Why is FFU air volume calculation the core of clean room design?
The air volume of FFU (Fan Filter Unit) is one of the most basic parameters in clean room design. The size of the air volume directly determines three key results:
Whether the cleanliness can meet the standard : insufficient air volume, insufficient number of air changes, and cleanliness cannot be guaranteed
How to determine the number of FFU : divide the total air volume demand by the air volume of a single unit to get the required number of FFU units
Long-term operating costs : The air volume design is too large, the number of equipment is large, and the energy consumption is high; the design is too small, and the cleanliness is not up to standard.
Therefore, mastering the calculation method of FFU air volume is the first step in clean room design and FFU selection.
1. How to calculate the air volume of a single FFU?
The air volume of a single FFU is determined by the effective area of the filter and the design surface wind speed .
Calculation formula:
Air volume (m³/h) = filter width (m) × filter length (m) × surface wind speed (m/s) × 3600 (seconds)
Among them, the face velocity is the speed of air passing through the filter section. The industry standard face wind speed is 0.45m/s。
Take the 1175×575mm specification FFU as an example:
The actual size of the high-efficiency filter corresponding to this specification FFU is approximately 1170×570mm.
Air volume = 1.17 × 0.57 × 0.45 × 3600 ≈ 1080 m³/h
Common specification FFU air volume reference value:
FFU specifications |
Corresponding air volume (approx.) |
575×575mm |
520m³/h |
1175×575mm |
1080m³/h |
1175×875mm |
1650m³/h |
1175×1175mm |
2210m³/h |
NOTE: of the filter The actual size (nominal size minus frame size) should be used when calculating air volume, not the nominal size.
2. What is the face wind speed?
The choice of face velocity depends on the type and cleanliness level of the clean room.
Clean room type |
Face wind speed range |
Typical values |
One-way flow clean room (ISO Class 5/Class 100 and above) |
0.35-0.55m/s |
0.45 m/s |
Non-unidirectional flow clean room (ISO Class 6-8/Class 1,000 and below) |
0.30-0.45 m/s |
Determine based on the number of air changes |
Selection tip : The face wind speed is not fixed. The higher the cleanliness level, the greater the dependence on unidirectional flow, and the face wind speed should take a higher value. At the same time, it is necessary to consider the attenuation of air volume caused by the increase in resistance after the filter is used.
3. How to calculate the total air volume requirement of a clean room?
The total air supply volume required for a clean room is determined by the volume of the clean room and the number of air changes .
Calculation formula:
Total air volume (m³/h) = clean room area (m²) × floor height (m) × number of air changes (times/h)
Reference for the number of air changes at different cleanliness levels:
Cleanliness level |
Air exchange rate reference |
ISO level 5 (level 100) |
≥300 times/h |
ISO level 6 (level 1,000) |
≥120 times/h |
ISO level 7 (level 10,000) |
≥60 times/h |
ISO level 8 (level 100,000) |
≥30 times/h |
Calculation example :
For a class 10,000 clean room (ISO class 7) with an area of 100m² and a floor height of 2.8m, the air change rate is 60 times/h:
Total air volume = 100 × 2.8 × 60 = 16,800 m³/h
4. How to determine the number of FFU?
After obtaining the total air volume demand of the clean room and the air volume of a single FFU, the required number of FFU can be calculated.
Calculation formula:
Number of FFU = Total air volume demand of clean room ÷ Air volume of single FFU
Continuing from the previous example :
The total air volume is 16,800 m³/h, and 1175×575mm FFU is selected (the air volume of a single unit is about 1080 m³/h):
Number of FFU = 16,800 ÷ 1080 ≈ 15.6 units
The actual deployment needs to be rounded up, and equipment occlusion and redundancy must be taken into consideration. It is recommended to deploy 17-18 units.。
Another calculation method : For one-way flow clean rooms (class 100), it can also be calculated through the area coverage rate and the coverage area of a single FFU. Number of FFU = clean room ceiling area × coverage rate ÷ effective coverage area of a single FFU.
5. Three key considerations for air volume calculations
1. Use actual dimensions, not nominal dimensions
When calculating FFU air volume, the actual size of the filter (the net size of the filter element after deducting the outer frame) must be used instead of the nominal size.
2. Consider the increase in filter resistance
The high-efficiency filter gradually accumulates dust during use, the resistance increases, and the air volume will decrease accordingly. It is recommended to reserve when selecting a filter more than 30% static pressure margin to ensure that the designed air volume can still be maintained after the filter is used.
3. Distinguish between unidirectional flow and non-unidirectional flow
Unidirectional flow clean room (ISO level 5 and above) : rely on face wind speed to ensure cleanliness, and calculate air volume based on face wind speed
Non-unidirectional flow clean room (ISO level 6 and below) : rely on the number of air changes to ensure cleanliness, and calculate the total air volume based on the number of air changes
6. FFU air volume test method
The air volume test needs to be performed before the FFU leaves the factory or after installation to verify whether it reaches the design value. There are three main detection methods commonly used in the industry:
1. Wind chamber method
Installing the FFU in a dedicated detection air chamber or air volume test bench for measurement is a more accurate and widely recognized volumetric air volume detection method.
2. Wind mask method
Use a special air hood to completely cover the entire air outlet surface of the FFU and measure the air volume directly. This is the most commonly used and direct method of air volume measurement in on-site testing.
3. Anemometer grid method
Divide grid points at the FFU air outlet, measure the wind speed point by point and calculate the average value, then multiply it by the air outlet area to get the air volume.
Quick review of the whole process of FFU air volume calculation
step |
operate |
formula/method |
first step |
Determine clean room cleanliness level |
Clarify whether it is ISO level 5 (level 100) or ISO level 6-8 (level 1,000/100,000) |
Step 2 |
Determine face velocity or air exchange rate |
For class 100, take 0.45m/s, for class 1,000, take ≥120 times/h, and for class 10,000, take ≥60 times/h. |
Step 3 |
Calculate the air volume of a single FFU |
Filter area × face velocity × 3600 |
Step 4 |
Calculate total cleanroom air volume requirements |
Area×floor height×number of air changes |
Step 5 |
Calculate the number of FFU required |
Total air volume ÷ single air volume |
Step 6 |
Consider redundancy and layout |
Round up and add 10%-15% redundancy |
Aili Industry—Full industry chain FFU manufacturer
Aili Industrial has been deeply involved in the field of clean room equipment for nearly two decades. From fan winding, box processing to control system development, the entire industry chain independently manufactures FFU fan filter units. The products cover standard specifications such as 575×575mm, 1175×575mm, 1175×875mm, 1175×1175mm and non-standard customized sizes. The air volume of a single unit covers 520-2210m³/h and above. It supports EC/AC motors, multiple control methods and filter level selections. Aili Industrial can provide professional FFU selection and layout solutions based on your cleanliness level, workshop area and air volume requirements, adapting to the different cleaning needs of 13 major industries including semiconductors, panel displays, pharmaceuticals, food, and data centers.
If you need FFU air volume calculation or selection plan, please contact the professional technical team of Aili Industry.
