How many FFUs should be used in a cleanroom?

  • 05, 03, 2021

Maintaining ventilation efficiency is one of the most important things in the cleanroom. Particularly, the air velocity and the air change rates must be controlled in order to meet the cleanliness requirements. Using FFUs can ensure the air velocity and the air change rates in the cleanroom. The question is "How many FFUs should be used in a cleanroom?" to meet the optimal air change rate and air velocity.

1. Air change rate

A critical factor in cleanroom design is controlling air-change per hour (ACH), also known as the air change rate, or ACR. This refers to the number of times each hour that filtered outside air replaces the existing volume in a building or chamber.

In a normal home, an air-conditioner changes room air 0.5 to 2 times per hour. In a cleanroom, depending on classification and usage, air change occurs anywhere from 10 to more than 600 times an hour.

Air change rate is a prime variable in determining ISO and Federal cleanliness standards. To meet optimal standards, air change rate must be painstakingly measured and controlled.

The air change rate is based on the operation of fan filter units and other HVAC systems. 

2. FFUs calculation

To calculate the number of FFUs required for a cleanroom, we need to know (1) the size of each room (L x W x H) and (2) the cleanliness grade for cleanroom.

Each grade has an air change range per hour. Once the required air change figure is established, the number of required FFUs can be calculated by using this formula:

No. of FFUs = Air Changes/Hour * Cleanroom Volume  / FFU exhaust air volume

with Cleanroom Volume (), FFU exhaust air volume (/h)

For example, suppose that to meet ISO 7 standard using the low-end air change recommendation (60/hour) inside (3 x 12 x 10 m) cleanroom, with 360 m³ of volume and 1080 m³/h of the FFU air volume, requires 20 FFUs

No. of FFUs = 60 * (3*12*10) / 1080 = 20

To meet the same standard using the high­end air change recommendation (480/hour) requires 30 FFUs. For your calculations, round up fractions to avoid under­performance.

Calculating the correct number of FFUs is not only ensuring the cleanroom cleanliness standards such as air change rate, air velocity but also reducing the FFU installation and maintenance cost. Such a big saving for your company!


Vietnam Cleanroom Equipment (VCR) specializes in providing cleanroom equipment for construction contractors. We provide high-quality products with competitive prices and large quantities nationwide. The equipment includes:

Differential pressure gauge, FFU Fan Filter Unit, Pass box, Clean room air filter, HEPA box, Clean booth, cleanroom steel door, Isolator cabinet, and other equipment

For details, please refer to Vietnam cleanroom equipment official website

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