ISO 5	240-480	40-80	35-70%
ISO 6	150-240	25-40	25-40%
ISO 7	60-90	10-15	15-20%
ISO 8	5-48	1-8	5-15%
ISO 9	0-5	0-5	5-10%

Ceiling coverage refers to how much of the cleanroom ceiling is composed of fan filter units. As you can see from the chart, in the lower classes nearly the entire ceiling of the cleanroom will be FFUs in order to generate the required airflow. If you need to upgrade your cleanroom in order to meet the standards of a lower ISO class, that will require adding fan filter units. 

In addition to the air change rate, airflow patterns and exchange efficiency also have real effects on how well the cleanroom performs, how much it costs to run, and what the return on investment is for the cleanroom. The goal for every cleanroom application is to optimize the air change rate in order to meet the necessary cleanliness requirements.