How will the airflow change when the air supply distance increases?

After the clean air blows out from the air outlet surface, it needs to travel a certain distance to reach the product location. This distance may be tens of centimeters or more than one meter. After the distance increases, the airflow will not maintain the same speed and direction at the outlet surface, but may change due to diffusion, entrainment of background air and structural interference.

It is a common misunderstanding in selection and acceptance to directly regard the wind speed at the air outlet surface as the wind speed at the working position. The following will start from the aspects of diffusion and entrainment, structure and interference, coverage, and measurement methods.

Clarify the air supply surface and working surface

Before judging the air supply distance, two locations must be marked on the drawing or on site:

• Air supply surface : the air outlet surface of the filter;

• Work surface : The location where the product is actually exposed.

The distance between the two is the air supply distance. If there are obstructions, grilles or equipment structures in the middle, these should also be marked. The distance between the air supply surface and the work surface is also related to the front window opening of the workbench and the layout of the workbench. For details, please refer to Will the height of the workbench front window affect the clean airflow?。

Airflow diffuses and entrains background air

After the supply air leaves the outlet surface, the airflow boundary will gradually spread. In an open environment, the supply air will mix with the surrounding air, entraining background air into the airflow. The greater the distance, the greater the entrainment volume, and the composition and speed of the airflow when it reaches the working surface will change.

If the background air itself contains particles, entrainment will bring the particles into the protected area. This is especially obvious in open workstations and local clean areas. For related methods, please refer to How to establish a local clean area in an open workstation?。

Speed ​​may decrease as distance increases

In open or semi-open conditions, air velocity decreases with distance. The degree of attenuation depends on the size of the air outlet surface, the air outlet speed and the surrounding environment. The larger the air outlet surface, the higher the speed, and the attenuation is relatively slow; the smaller the air outlet surface, the lower the speed, and the faster the attenuation.

The degree of attenuation cannot be uniformly applied to a fixed ratio and should be judged based on the specific equipment structure and environmental conditions. If you are not clear about the difference between the wind speed at the outlet surface and the wind speed at the working position, you can see Why does the measured average wind speed not mean that the air flow is uniform?。

Structural occlusion changes the direction of airflow

If there are beams, lamps, pipelines, mechanical structures or high fixtures in the middle of the air supply path, the direction of the air flow may be changed, resulting in local backflow or dead ends. Such changes are difficult to predict completely by calculation, and the airflow should be observed under the actual structure.

As the air supply distance increases, the possibility of obstruction in the middle of the path also increases. For airflow path planning in a closed cavity, please refer to How to plan clean air supply for closed equipment cavities? , to understand the impact of structure on airflow.

The relationship between air supply distance and coverage

As the air supply distance increases, the airflow coverage will also change. At short distances, the coverage is small but the wind speed is high; at long distances, the coverage is expanded but the wind speed is reduced. If the protected area is large, you need to confirm whether the wind speed at the far end still meets the requirements.

For the coverage of clean screen or air supply end, please refer to How does clean air supply coverage affect the number of equipment? , to understand coverage versus number of devices.

Cross airflow exacerbates distance effects

The impact of lateral airflow on long-distance air supply is more obvious. If there are air-conditioning vents, people moving around, or exhaust from adjacent equipment near the working location, the supply air flow may be deflected before reaching the product.

When making judgments, the source and direction of external interference should be recorded, and then decide whether it is necessary to adjust the air supply direction, add baffles, or adjust the position of the workstation. For processing of lateral interference, please refer to How to identify lateral interference in unidirectional flow areas? learn.

Measure at the work surface, not just the wind surface

To determine the impact of air supply distance on air flow, the wind speed and direction should be measured at the working surface instead of just the wind surface. When measuring, please note:

• Record the distance between the measuring point and the air outlet surface;

• Measured under normal operating conditions;

• Cover the corresponding locations of key workstations;

• Also observe the direction of airflow;

• Document external disturbances and structural occlusions.

For sampling point layout, please refer to Why can’t the sampling point be located just below the air supply outlet? , to avoid measuring points concentrated near the wind outlet surface.

You cannot just increase the outlet wind speed to compensate for long distances

When the air supply distance is too large and the wind speed on the working surface is insufficient, some sites will increase the fan speed to compensate. However, increasing the outlet wind speed may increase disturbance and energy consumption, and may not necessarily improve the far-end coverage effect.

If distance is the root cause, you should consider adjusting the position of the air outlet surface, adding air supply units, or changing the workstation layout instead of simply increasing the wind speed. The influence of excessive air volume can be referred to Is excessive FFU air volume good for cleanliness?。

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

A clean screen is installed at a higher position above the workstation. The wind speed on the air outlet surface is normal, but the wind speed at the product location is low. The investigation found that the air supply distance was relatively large and the airflow had spread before reaching the product. Adjustment method: Reduce the height of the air outlet surface or adjust the position of the work station so that the working surface is within the effective coverage range. After adjustment, the wind speed of the product position is restored.

Another project has a large air supply distance, is blocked by pipelines in the middle, and the air flow direction is changed. Adjustment method: adjust the position of the pipeline and add a local air supply unit instead of continuing to increase the fan speed. After adjustment, the airflow direction of the working surface is restored.

These two situations illustrate that as the air supply distance increases, the airflow speed and direction will change. It should be measured at the working surface instead of just reading the wind surface reading.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

The air supply distance affects the actual wind speed and airflow direction of the working surface. Aili Industrial has collaborative capabilities in fans, motors, filters, control systems and complete machine assembly links. It can combine the air outlet size, working distance and background conditions to check the wind speed and coverage of the target working point, helping to determine whether to adjust the installation position, change the air supply form or add an air supply unit, instead of just looking at the equipment nameplate parameters. Specific models can be viewed FFU clean air supply products , the supporting equipment can be viewed EFU equipment end fan filter unit , please refer to the filter package. HEPA and ULPA filters。