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How is the number of FFUs in a clean room calculated? Learn about the common engineering algorithm in this article!
2026/10/20
Today, I will present a set of standard calculation methods for FFUs that are universally applicable in the field and recognized by both the client and the supervisor.
There are two completely different calculation methods for different cleanliness levels of cleanrooms. For ISO 7 to ISO 9 (thousand-level, ten-thousand-level, and hundred-thousand-level), the air change rate method (commonly used for 90% of ordinary cleanrooms) is adopted; for ISO 5 and above (hundred-level and higher), the particle balance + ceiling coverage method (specific for semiconductor and precision workshops) is used.
01 General mainstream algorithm:
The air change rate method is applicable to cleanrooms of ISO 7 to ISO 9 (thousand-level, ten-thousand-level, and hundred-thousand-level), and is the preferred algorithm for project bidding and drawing design.
1. Core calculation formula
Total room air circulation volume = Room volume × Design air change rate
Theoretical number of FFUs = Total air circulation volume ÷ Air volume per unit FFU operation
2. Air change rate values for different cleanliness levels
ISO 9 / thousand-level: 15 to 25 times/hour
ISO 8 / ten-thousand-level: 25 to 45 times/hour
ISO 7 / hundred-thousand-level: 45 to 70 times/hour
To compensate for filter resistance attenuation, room leakage, and air loss, all projects are recommended to add 10% to 20% of air volume reserve. For GMP workshops in pharmaceuticals, the upper limit is recommended.
3. Complete practical case
Scenario: A ten-thousand-level (ISO 8) electronic cleanroom, with dimensions of 10m x 6m and ceiling height of 2.8m, selects 1200 x 600 FFUs (rated at 1000 m³/h), with an intermediate air change rate of 35 times/hour, and adds 15% air volume reserve.
① Calculate room volume: 10 x 6 x 2.8 = 168 m³
② Basic total air volume: 168 x 35 = 5880 m³/h
③ After adding reserve: 5880 x 1.15 = 6762 m³/h
④ Theoretical number of FFUs: 6762 ÷ 1000 = 6.76 units
⑤ Engineering value: Round up to 7 units
Key misunderstanding correction: Cleanrooms of thousand-level, ten-thousand-level, and hundred-thousand-level do not need to be fully covered with FFUs! Only the air change rate needs to be met, and the remaining ceiling is sealed with blind plates to significantly reduce costs. Only high-level workshops of hundred-level and above need high coverage layout.
02 Special algorithm for high-level cleanrooms:
Coverage + particle balance method is applicable to ISO 5 and above semiconductor, precision optical, and chip cleanrooms.
For these high-level workshops, the air change rate alone cannot precisely control the cleanliness level. The core is to look at the ceiling FFU coverage rate, combined with the principle of particle balance for design.
1. Core principle
Through the efficient air supply of FFUs, continuously dilute and neutralize the dust particles generated by personnel, equipment, and walls, keeping the indoor particle concentration stable and meeting the standards. The particle concentration of the HEPA filter air supply is approximately 0, and only a reasonable air supply coverage is required.
2. Industry standard coverage rate for general use
ISO 5 hundred-level cleanroom: Ceiling FFU coverage rate 60% to 80%
3. Coverage calculation formula
Coverage = (FFU single area x number of units) ÷ Cleanroom ceiling area. For example: Total ceiling area 100㎡, requiring 70% coverage, 1200 x 600 FFU single area 0.72㎡, the required number of FFUs = (100 x 70%) ÷ 0.72 ≈ 98 units
03 Standard process for project implementation 9 steps
Determine the cleanliness level, and determine whether to use the ventilation frequency method or the coverage method to accurately measure the length, width, and ceiling clear height of the room (excluding the mezzanine) based on the industry-selected standards. The ventilation frequency is calculated to determine the room volume, the basic total supply air volume, and add 10% to 20% of the engineering air volume margin. Select the FFU specification, confirm the actual operating air volume (critical!) Calculate the theoretical number of units and round up. Combine the ceiling bracket modulus arrangement. Verify the actual ventilation frequency in reverse, ensuring compliance.
04 Common Misunderstandings
❌ Misunderstanding 1: Using the maximum rated air volume of the FFU for calculation
In engineering, the FFU will not operate at full load for a long time. The actual operating air volume should be calculated at around 80% of the actual air volume. Otherwise, it will result in insufficient air volume and failure to pass the acceptance.
❌ Misunderstanding 2: Including the fresh air volume in the air volume of the FFU
The FFU is only responsible for indoor circulation supply air. The fresh air is calculated separately and is only used to maintain room positive pressure, compensate for exhaust air, and provide oxygen for personnel. It has nothing to do with the ventilation frequency of the FFU.
❌ Misunderstanding 3: Placing FFU in all levels
Only the hundred-level and higher-grade workshops require high coverage. Placing FFU in full coverage for the conventional thousand-, ten-thousand-, and one-hundred-thousand-level workshops is a serious waste of cost.
❌ Misunderstanding 4: Not leaving air volume margin
During the use of the filter, dust accumulation will increase the resistance, and the air volume will gradually decrease year by year. Not leaving a margin will likely result in the failure to meet the cleanliness level in the later stage.
❌ Misunderstanding 5: Calculating the volume of the room based on the total height of the room
Only the effective clear height below the ceiling is calculated. The enclosed space of the ceiling layer is not included in the volume of the cleanroom.
05 Quick Summary
Ordinary cleanroom (ISO7-9): Calculate the air volume using the ventilation frequency method → Determine the number of units → Arrange blind plates. This is the most cost-effective method.
High-grade cleanroom (ISO5): Coverage is the main factor, and particle balance verification is conducted to ensure air flow cleanliness.
Core of the project: Must add air volume margin, calculate the actual operating air volume, and distinguish between circulating air and fresh air.
