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How do you calculate air changes per hour?
We get a lot of questions about HVAC calculations and the airflow requirements of a cleanroom. Cleanroom HVAC engineering is not an easy thing. It takes a mix of engineering skills, understanding the particle-generating potential of the process, and experience.
For our cleanroom airflow calculator*, we assumed 10 to 30 air changes per hour (ACH) for an ISO 8; 30 to 65 ACH for an ISO 7; 80 to 150 ACH for an ISO 6; 200 to 450 ACH for an ISO 5. If there is a significant generation of particles in the process, the higher number in the range is selected. This is a rule of thumb only. The air changes per hour and CFM must be calculated by an HVAC engineer based on experience and understanding of the particle-generating potential of the process.
*Various recommendations for air change ranges can be found on the Internet.
The amount of air is different in an ISO 6 and ISO 8 cleanroom. This means that the HVAC system must be capable of conditioning more than double the air. However, classification alone isn’t sufficient for calculating the airflow.
ISO 14644-1:2015 does not specify the air changes per hour (ACH) for each cleanroom class because it depends on many factors. Air changes per hour is the number of total replacements of a room’s air in one hour. ISO 14644-1:2015 can only tell you the result that you must aim for: the maximum concentration limits for particles. For example, for ISO 7, particles smaller than 0.5 microns (≥0.1 µm, ≥0.2 µm, ≥0.3 µm) are not taken into consideration. The concentration of particles of ≥0.5 µm should be below 352,000; particles of ≥1 micron should be below 83,200; and particles of ≥5 microns should be below 2,930.
That being said, the ISO cleanliness level (ISO 8, ISO 7, ISO 6 and ISO 5) gives a hint on the ACH range required. Notice that the term “range” is used—not “value”. Even if both ISO 7, two cleanrooms with activities that generate few particles versus one that generates a lot of airborne particles, will not require the same air changes per hour.
It is easy to make a room clean if no one is inside, with no equipment, and no material movement. But operations occur in cleanrooms and must be accounted for in the HVAC calculation. Below are some other elements that influence the required airflow.
The volume of air in the cleanroom will influence the amount of airflow needed. The bigger the room, the more air you need. The width, length, and height of the classified rooms and their layout must be used for HVAC calculations. People often forget that the height of the room directly impacts the CFM (airflow). One way to save on costs is to lower the ceiling. You can see the difference by comparing the airflow in the calculator using the same inputs but varying the size of the room or the height of the ceiling.
The airborne contamination level of a cleanroom depends largely upon the activities in the room and the personnel. People are responsible for most of the particles generated in a cleanroom. Airborne particles, such as skin flakes, cosmetics, perfume, spittle, clothing debris (lint, fibers) and hair, are the usual suspects. When designing the HVAC system, the number of people working in the room at the same time must be taken into account. The more people working in the cleanroom, the more airflow is needed to get rid of the contaminants. People generate airborne contaminants, but also heat. The number of operators is also used to calculate the level of conditioning to compensate for the heat that they produce. People in the cleanroom usually wear coveralls to limit contamination. Therefore, it is important to maintain a comfortable environment, usually between 66.5°F and 70°F (19°C and 21°C).
Similar to people, equipment generates heat and dust. The heat gain produced by the equipment inside the cleanroom is used to determine the cooling required. The equipment in the room, along with the product manufacturing, generate dust that must be removed with the correct amount of air.
A fume hood or laminar flow cabinet needs constant air supply—just like the cleanroom. This air supply must be accounted for in the cleanroom HVAC calculations. Moreover, if the fume hood exhausts air outside the building, like with a BSC, the exhausted air must be replaced with fresh air. This fresh air will need to be conditioned (temperature and relative humidity). This requires a larger air make-up or air handling unit. If the air extracted from the hood is not accounted for in the HVAC calculations, there might not be enough air pushed into the room to maintain positive pressurization. On the flip side, the pressurization may become too negative and suck in some dirty air from the exterior of the room.
The required level of lighting will also affect the heat generated inside the cleanroom and therefore the cooling required. Regular office lighting of 300 lux versus high-precision lighting of 1200 lux will not generate the same quantity of heat.
The pressure must be greater in more stringent classified rooms, so the air leaks towards the lesser clean rooms. Positive pressure prevents dirty air from entering into the cleanroom. In a negative pressurized cleanroom, the opposite occurs; the airflow must be greater in the adjacent room.
If air can be re-circulated in the cleanroom, the outside weather only slightly impacts the HVAC system. However, for cleanrooms working with hazardous products, the air make-up can go as high as 100% of fresh air. In these types of cleanrooms, the HVAC systems are more complex. For example, in winter in some regions, the HVAC systems need to take the outside air at -22°F (-30°C) in winter, warm it up to 68°F (20 °C), remove the humidity, and bring it into the room, over and over.
Last but certainly not least, the degree of precision you need will also influence the design of the HVAC system. High-precision temperature control systems can control to ± 0.25°F (± 0.15°C) and to ± 2% for humidity. Cleanrooms rarely need that high degree of precision. In most cases, ± 2°F (± 1°C) precision for temperature and ± 10% for humidity is sufficient. The level of precision depends on the operations in the cleanroom.
You can read this article for a simplified CFM calculation example or try this cleanroom design calculator to get an approximation of the CFM, ACH per classified room, and quantity of lighting fixtures. It also gives you an estimate on the number of HEPA filters that are needed and the number of low air return you need for your cleanroom. Remember though these are only estimations. Airflow must be calculated by an HVAC engineer. Many of the elements listed above are not taken into account in the calculator.
This article was previously published in Cleanroom Technology.