How does the air volume change after ULPA accumulates dust?

There is no dispute that dust accumulation will increase the resistance of the filter element. But after the resistance rises, how the air supply volume will change depends on how your fan is controlled.

For a system running at a fixed speed, the air volume will decrease as the resistance increases. The fan speed remains unchanged, the system resistance increases, and the operating point moves to the left along the fan curve, resulting in a decrease in air volume.

The constant air volume control system will increase the speed within a certain range to maintain the air volume. After the resistance rises, the controller automatically compensates, and the air volume does not change much in the short term. However, the compensation capability has an upper limit. When it reaches the limit of the motor or control, the air volume will still drop.

So when asking 'How does the air volume change after dust accumulation?', you must first find out what kind of control method your system uses. The two behave differently and have different judgment methods.

Performance of the fixed speed system: natural attenuation of air volume

The fixed speed system has a simple structure and no complicated feedback adjustment. The fan runs at a fixed speed, and when the system resistance changes, the working point moves naturally.

In the early stage of dust accumulation, the resistance rise is small and the air volume does not drop significantly. After the operation time is extended, the pressure difference gradually increases, and the air volume attenuation will gradually appear.

There is a hiddenness in this process: the fan speed does not change, and the controller does not alarm. On the surface, the system looks 'everything is normal.' If the air volume is not measured regularly, it may not be discovered until the feedback from the process end is abnormal that the air supply volume is insufficient.

For fixed speed systems, the pressure difference can be used as a reference for dust accumulation, but the pressure difference itself does not tell you how much air volume is left . When the pressure difference rises from 200 Pa to 400 Pa, the air volume may only drop by 5% or by 15%, depending on the shape of the fan curve and the composition of the system resistance.

Therefore, for the maintenance of the fixed speed system, it is recommended to include air volume measurement as a regular inspection item. Only by recording the initial air volume and re-testing regularly can the attenuation trend be judged. Supporting For the FFU fan filter unit system, measuring the surface wind speed one by one is an effective way to determine the air volume status of each unit.

Performance of constant air volume system: speed compensation, but with boundaries

Constant air volume control uses feedback adjustment to increase the fan speed when resistance rises to maintain the target air volume.

The initial performance is that the rotation speed gradually increases and the air volume is stable. The pressure difference is rising, and the rotational speed is also rising, but the air volume reading has not changed much. At this time, the system is running well and does not require additional intervention.

The mid-term performance is that the speed is close to the upper limit. As dust accumulation continues, the rotational speed will approach the maximum allowed by the motor or controller. If you only look at the air volume at this time, it may still be within the range, but the system has no margin. Once the resistance goes up a little more, the air volume will start to drop.

The later manifestation is that the air volume cannot be maintained. After the speed reaches the top, the controller loses its adjustment ability and the system degrades to fixed-speed operation. The air volume begins to decrease as the resistance increases.

A blind spot of the constant air volume system: it 'hides' the resistance changes in the rotation speed. If you only monitor the air volume and do not record the rotational speed or power, the problem may not be discovered until the compensation capacity is exhausted. Therefore, it is recommended that the constant air volume system records the rotation speed, power and pressure difference at the same time to completely judge the status of the filter element.

Device-side applications such as The EFU equipment-side fan filter unit has a compact internal space, limited fan capacity and control range, and the compensation margin is usually smaller than that of large units. Pay special attention to this boundary during selection and maintenance.

How to distinguish the effects of changes in working conditions and dust accumulation

Whether it is a fixed speed or constant air volume system, there are other factors that will cause the air volume to change during operation. When judging the impact of dust accumulation, first eliminate working condition factors.

Fixed speed system: When the fan speed remains unchanged, if the air volume decreases, the system resistance is likely to increase - it may be dust accumulation in the filter element, or it may be blockage of the pre-filter, dust accumulation in the air duct, or changes in valve position. Comparing the pressure differences at various levels can help with positioning. If the pressure difference between the front stage also rises significantly, it means that the load comes from the upstream.

Constant air volume system: The increase in rotational speed may be caused by dust accumulation or increased resistance in other parts of the system. Also compare the pressure differences and operating modes at all levels. If the device itself switches operating modes (e.g. from production mode to standby mode), changes in rotational speed and air volume cannot be attributed to the filter element.

How you record is important. Only by comparing data in the same operating mode can you determine the true trend. If working conditions change frequently, it is recommended to note the time and reason for each adjustment in the maintenance ledger.

Project judgment example

The FFU in a certain clean room operates at a fixed speed, with an initial air volume of 800 m³/h and an initial resistance of the filter element of approximately 180 Pa.

After six months of operation, the pressure difference rose to about 320 Pa, and regular air volume measurements found that it dropped to about 720 m³/h, with an attenuation of about 10%. The particle monitoring in the process area is still normal, indicating that it is still within the acceptable range. Continue to track and be prepared to arrange replacement when the air volume drops to about 85% of the design value.

Another project uses constant air volume control, with an initial speed of 60% and an air volume of 800 m³/h. After six months of operation, the pressure difference also increased to about 320 Pa, the rotation speed automatically increased to about 78%, and the air volume remained stable at 800 m³/h. However, the control system records showed that the speed was close to the recommended upper limit of the unit. The maintenance team arranged spare parts and replacement plans in advance accordingly.

The dust accumulation levels of the filter elements of the two projects are similar, but due to different control methods, the performance is completely different. The focus of the fixed speed system is the attenuation of the air volume, while the focus of the constant air volume system is the rotational speed margin. involving For HEPA/ULPA air filter selection, please refer to the corresponding product information.

Selection and Acceptance Checklist

About the operation plan adaptation of Aili Industry

How the air volume changes after dust accumulation is directly related to the matching of the fan, control system, and filter.

Aili Industrial has independent manufacturing capabilities in fans, motors, control systems and filters. It can evaluate the filter element resistance curve, fan operating point and control strategy together during the planning stage. For a fixed-speed system, the target air volume and allowable attenuation range can be combined to match the fan; for a constant air volume system, the rotation speed and power margin can be checked to avoid insufficient compensation capabilities being discovered at a later stage of operation. When the filter is matched with equipment such as FFU and EFU, the control parameters and filter element data can be confirmed uniformly.

Providing the control method, target air volume and allowable maintenance limits when enquiring, will help confirm a more suitable solution. The specific configuration is subject to the technical documents and operating data confirmed by the project.