Why does the measured average wind speed not mean that the air flow is uniform?

When accepting clean areas, average wind speed is a common indicator. But if you only record an average, it can mask local problems. The wind speed is high in one location and low in another location. The average of the two may seem qualified, but the actual protection effect is not uniform.

The average wind speed can only indicate the overall level, but cannot indicate that each location meets the requirements. To determine whether the air flow is uniform, you need to check the distribution and differences of measuring points at the same time.

The calculation method of average wind speed determines that it cannot reflect local

The average wind speed is the arithmetic mean of the readings from multiple measuring points. If some measuring points are too high and others are too low, the average value may still fall within the qualified range, but the lower locations may already be below the protection requirements.

For example, in a group of measurement points, the wind speed is higher in the middle and lower on both sides. The average looks normal, but the product positions on both sides may be in weak protection zones. This problem is easily overlooked when only averaging is recorded.

High and low wind speeds may cancel each other out

Qualified average wind speed does not mean that every location is qualified. High and low data will cancel each other out when calculating the average, forming the illusion of 'qualification'. In actual use, the lower position is the risk point that needs attention.

When judging, the readings of each measuring point should be retained to observe whether the distribution is uniform, rather than just recording an average value. If you find that some measurement points are significantly low, you should examine the cause rather than masking the differences with averages.

The distribution of measuring points should be representative

The location and number of measuring points will affect the representativeness of the average wind speed. If the measurement points are concentrated in the center of the air outlet surface, edge positions may be missed; if the number of measurement points is too small, the single-point deviation will significantly affect the average value.

The measuring points should cover different locations on the air outlet surface, including the center, edge and areas corresponding to key workstations. For specific point layout methods, please refer to Why can’t the sampling point be located just below the air supply outlet? , understand the ideas of representative point layout.

Pay attention to the wind speed corresponding to key workstations

The actual exposed location of the product is the area that needs to be protected. If the wind speed is low at that location, the protection effect may not be sufficient even if the wind speed is normal at other locations.

When making judgments, the location of the measuring point should be matched with the location of the workstation to confirm that the key operating position is within the effective protection range. For on-site performance with insufficient number of FFU, please refer to What are the on-site manifestations of insufficient FFU quantity? , first check the coverage and air volume issues.

Local low-speed areas may be caused by structural occlusion

The low-speed zone does not necessarily come from the equipment itself, but may also be related to structural occlusion. Frames, light fixtures, ductwork, fixtures, and stacked items can all alter local airflow.

Please refer to the impact of stacking items on the workbench. Will stacking items on the workbench create a pollution dead zone? , the interference of operating arms can be referred to How will operating the arm interfere with the air supply to the workbench? . The investigation should be based on the on-site layout, rather than just looking at wind speed readings.

Airflow direction is equally important as wind speed

The average wind speed only reflects the speed, not the direction of the airflow. If the airflow direction deviates from the position of the product, even if the wind speed is qualified, the protection effect may not be sufficient.

When judging, you should also observe the direction of air flow to confirm whether the air passes through the product first and then leaves the contaminated area. What particle problems will be caused by air flow short circuit, please refer to What particle problems can airflow short circuits cause?。

The uniformity index should correspond to the test method

Different testing solutions may use different uniformity indicators, such as relative standard deviation, maximum and minimum deviation, or measurement point pass rate. When comparing different projects, you should confirm whether the indicators and calculation methods used are consistent.

If you are not familiar with wind speed conversion, you can refer to How to convert face wind speed and air volume? , first confirm whether the calibers of area and wind speed are consistent.

Reconfirm after the operating status changes

Dust accumulation on filters, fan speed adjustments, air supply distance changes and external interference may all change airflow distribution. After the operating status changes, the original average wind speed data may no longer be applicable.

For the changes in airflow when the air supply distance increases, please refer to How will the airflow change when the air supply distance increases? , understand the effect of distance on wind speed and direction.

Equipment selection and terminal matching

Please refer to the room background air supply 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 devices require partition duty and troubleshooting, they can be combined FFU group control system provides unified management.

Project judgment example

The average wind speed was qualified during the acceptance of a certain project, but the particles at the workstation were occasionally high during production. The investigation found that the wind speed on both sides was low and the wind speed in the middle was high. The average value concealed local problems. Adjustment method: Adjust the filter arrangement and the position of the air outlet surface to make the wind speed at each measuring point even. Workstation protection is restored after adjustment.

The measuring points of another project are concentrated in the center of the wind outlet, and the average wind speed is qualified. However, the actual workpiece position is at the edge of the table and the wind speed is low. Adjustment method: Re-distribute points according to key workstation locations, and confirm that the effective protection area covers the actual operating position. After adjustment, the edge wind speed meets the requirements.

These two situations illustrate that the average wind speed can only reflect the overall level. To judge the airflow protection effect, you need to look at the distribution of measuring points, local differences and the location of key workstations at the same time.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

The average wind speed is qualified but the local protection is insufficient, which is a common problem in clean workstations. Aili Industrial has collaborative capabilities in fans, motors, filters, control systems and complete machine assembly. It can check equipment layout, operation settings and filter configurations based on project layout methods, key workstation locations and airflow organization requirements to help determine whether uniformity meets actual protection needs, rather than just looking at an average value. Specific models can be viewed FFU clean air supply products , supporting equipment can be viewed EFU equipment end fan filter unit , please refer to the filter package. HEPA and ULPA filters。