How Many FFU Needed for a Cleanroom?

FFU stands for fan filter unit, which is a device that consists of a fan and a filter, usually HEPA or ULPA, that provides filtered air to a cleanroom. The number of FFU needed for a cleanroom depends on several factors, such as:
• The size and layout of the cleanroom
• The cleanliness level and air changes per hour (ACH) required for the cleanroom
• The type and efficiency of the air filtration system
• The type and direction of the airflow pattern

There is no universal formula for calculating the number of FFU needed for a cleanroom, as different standards and guidelines may have different recommendations based on the application, process, and product requirements. However, a general formula that can be used as a reference is:
No. of FFUs = (ACH / 60) x (Cleanroom Volume / FFU Air Volume)
• ACH is the air changes per hour, which is a measure of how many times the air in a cleanroom is replaced by filtered air in one hour. ACH is determined by the cleanliness level of the cleanroom, according to the ISO 14644-1 or FS 209E standards. For example, an ISO 5 cleanroom requires roughly 240-480 ACH, while an ISO 7 cleanroom requires roughly 60-90 ACH1
• Cleanroom Volume is the volume of the cleanroom in cubic meters (m3) or cubic feet (ft3). It is calculated by multiplying the length, width, and height of the cleanroom. For example, a cleanroom measuring 12’ x 12’ x 7’ has a volume of 1008 ft3 or 28.5 m3.
• FFU Air Volume is the volume of air that a single FFU can deliver in cubic meters per hour (m3/h) or cubic feet per minute (CFM). It depends on the size, speed, and performance of the FFU. A typical 2’ x 4’ FFU can deliver about 650 CFM or 1105 m3/h at medium speed 2.

Using this formula, we can estimate the number of FFU needed for a cleanroom with a given size, cleanliness level, and FFU type. For example, suppose that we want to build an ISO 5 cleanroom measuring 12’ x 12’ x 7’ with 2’ x 4’ FFUs. Using the low-end ACH recommendation (240/hour) and the typical FFU air volume (650 CFM), we can calculate the minimum number of FFUs required as follows:
Minimum No. of FFUs = (240 / 60) x (1008 / 650) = 6.20
Rounding up the fraction, we get 7 FFUs as the minimum number required. Similarly, using the high-end ACH recommendation (480/hour), we can calculate the maximum number of FFUs required as follows:
Maximum No. of FFUs = (480 / 60) x (1008 / 650) = 12.41
Rounding up the fraction, we get 13 FFUs as the maximum number required. Therefore, the range of FFUs needed for this cleanroom is between 7 and 13.
It is important to note that this formula is only an approximation and does not account for other factors that may affect the number of FFU needed, such as the design and layout of the cleanroom, the type and direction of the airflow, the temperature and humidity conditions, the product and process specifications, etc. Therefore, it is recommended to consult with a professional cleanroom designer or engineer who can perform a detailed analysis and simulation of the cleanroom system and provide a customized solution that meets the specific needs and goals of the cleanroom application.

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