The number of FFU is calculated based on the number of air changes or the air supply area? The difference between different airflow forms
Why are there two methods for calculating the number of FFU?
In clean room design, there are usually two common methods to determine the number of FFU: calculation based on the number of air changes and calculation based on the air supply area. The results obtained by the two methods may be different, and there are often questions on projects: Which one should be used?
Both methods have their own applicable scenarios, depending on the air flow pattern and process control objectives of the clean room. The air exchange rate method starts from the room volume and air exchange requirements, and is suitable for describing the air supply needs of the entire room. The air supply area method starts from the protection area and target wind speed, and is suitable for one-way flow areas or local protection areas.
Which method to choose, you must first clarify the air flow pattern of the clean room and the objects that need to be protected. Aili Industrial has complete production capabilities in FFU complete machine and airflow design, and can provide quantity accounting support based on the project's airflow form. For the basic structure of the FFU machine, you can refer toAili Industrial FFU fan filter unit。
What scenarios is the ventilation rate method suitable for?
The number of air changes method multiplies the room volume by the number of air changes to obtain the required air supply volume, and then divides it by the air volume of a single FFU to obtain the number of FFU. This method is suitable for non-unidirectional flow clean rooms or mixed flow clean rooms. The air flow in the room is not completely unidirectional and particle control mainly relies on dilution.
The key to the ventilation rate method is to determine the appropriate number of ventilations. The number of air changes is related to the cleanliness level, dust volume, number of personnel and process requirements. The higher the cleanliness level and the greater the amount of dust generated, the higher the frequency of air changes is usually required.
The air change rate method is suitable for overall room cleanliness control, but it is not necessarily suitable for local protection of key workstations. If the process requires a higher degree of cleanliness in critical areas, calculating room air changes alone may not be sufficient.
What scenarios is the air supply area method suitable for?
The air supply area method multiplies the protection area by the target wind speed to obtain the required air supply volume, and then divides it by the air volume of a single FFU to obtain the number of FFU. This method is suitable for one-way flow clean rooms or local protected areas. The clean airflow covers the protected surface at a certain wind speed and brings the particles to the return air.
The key to the air supply area method is to determine the protection area and target wind speed. The protected area may be the entire suspended ceiling area, or it may be a local area above key workstations or equipment. The target wind speed depends on the cleanliness level and process requirements. Unidirectional flow areas usually have a clear wind speed range.
The air supply area method is suitable for areas that require continuous air flow coverage, but it needs to be confirmed whether the FFU arrangement can form a uniform unidirectional flow. If there are gaps between FFUs or airflow is blocked, actual coverage may be insufficient.
Why are the results of the two methods different?
The two methods calculate air supply demand from different perspectives, and the results may be different. The air change method focuses on the overall volumetric air change in the room and is suitable for diluting pollution. The air supply area method focuses on the wind speed coverage of the protective surface and is suitable for controlling the air flow direction.
If the room is large and the floor height is high, the air exchange rate method may get more FFU. If the protection area is concentrated and the target wind speed is high, the air supply area method may obtain more FFU. In actual projects, it may be necessary to calculate using two methods at the same time, take a reasonable value, and then combine it with the layout correction.
For clean rooms with different cleanliness level partitions, partition calculations can be performed. High cleanliness level areas may be based on the air supply area method, and auxiliary areas may be based on the air exchange rate method.
Why layout fixes are important
No matter which method is used to calculate, the final number of FFU still needs to be combined with layout correction. The ceiling module, lamp position, fire protection facilities, return air vents and maintenance channels will all affect the actual layout of the FFU. The calculated quantity is a theoretical requirement, and the arrangement quantity must take into account the installation conditions.
If the ceiling module does not exactly match the FFU size, you may need to adjust the number of FFUs or choose a different size FFU. If lighting fixtures and fire protection equipment occupy ceiling space, the FFU location may need to be adjusted. If the position of the return air outlet affects the airflow organization, the FFU may need to be increased or adjusted.
After the layout is revised, the air supply volume and airflow coverage should be rechecked to see if they meet the targets. For projects with limited space, you can refer toUltra-thin FFU and70mm ultra-thin EFU understands compact layout solutions.
How airflow patterns affect selection
The air flow patterns in clean rooms are usually divided into unidirectional flow, non-unidirectional flow and mixed flow. The airflow in a unidirectional flow clean room flows in parallel directions, which is suitable for areas with high cleanliness levels. It is usually calculated according to the air supply area method. The airflow mixing in non-unidirectional flow clean rooms is suitable for general clean areas and is usually calculated based on the number of air changes. The mixed flow clean room combines two forms, with the key area designed for unidirectional flow and the auxiliary area designed for non-unidirectional flow.
After clarifying the air flow pattern, select the appropriate method and then modify it based on the process requirements and layout conditions.
Issues that should be confirmed in advance during project communication
At the FFU quantity calculation stage, it is recommended to confirm the following with the designer and FFU manufacturer: clean room airflow form; target cleanliness level and detection status; room volume and floor height; protection area and key work station locations; target wind speed or number of air changes; ceiling modulus and layout conditions; locations of lamps, fire protection and return air outlets; whether zoning calculations are required; calculation methods and result verification.
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in conclusion
The FFU number calculation method should be selected based on the air flow pattern and process goals. The number of air changes method is suitable for the volumetric ventilation needs of rooms with non-unidirectional flow, and the air supply area method is suitable for unidirectional flow areas and local protection. Both methods can be used in combination and the final quantity determined after layout correction.
Aili Industrial has complete production capabilities in FFU complete machine and airflow design, and can provide quantity accounting support based on the project's airflow form. If you need to know more about FFU quantity design, you can visitAili Industrial FFU fan filter unit , you are also welcome to contact Aili Industrial for project support.
