What does FFU external static pressure mean?

When selecting FFU, two pressure figures often appear on the sample: external static pressure and fan maximum pressure. These two numbers are not the same thing. The external static pressure of the machine describes how much pressure the whole machine can exert to overcome the external air path resistance under the specified flow rate and test configuration; the maximum pressure of the fan is the limit value of the fan body under specific conditions. Mixing the two can easily overestimate the actual air supply capacity of the equipment in the project.

The following will start with the internal and external boundaries of the machine, working point reading, performance curve, and adjustment margin.

Distinguish between internal resistance and external resistance

When FFU is running, the pressure provided by the fan must overcome two types of resistance at the same time:

• Internal resistance : the loss caused by the filter and the internal structure of the box;

• External resistance : losses caused by external air ducts, static pressure boxes, return air channels or connecting components.

The static pressure outside the machine is that part of the pressure that can be used to overcome the resistance outside the machine after deducting the losses inside the machine. If different samples use different complete machine boundaries, the static pressure data cannot be directly compared. Some samples include the filter inside the machine, and some include some connectors outside the machine. The caliber should be unified before comparison.

If you are not clear about the test configuration of air volume with filter, you can see Is FFU's nominal air volume tested with a filter? , first confirm the test object corresponding to the sample data.

The static pressure outside the machine is not the maximum pressure of the fan

The maximum pressure of the fan is the limit value of the fan body. It is usually measured under specific test conditions. It does not mean that it can provide the same pressure after being installed on the complete machine and equipped with a filter. The resistance inside the machine will consume part of the pressure, and the remaining part is the static pressure outside the machine.

If the maximum pressure of the fan is directly used to estimate the external air duct capacity, the actual available pressure may be overestimated, resulting in a smaller air duct selection or insufficient air volume. When making judgments, you should look at the overall machine performance curve instead of just looking at a maximum pressure number.

Check remaining capacity using work points

When selecting a project, you should first determine the required air volume and external resistance, and then check the available air volume under the corresponding pressure. This correspondence is the working point.

For example, the project requires FFU to provide the target air volume while connecting a section of air duct. If you only view the maximum air volume, you may ignore the duct resistance; if you only view the maximum pressure, you may ignore the target air volume. The correct approach is to find the external static pressure corresponding to the target air volume on the performance curve and confirm whether the equipment can meet it.

If you are not familiar with how to read performance curves, you can read Why should we look at the complete performance curve when selecting FFU? , learn how to check air volume and pressure at the same time.

Changes in filter resistance will affect the external static pressure margin

The filter will gradually accumulate dust during use and the resistance will increase. When the filter is new, the resistance inside the machine is low, and the static pressure margin outside the machine is relatively sufficient; as the filter accumulates dust, the resistance inside the machine increases, and the pressure margin left for the air path outside the machine will decrease. If the resistance of the external air path of the project is large, the air volume may be insufficient at the end of the filter.

When selecting the filter, you should confirm the working points under the initial and final resistance of the filter, and leave a reasonable adjustment margin. 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 may require higher pressure to maintain flow. For details, please refer to How does insufficient filter area affect fan selection?。

External air path resistance should be calculated according to actual conditions

External air path resistance includes losses along the duct, elbows, reductions, static plenums and end components. These resistances are related to air volume, wind speed and duct size and cannot be estimated empirically.

If the FFU is connected to a long air inlet duct, you cannot just look at the maximum air volume. Check whether the inlet line resistance causes the equipment to deviate from the target operating point. Please refer to the specific air volume conversion. How to convert face wind speed and air volume? , the effective area diameter can be referred to How should we look at the effective air outlet area of ​​FFU?。

Adjustment margin and operating upper limit

Fan speed adjustment can compensate for some resistance changes, but the adjustment range is limited. If the external resistance of the machine is too large, the target air volume may not be achieved by simply increasing the rotation speed, and the power, noise and filter load will also be increased.

When selecting the model, you should check the power and noise range after speed adjustment to confirm whether long-term operation is still within the allowable conditions of the equipment. If multiple FFUs need zone duty and speed adjustment, they can be combined The FFU group control system manages unified management to avoid regional imbalance caused by manual adjustment.

Coordination with suspended ceiling installation and interfaces

The application scenarios of external static pressure are related to the installation method. If the FFU is installed directly on the ceiling with a clean area below, usually no additional external pressure is required; if the FFU is connected to an air duct, static plenum box or return air channel, you need to check whether the external static pressure is sufficient.

For installation dimensions and ceiling interfaces, please refer to How does the FFU appearance size match the ceiling module? . 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 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, and then determine the external static pressure requirements.

Project judgment example

In a certain project, FFU was connected to a section of air inlet duct. The nominal air volume of the sample met the requirements, but the actual air volume after installation was low. The investigation found that the sample air volume was free air supply data and did not take into account the air duct resistance. Adjustment method: Find the working point with air duct according to the performance curve, replace it with a model with higher external static pressure, and optimize the air duct path. The air volume is restored after adjustment.

In another project, after replacing a high-efficiency filter, the resistance inside the machine increased, and the static pressure margin outside the original fan was insufficient, resulting in a decrease in the wind speed at the workstation. Adjustment method: Check the working point under the initial resistance of the new filter, adjust the fan speed and confirm that the power and noise are within the allowable range.

These two situations illustrate that the external static pressure is not the maximum pressure of the fan and must be understood in conjunction with the filter configuration, external air duct and operating status.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

The external static pressure of the FFU machine involves multiple variables such as the internal filter, box structure, fan curve and external air duct. It is difficult to judge whether the project matches the project by looking at only one pressure number. Aili Industrial has collaborative capabilities in the fan, motor, control and complete machine assembly links. It can combine the external air path, target air volume and filter configuration of the project to check the air volume, static pressure, power and noise of the complete machine at the target working point, helping to reduce the deviation between sample data and actual working conditions. Specific models can be viewed FFU clean air supply products , supporting equipment can be viewed EFU equipment end fan filter unit , filter parameters can be referred to HEPA and ULPA filters。