How to reduce local retention in non-unidirectional flow clean rooms?

Non-unidirectional flow clean rooms rely on the mixing and dilution of the air supply jet and indoor air to reduce the concentration of pollutants, rather than relying on unidirectional airflow to push and discharge pollutants. This method is widely used in cleanliness scenarios of level 10,000 and below, but it is also more prone to local retention problems.

Air stagnation can occur on the backs of equipment, in corners of rooms, under shelves, and in airflow short-circuit areas, causing particle accumulation. Increasing the nominal air exchange rate will not necessarily resolve these blocked or short-circuited areas. The following will start with the causes of retention, investigation methods, optimization of air supply and return, and equipment layout.

First understand the airflow characteristics of non-unidirectional flow

High-efficiency filters are arranged on the ceiling or side walls of non-unidirectional clean rooms, and the supply air is mixed with indoor air and then discharged from the return air outlet. Since the air supply area and return air area are not fully covered, uneven air flow speed will occur indoors. Before the supply air jet reaches the return air outlet, it may be blocked by equipment, shelves or walls, forming a vortex or stagnation area.

Sufficient ventilation does not mean effective ventilation at every location. If air is not circulating well in a localized area, particle concentrations at that location may be consistently high. When the number of air changes is insufficient or the air flow is turbulent, it is easy to form a 'clean dead corner', causing local pollutants to exceed standards.

Find weak loop locations from layout

To investigate local detention, priority should be given to the following locations:

• the back of a tall cabinet or large piece of equipment;

• Room corners and near columns;

• Under equipment and on the bottom of shelves;

• The area where the return air outlet is blocked;

• Distal locations that cannot be reached by the supply air jet.

Combining particle trend and airflow observations can locate stagnation areas. If the particle concentration at a certain location is consistently higher than the surrounding area and is not clearly related to personnel activities, it may be due to poor air circulation. For the problem of dead corners caused by stacking items on the workbench, please refer to Will stacking items on the workbench create a pollution dead zone? , to understand the impact of occlusion on airflow.

Return air layout is the key to reducing detention

Non-unidirectional flow clean rooms reduce local retention and optimize the return air layout, which is more effective than simply increasing the air supply volume. If the location of the return air outlet is unreasonable, even if the total air volume is sufficient, some areas may not be effectively covered.

The return air vents should be distributed as evenly as possible to avoid being concentrated on one side of the room. The distance and direction between the supply air jet and the return air outlet will affect whether the airflow can cover the work area. If return air vents are blocked by equipment or shelves, air may not be able to escape from the area, causing stagnation.

Please refer to the return air layout How to convert the clean area area into air supply demand? , first confirm the air volume requirement, and then check the return air capacity. If the return air channel is occupied or the resistance is too high, even if the air supply volume reaches the standard, the actual effective ventilation will decrease.

Equipment placement affects airflow paths

When large equipment divides a room into two sides, the original return air is concentrated on one side, and stagnation may occur on the other side. Equipment should be placed as much as possible to allow airflow to bypass the equipment and reach the return air area.

If equipment must be placed against a wall or side by side, verify that there is adequate airflow behind and below the equipment. If necessary, add auxiliary air return vents near the equipment or adjust the position of the return air vents.

For the impact of equipment blocking on airflow, please refer to Why is it easy for contamination to accumulate in the return flow area? , understand the reasons for the formation of backflow and the treatment direction.

Airflow short circuits impair effective ventilation

An airflow short circuit is when the supply air enters the return air or exhaust path before it has effectively passed through the protected area. Shortcuts may occur if the air supply inlets and return air inlets are too close to each other, if the equipment is blocked, or if there are obvious low-resistance passages.

When a short circuit occurs, the total air volume seems to be sufficient, but the particle removal at the workstation is insufficient, recovery is slow, or regional differences increase. Specific performance can be referred to What particle problems can airflow short circuits cause?。

To improve the airflow short circuit, priority should be given to adjusting the air supply and return air position and airflow path, rather than blindly increasing the air volume. After adjustment, the air volume and pressure difference should be re-measured to ensure that other areas are not affected.

The relationship between the location of pollution sources and the direction of air flow

If the source of contamination is located downstream of the air flow or within a stagnation zone, particles may accumulate locally and circulate repeatedly. Dust-generating equipment or processes should be arranged on the return air side as much as possible so that particles can be taken away by the airflow as quickly as possible after being generated.

Areas with frequent human activities should also maintain a reasonable distance from sensitive workstations. The airflow caused by the movement of people may change the local circulation path, causing stagnation in originally stable areas.

Number of air changes and actual effective air change

The number of air changes is a nominal value, and the actual effective air change depends on whether the air flow can cover and leave each workstation. Increasing the nominal air exchange rate will not necessarily resolve areas that are blocked or have shorted airflow.

To determine whether effective ventilation is sufficient, the following information should be combined:

• Measured wind speed and air flow direction in each area;

• Return air volume and pressure difference;

• Particle concentration trends and recovery times;

• Equipment occlusion and spatial layout changes.

The problem of average wind speed masking local differences can be referred to Why does the measured average wind speed not mean that the air flow is uniform? , avoid judging the overall effect by just looking at an average value.

Rectification plan and verification

Rectification directions to reduce local detention include:

• Adjust the position of the return air outlet or add an auxiliary return air outlet;

• Optimize equipment spacing and space layout;

• Adjust the air supply direction or increase local air supply;

• Clear the obstructions on the return air duct;

• Adjust the location of the pollution source so that it is close to the return air side.

After rectification, it should be re-verified under actual operating conditions, covering all work stations and weak locations. Verification includes wind speed, air flow direction, pressure differential and particle trends. If the one-way flow area is involved, please refer to How to identify lateral interference in unidirectional flow areas? , learn about interference identification methods.

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 particles in a certain non-unidirectional flow clean room continued to be high, and the original plan was to increase the number of air changes. The investigation found that the back of large equipment has been in an airflow stagnation area for a long time, and the return air outlets are concentrated on one side, and effective circulation cannot be formed on the other side of the equipment. Adjustment method: Add an auxiliary return air outlet near the back of the equipment, and adjust the distance between the equipment so that the airflow can bypass the equipment and reach the return air area. After adjustment, the particle trend decreased significantly without increasing the total air supply volume.

In another project, the particles in the corners of the room were higher because the return air outlet was blocked by shelves. Adjustment method: remove the obstruction and adjust the position of the return air outlet to be evenly distributed. After adjustment, the airflow in the corners is restored and the particles decrease.

These two situations illustrate that: to reduce local retention in non-unidirectional flow clean rooms, we need to start with the air supply and return air layout, equipment placement and air flow paths, rather than just increasing the nominal number of air changes.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

Local retention in non-unidirectional flow clean rooms is often related to the return air layout and equipment blocking, rather than simply insufficient air volume. Aili Industrial has collaborative capabilities in fans, motors, filters, control systems and complete machine assembly. It can combine the project space layout, return air conditions and pollution source locations to check the equipment layout, return air path and operation settings to help determine whether to adjust the position of the air outlet, optimize the equipment spacing or increase local air supply, rather than just adding equipment based on the number of air changes. 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。