Why should we look at the complete performance curve when selecting FFU?

When choosing FFU, many items only look at two numbers: maximum air volume and maximum pressure. But the maximum values ​​do not necessarily occur at the same time. The maximum air volume near free air supply and the maximum pressure under high resistance state correspond to different working conditions. If you only compare these two extreme values, it is easy to choose a model that 'looks good' on the sample, but has insufficient air volume under the actual resistance of the project.

The complete performance curve can display the air volume, static pressure, power and operating range under different resistances, helping to determine whether the equipment is suitable for the target operating point. Let’s start with the following aspects: curve reading, working point selection, filter resistance, and adjustment margin.

The maximum values ​​do not necessarily appear at the same time

FFU performance curves usually have air volume on the horizontal axis and static pressure on the vertical axis, with power or rotational speed marked at the same time. Each point on the curve represents a combination of air volume and static pressure. When the fan is supplying air freely, the air volume is the largest, but the static pressure is close to zero; as the system resistance increases, the air volume decreases and the static pressure increases.

If you only look at the maximum air volume, you may ignore the actual filter resistance and external air path resistance of the project; if you only look at the maximum pressure, you may ignore the actual operating point at the target air volume. The correct approach is to first determine the working points required for the project, and then find the corresponding position on the curve.

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 boundary between the internal resistance and the external resistance.

The working point depends on both air volume and pressure.

When selecting a project, two inputs need to be clarified: target air volume and system resistance. The target air volume is determined by the protection range, air flow form, ventilation demand or exhaust compensation; system resistance includes filters, boxes, air ducts and return air channels.

On the performance curve, the intersection of target air volume and system resistance is the operating point. This point should be within the stable operating range of the equipment, with a certain adjustment margin. If the intersection point falls in the steep drop section of the curve, it means that the system resistance is close to the upper limit of the fan capacity, and the air volume is very sensitive to resistance changes, and the filter may decrease significantly after dust accumulation.

If you are not familiar with the method of determining target air volume, you can read How to convert the clean area area into air supply demand? and How to convert face wind speed and air volume? , first calculate the air volume requirements clearly.

Effect of filter resistance changes on operating point

The filter is the main source of resistance in the FFU system, and resistance increases with dust accumulation. When the filter is new, the working point is to the left and the air volume is high; when the filter accumulates dust, the resistance increases, the working point moves along the curve, and the air volume decreases. If the curve drops steeply, the decline may be significant.

When selecting the filter, you should confirm the working points under the initial resistance and final resistance of the filter, and leave an 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?。

The curve should correspond to the actual configuration

Different filters, different box structures and different rotation speeds will change the performance of the entire machine. The sample curve should indicate the test configuration: whether it has a filter, what grade, what size, and what the external pressure conditions are. If the configuration corresponding to the curve is different from the actual configuration of the project, it cannot be applied directly.

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 diameter of the effective air outlet area can be referred to How should we look at the effective air outlet area of ​​FFU?。

Adjustment margin and usage changes

FFU can usually adjust the speed, but the adjustment range is limited. If the operating point is close to the end of the curve, increasing the speed may not achieve the target air volume, and will also increase power, noise and filter load. When selecting the model, you should check the power and noise range after speed adjustment to confirm that long-term operation is still within the allowable conditions of the equipment.

If the project is constructed in phases or requires zoning duty, please refer to What are the expansion conditions for using independent modules in clean systems? , unify interfaces and operation strategies in advance. Multiple FFUs can be combined The FFU group control system manages unified management to avoid regional imbalance caused by manual adjustment.

Coordination with equipment interfaces and installation methods

The performance curve must also be understood in conjunction with the installation method. The FFU is installed directly on the ceiling and usually does not require additional external pressure; if it is connected to an air duct, static plenum box or return air channel, it is necessary 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 partial 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 air flow path, and then determine the fan operating point.

How to compare two FFUs

When comparing two FFUs, you can follow the following steps:

• Unify target air volume and system resistance conditions;

• Find the working points of the two devices under the same conditions;

• Compare the air volume, static pressure, power and speed at the working point;

• Check the air volume margin under the final resistance of the filter;

• Check whether the noise, power and adjustment range meet project requirements;

• Confirm that the curve version is consistent with the order model.

If there is insufficient air volume or low wind speed at the site, you can 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. If excessive air volume causes disturbance, please refer to Is excessive FFU air volume good for cleanliness?。

Project judgment example

In a certain project, two FFUs were compared, and the maximum air volumes were similar. However, the air volume of one of them dropped significantly under the resistance of the project. It turns out that only the first row of numbers in the sample was compared, and the complete curve was not viewed. Adjustment method: Re-search the air volume, static pressure and power of the two devices according to the target working point, and select the model that better matches the project resistance. After adjustment, the wind speed of the workstation meets the requirements.

After replacing a high-efficiency filter in another project, the working point of the original fan was close to the steep drop section of the curve, and the air volume dropped significantly. Adjustment method: Check the working point under the initial resistance of the new filter, select a higher static pressure model and adjust the speed. After adjustment, the air volume and noise are within the allowable range.

These two situations illustrate that FFU selection cannot only look at the maximum air volume or maximum pressure. Only the complete performance curve can reflect the operating capability under actual working conditions.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

The FFU performance curve is not a separate graph. It is related to the motor, fan, filter, box structure and control method. Aili Industrial has collaborative capabilities in the motor, fan, control and complete machine assembly links. According to the project's target air volume, filter configuration and external resistance, it can check the working point of the complete machine under initial resistance and final resistance, and provide corresponding air volume, static pressure, power and speed data to help match the sample curve with on-site working conditions during model selection. 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。