How should we look at the effective air outlet area of ​​FFU?

The effective air outlet area of ​​FFU is the area actually responsible for filtering air supply. It is not equal to the outer box area, nor is it equal to the filter frame area. If the wrong area diameter is used, the air volume, surface wind speed and coverage rate will be calculated incorrectly, which will affect the number of equipment, wind speed judgment and acceptance conclusion.

The following will start with the definition of area, measurement diameter, conversion method, and the impact of model selection.

First distinguish several different areas

FFU involves multiple area concepts, which are easy to confuse:

• Box appearance area : the length times the width of the equipment shell, used for installation and ceiling opening verification;

• Filter frame area : the peripheral size of the filter frame, used for installation interface verification;

• Filter element expansion area : the actual expanded area of ​​the filter material, which affects resistance and dust holding capacity;

• Effective air outlet area : the surface area that is actually responsible for filtered air supply and can pass airflow, used for calculation of air volume and surface wind speed.

These areas have different values ​​and different uses. When calculating face wind speed, the effective air outlet area should be used instead of the box external area. If you are still unclear about the relationship between installation dimensions and ceiling modulus, you can see How does the FFU appearance size match the ceiling module? , first distinguish the installation size and air supply size.

The sample area needs to be clearly defined

The definition of area may be different in samples from different manufacturers. Some mark the outer frame area of ​​the filter, some mark the front area of ​​the filter element, and some mark the actual ventilation area. When comparing equipment, the calibers used should be unified, otherwise different average wind speeds will be calculated for the same air volume.

For example, two FFUs have the same appearance but different filter frames and internal structures, so the actual air supply areas may be different. Station coverage capabilities cannot be compared solely by box area. If the sample only gives the outer dimensions, the definition of the effective air outlet area or measurement surface should be further confirmed.

Conversion relationship between face wind speed and air volume

On the corresponding measurement surface, the air volume can be calculated by multiplying the average wind speed by the effective area. If the wind speed unit is m/s and the area is m², the result is m³/s; convert to m³/h and multiply by 3600. The premise is that the wind speed and area come from the same ventilation section.

For specific conversion methods, please refer to How to convert face wind speed and air volume? . If the area diameter is used incorrectly, the converted air volume will deviate greatly, which will affect the judgment of the FFU quantity.

Pay attention to the effects of edges and stencils when measuring

Protective nets, frames and mounting seams can affect local measurements. After estimating the average wind speed, you still need to check the wind distribution, and the average value cannot be used to cover up the low-speed areas near key workstations.

When measuring, please note:

• The measuring points should cover different positions on the air outlet surface to avoid a single point representing the whole;

• Record the distance between the measurement surface and the FFU air outlet surface;

• Confirm whether the protective net, diffuser plate and frame affect the local wind speed;

• Focus on the areas corresponding to key workstations rather than just looking at the central average.

If the edge wind speed is found to be significantly lower, the frame shielding and sealing should be checked. For detailed investigation, please refer to Why does the measured average wind speed not mean that the air flow is uniform?。

The impact of effective air outlet area on model selection

The effective air outlet area affects air volume calculation, face wind speed judgment and coverage estimation. The area is not clearly defined, which may lead to:

• The number of units is estimated based on the overall area, but the actual effective air supply is insufficient;

• The surface wind speed is calculated based on the outer frame area and the filter working point is misjudged;

• Select the model based on the nominal air volume, ignoring the actual working point with the filter;

• Acceptance is based on average wind speed, leaving out local low-speed areas.

If you have any questions about judging the number of FFU, please refer to What are the on-site manifestations of insufficient FFU quantity? . If excessive air volume causes disturbance, please refer to Is excessive FFU air volume good for cleanliness?。

Relationship with filter head-on wind speed

The wind speed facing the filter is usually divided by the actual air volume passing through the filter by the corresponding windward area. Note that the front area of ​​the filter is different from the unfolded area of ​​the pleated filter material. Using different areas will cause numerical distortion.

Excessive headwind speed may increase resistance, power consumption and noise, and may cause efficiency or sealing to withstand operating conditions beyond the originally confirmed range. For specific impacts, please refer to What will be the impact if the wind speed facing the filter is too high? . If the filter area is insufficient, the fan may require higher pressure or speed to maintain flow, please refer to How does insufficient filter area affect fan selection?。

Check with FFU nominal air volume

Whether the FFU nominal air volume includes the filter load shall be subject to the model data and test conditions. The air volume of the no-load fan cannot be directly used as the air volume of the whole machine. For relevant judgments, please refer to Is FFU's nominal air volume tested with a filter?。

When comparing quotations, the load conditions should be unified, and the filter size, initial resistance and measurement method should be checked. If only the maximum air volume is given, further target operating point data should be obtained. The complete performance curve is available for reference Why should we look at the complete performance curve when selecting FFU?。

Relationship with air supply coverage

Clean air supply coverage affects the effective air outlet area and protection continuity, thereby affecting the number of equipment. The coverage rate cannot be used without the effective air outlet size, target airflow and return air conditions of the equipment, nor can the entire room area be directly calculated according to a fixed ratio.

For specific calculation methods, please refer to How does clean air supply coverage affect the number of equipment? . If the project is constructed in phases, please refer to What are the expansion conditions for using independent modules in clean systems? , unify the ceiling module and equipment interface in advance.

Equipment selection and interface coordination

When selecting equipment, please refer to the background air supply of the room. FFU clean air supply products , please refer to the local protection of the equipment side. EFU equipment side fan filter unit . The logic of division of labor between the two lies in The division of labor between device-side EFU and room FFU is explained in detail.

Filter selection should correspond to target particle size and actual resistance.The HEPA and ULPA filter pages have parameters for different levels of products. When selecting, check the efficiency of the specific model at the target air volume. If you are still unclear about the difference between space rating and filter efficiency, you can read What is the difference between cleanliness level and filtration efficiency?。

If multiple FFUs require partition duty and troubleshooting, they can be combined FFU group control system provides unified management. If it involves sealing the equipment cavity, please refer to How to plan clean air supply for closed equipment cavities? , first confirm the cavity boundary and airflow path.

Project judgment example

For a certain project, the air volume was estimated based on the FFU outer area and the wind speed on the outlet surface was considered to be sufficient. On-site measurements found that the actual effective air outlet area was smaller than the external area, and the wind speed at key workstations was low. Adjustment method: Recalculate based on the effective air outlet area, and adjust the equipment layout and operating speed. After adjustment, the wind speed at the workstation returned to stability.

Another project compared two FFUs. The appearance and dimensions are the same, but the filter frame of one is wider and the effective air outlet area is smaller. The original plan was compared based on the appearance area, and it was believed that the coverage capabilities of the two units were the same. Actual measurements found that the FFU with a smaller effective area has higher headwind speed and increased resistance under the target air volume. Adjustment method: Compare again according to the actual effective air outlet area and the working point with filter, and select the matching model.

These two situations illustrate that the effective air outlet area of ​​FFU is not the appearance size, but a key parameter that affects air volume, face wind speed and coverage rate.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

Aili Industry covers motor manufacturing, independent research and development and production of fans, control system development, sheet metal processing, air filter manufacturing and complete machine assembly. For the FFU effective air outlet area and air volume calculation project, FFU, EFU, HEPA/ULPA filters, group control systems, air volume static pressure and installation interfaces can be uniformly checked to connect equipment configuration with on-site evaluation boundaries and reduce the disconnect between component parameters and complete machine operating conditions.

learn FFU clean air supply products；EFU equipment side fan filter unit；HEPA and ULPA filters；FFU group control system checks the installation space, operating conditions and filtered air supply configuration.