Is FFU's nominal air volume tested with a filter?

When purchasing FFU, the sample usually has an air volume number written on it, such as 1200m³/h, 1500m³/h. But is this number the air volume of the unloaded fan, or the air volume of the entire machine after the filter is installed? Is the air duct connected during the test, what is the static pressure outside the machine, and what gear is the speed set to? If these problems are not confirmed, it is easy to regard the nominal air volume as the actual air volume during model selection, leading to deviations in the estimation of the number of units.

The following will start from the test configuration, operating point, filter resistance, and selection comparison.

First distinguish between fan data and complete machine data

FFU nominal air volume may come from different test objects:

• Free air supply from the fan : only the fan body is measured, no filter is installed, and the air duct is not connected;

• Fan plus filter : Install the filter, but do not connect to the external air duct;

• The whole machine has a filter and external pressure : install the filter and simulate the actual external resistance.

These three configurations correspond to different resistances and different air volume results. The free air supply data of the fan is usually the highest, and will decrease after installing the filter, and will continue to decrease when the external air duct is connected. If the sample does not indicate the test configuration, you cannot directly compare the air volume figures of different models.

If you are still unclear about the meaning of external static pressure, you can read What does FFU external static pressure mean? , first distinguish the difference between internal resistance and external resistance.

Effect of filter resistance on air volume

The filter is one of the main sources of resistance in the FFU system. Filters with different efficiency levels, different filter material areas, and different folding structures may have significant differences in initial resistance. If the same fan is equipped with different filters, the actual air volume will be different.

For example, H13 and H14 filters may have different resistances at the same air volume; filters with smaller filter areas have higher head-on wind speeds and greater resistance. If the nominal air volume is measured with a low-resistance filter, the actual air volume may be significantly lower than the sample number after switching to a high-resistance filter.

For the relationship between filter resistance and headwind speed, 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 requires higher pressure to maintain the flow. For details, please refer to How does insufficient filter area affect fan selection?。

Speed ​​and operating point cannot be ignored

FFU usually has multiple speed gears or adjustable voltage. Which speed does the nominal air volume correspond to? Is it the highest gear or the rated gear? If the sample only gives the maximum air volume without specifying the speed and power, and it runs at a lower speed after purchase, the actual air volume may be lower than expected.

When comparing different models, the comparison should be made under the same rotation speed or the same input conditions, and the air volume, static pressure and power of the target operating point should be checked. The complete performance curve can show the air volume range under different resistances. For details, please refer to Why should we look at the complete performance curve when selecting FFU?。

External pressure conditions should be uniform

Even if both FFUs are equipped with filters, the air volume figures cannot be directly compared if the external pressure conditions are different at the time of testing. Some samples are measured under free air supply conditions, and some are measured under specified external static pressure. When the project is actually used, the FFU may be connected to air ducts, static plenums or return air ducts, which will increase external resistance.

When purchasing, you should confirm the external pressure conditions corresponding to the nominal air volume and check whether the actual resistance of the project is close. If the actual resistance is greater, the air volume corresponding to the working point should be found according to the performance curve instead of directly using the nominal maximum value.

How to check nominal air volume

To check the FFU nominal air volume, follow the steps below:

• Confirm the test object indicated in the sample: fan, complete machine or with external pressure of the machine;

• Confirm filter configuration: grade, size, initial resistance;

• Confirm the speed or input conditions;

• Confirm the external pressure conditions;

• Find the air volume, static pressure and power of the target operating point;

• If necessary, ask the supplier to provide performance data for the corresponding configuration.

If the sample only gives an air volume number and no description of test conditions, further detailed information should be requested. Please refer to it when purchasing FFU clean air supply products , check the performance of specific models under different configurations.

Relationship with effective air outlet area and face wind speed

The nominal air volume needs to be combined with the effective air outlet area to calculate the face wind speed. If the air volume is no-load data and the area is calculated based on the overall shape of the machine, the obtained surface wind speed will deviate from the actual value. For the distinction of area diameter, please refer to How should we look at the effective air outlet area of ​​FFU? , please refer to the air volume conversion How to convert face wind speed and air volume?。

Impact on model selection and quantity estimation

Use the maximum air volume of unloaded fans to estimate the number of units. After installing a high-efficiency filter, the total air volume may be lower than expected. At this time, problems such as insufficient coverage and low wind speed at the workstation may occur. If similar behavior has already occurred on site, please refer to What are the on-site manifestations of insufficient FFU quantity? , first check whether it is a quantity problem or a working point offset.

On the contrary, if the actual air volume at the working point is higher than the demand, it may lead to excessive wind speed and increased disturbance. For relevant judgments, please refer to Is excessive FFU air volume good for cleanliness?。

Interface coordination with filters and group control

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 multiple FFUs need zone duty and speed adjustment, they can be combined FFU group control system provides unified management. For local protection on the equipment side, please refer to EFU equipment side fan filter unit , the division of labor logic between the two is The division of labor between device-side EFU and room FFU is explained in detail.

If it involves equipment cavity or ceiling installation, please refer to How to plan clean air supply for closed equipment cavities? and How does the FFU appearance size match the ceiling module? , first confirm the installation interface and air flow path.

Project judgment example

For a project, the number of FFU was estimated based on the maximum air volume of the sample. After installation, it was found that the wind speed at the workstation was low. The investigation found that the sample air volume was the data of the unloaded fan, and the actual air volume dropped by about 20% after the H14 filter was installed. Adjustment method: Recalculate according to the working point with filter, add some FFU and adjust the operating speed. The wind speed at the workstation recovered after adjustment.

Another project compared two FFUs. The nominal air volumes were similar, but one was measured under free air supply, and the other was measured under specified external static pressure. After comparing under the same conditions, the air volume at the actual working point is significantly different. Adjustment method: Re-select the model after unifying the test conditions, and select a model that better matches the target operating point.

These two situations illustrate that the FFU nominal air volume must be understood in conjunction with the test configuration, and the no-load data cannot be directly used as the air volume of the entire machine.

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 nominal air volume and working point verification project, FFU, EFU, HEPA/ULPA filters, group control systems, air volume static pressure and installation interfaces can be uniformly checked to connect the equipment configuration with the on-site evaluation boundary and reduce the disconnect between component parameters and the working conditions of the entire machine.

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.