How to check the residual pressure of ULPA fan according to the final resistance?

When choosing a fan, many people only consider one point: a new filter element is installed, the system is clean, and the air volume reaches the standard. If this point is correct, the equipment can rotate, but whether the air volume is enough after running for a few months is another matter.

When checking the fan residual pressure, the real question to be answered is: from the new filter element to the final resistance, can the fan still maintain stable air volume. There is a process of rising resistance, which is often ignored.

The resistance characteristics of ULPA determine that it is more sensitive than ordinary filters. The filter material is dense and the resistance is high, so the pressure difference increases greatly after dust accumulation. If the fan is selected based only on the initial resistance, the air volume may be insufficient in the middle of operation and the end protection effect will be reduced.

System pressure loss needs to be disassembled and calculated

The first step in checking the residual pressure is to break the total resistance of the system into several pieces, rather than generally estimating a number.

Filter element body resistance. This is the area that changes the most. From the initial resistance to the final resistance, it may double or even more. The specific value is based on the resistance curve of the selected ULPA model, and cannot just use one point on the sample.

Pre-filter section resistance. The primary effect, medium effect and chemical filtration section (if any) have their own initial resistance and final resistance. These segments can also collect dust, and maintenance cycles and ends are out of sync.

Box and air duct resistance. Including internal channels, air ducts, elbows, valves, and diffusers of the FFU box. This part is relatively fixed, but will change when the air volume changes.

Terminal connections and plenum resistance. The pressure loss in high-efficiency air supply inlets, diffuser structures, and internal air ducts of equipment can easily be missed.

Adding these pieces together is the total resistance that the fan really has to overcome. Calculating only the filter element, or only calculating the initial resistance state, will underestimate the fan demand. involving HEPA/ULPA air filter items , the pressure loss decomposition method is the same. For

The initial resistance working condition and the final resistance working condition should be checked separately.

This is the core action of residual pressure verification.

Initial resistance working conditions : new filter element, clean system, and all filter sections are new. In this state, the total resistance is the lowest, the fan working point is on the far right, and the air volume is the largest. The purpose of the verification is to confirm that the fan will not blow the air volume too far and that the initial energy consumption is within a reasonable range.

Final resistance working condition : The end ULPA accumulates dust to the agreed final resistance, and other filter sections are also considered to be close to the replacement point. In this state, the total resistance is the highest, the fan operating point moves to the left, and the air volume decreases. The purpose of the calibration is to confirm whether the fan is still capable of maintaining the target air volume.

Both operating conditions must fall within the operating range of the fan curve. If the air volume under initial resistance conditions exceeds the standard and the air volume under final resistance conditions is insufficient, it means that the fan selection range is too narrow and needs to be adjusted. For specific curve reading methods, please refer to the relevant instructions on the resistance curve.

In actual operation, extreme values ​​will not appear in the two working conditions at the same time. However, the margin left is reliable when calculating based on the most unfavorable combination during calibration.

How to read fan curve

To check the residual pressure, you need to put the system resistance curve and the fan curve on the same graph.

System resistance curve : horizontal axis air volume, vertical axis pressure loss. From the initial resistance condition to the final resistance condition, the curve will move upward. The amount of movement depends on changes in filter resistance and changes in resistance in other parts of the system.

Fan curve : The PQ curve provided by the manufacturer represents the static pressure that the fan can provide under different air volumes.

The intersection of the two curves is the working point. The initial resistance condition corresponds to one intersection point, and the final resistance condition corresponds to the other. The air volume corresponding to the two intersection points should be within the allowable design range.

If the intersection point of the final resistance condition is too far to the left , it means that the fan cannot maintain sufficient air volume under this resistance. Treatment directions include: selecting a fan with higher static pressure, increasing the effective area of ​​the filter element to reduce resistance, optimizing the pressure loss in other parts of the system, or adjusting the final resistance setting value.

Supporting In the FFU fan filter unit system, the air volume and fan are designed in one piece, and the curve matching is relatively straightforward. Device-side applications such as The EFU equipment-side fan filter unit has a compact internal space and a small range of optional fans, so it is necessary to leave room for calibration.

The final resistance value itself must also be reasonable

Residual pressure check and final resistance setting influence each other.

If the final resistance is set too high , the fan needs to provide greater static pressure, which may exceed the capability. Or the fan can barely hold up, but the energy consumption and noise have increased.

If the final resistance is set too low , the filter element will have to be replaced before it is fully used, which will increase operating costs.

A reasonable final resistance should be a balance between fan capacity, system energy consumption and maintenance costs. There is no unified answer to this point, it depends on project conditions and operating strategies. For specific setting methods, please refer to the relevant instructions on final resistance.

It is recommended to use the final resistance as a variable during calibration. For example, check according to 450 Pa, 550 Pa, and 650 Pa to see which value the fan operating point is within a reasonable range and the maintenance cycle is acceptable. This is more reliable than taking a number directly.

The dust accumulation process is not linear

From the initial resistance to the final resistance, the resistance does not rise at a uniform speed. Dust accumulation is rapid at the beginning of operation and may level off later; or conversely, if there is sudden pollution upstream, the resistance may jump up in the short term.

It is okay to use linear estimation when checking, but extra margin must be left. Especially for systems with unstable upstream loads or without reliable front-stage protection, the actual resistance may rise faster than expected.

Continuously recording the pressure difference and air volume during operation can help determine the actual dust accumulation speed. If it is found that the resistance rises faster than expected, replacement can be arranged in advance, or the upstream cause can be investigated at the system level. involving ULPA ultra high efficiency filter or When selecting HEPA high-efficiency filters , it is recommended to use operating data as a reference for subsequent calibration.

Project judgment example

In a clean room renovation project, the original system used H14 and planned to upgrade it to U15. The fan remains unchanged, only the filter element is changed.

Preliminary calculation: The initial resistance of U15 at the target air volume is about 25% higher than that of the original H14. According to the original fan curve, the air volume can still be maintained near the design value under the initial resistance condition. The project team believes it can be implemented.

Further verification based on the final resistance working condition: the final resistance of U15 is about 80% higher than the initial resistance. With the increase in resistance in pre-filtration and other sections, the total system resistance under the final resistance working condition is about 60% higher than the initial resistance working condition. According to the fan curve, the air volume will drop by about 20% under this working condition.

This result shows that although it can run in the early stage, the air volume will be insufficient in the later stages of operation. There are two treatment directions: choose the U15 structure with a larger effective filtration area to reduce the increase in resistance; or replace the fan with a higher static pressure. The project team ultimately chose the former because the equipment space allowed and the amount of changes was smaller.

The key to this case is: drawing conclusions based on initial resistance is very risky. Checking the final resistance conditions cannot be omitted.

Selection and Acceptance Checklist

About Aili Industrial’s fan residual pressure package

The fan residual pressure check is essentially to check the filter element resistance, system pressure loss and fan capacity at the same working point. If this work can be completed during the planning stage, subsequent operations will be much more stable.

Aili Industrial has independent manufacturing capabilities in fans, motors, control systems and filters. When doing ULPA projects, the filter element resistance curve, system pressure loss and fan PQ curve can be evaluated together. When air supply equipment such as FFU and EFU are matched with filters in the same system, the initial resistance and final resistance conditions can be checked together to reduce the deviation of the air volume from the design value after installation. For renovation projects, you can also check whether the fan needs adjustment based on the original system parameters and new filter element specifications.

When enquiring, provide the design air volume, system pressure loss conditions, expected final resistance and installation space to help confirm the appropriate configuration. The specific selection is subject to the technical documents and test results confirmed by the project.