What is the difference between the initial resistance and final resistance of ULPA filter?

These two words often appear together, but if they are used without distinction, it is easy to cause problems.

Initial resistance is the pressure loss of a clean new filter element at a specified air volume and is used to establish a performance baseline. The final resistance is the operation and maintenance limit specified by the project or product. At this line, it should be considered and dealt with.

One is the starting point and the other is the end point. Sandwiched in the middle is the entire operation process of the filter element from new to old, from clean to dusty.

Initial resistance is the performance baseline, not the product upper limit

When a new filter element is installed, the measured pressure difference is the initial resistance. The significance of this value is that it is the starting point for all subsequent comparisons.

However, the initial resistance must be recorded together with the test conditions. The readings will be different depending on the air volume, temperature, humidity and measuring point location. The same product may measure 20 Pa at low air volume, and may measure 50 Pa at high air volume. Both numbers are correct, but they describe different working conditions.

In actual projects, the initial resistance is used for two purposes: to check whether the system operating point is within the design range, and to establish a baseline for subsequent judgment of the filter element status. If the initial data is not recorded and the pressure difference is found to have increased after running for a period of time, it will be impossible to determine how much the increase has been and whether it is abnormal.

Supporting FFU fan filter unit system, when recording the initial resistance, it is recommended to record the unit gear and single table wind speed at the same time; the equipment end application is as follows: For the EFU equipment end fan filter unit , the operating status of the equipment must also be recorded for subsequent comparison.

The final resistance is the maintenance limit, not the product damage point

The final resistance is the operation and maintenance boundary specified by the project or product. At this point, the filter is due for treatment - but 'treatment' does not necessarily mean 'must replace'.

There are several things to consider when setting the final resistance:

Fan capacity. After the filter element resistance increases, can the fan still maintain the target air volume? If the final resistance is set too high, the fan may not be able to provide enough air before it reaches this line.

other parts of the system. The capabilities of the air conditioning system, air ducts, and terminal regulators must all be considered. The actual usage conditions of end users also affect the setting of this limit.

Cleanliness requirements. If the project has strict particle control requirements, it may be necessary to arrange replacement during the rising resistance stage instead of waiting for a fixed value.

Therefore, the final resistance is not a number that can be applied arbitrarily. It needs to be determined according to the actual conditions of the project and stated in the contract and maintenance documents. Air volume, fan capacity and cleanliness requirements cannot be set independently.

How to use these two values?

Putting the initial resistance and the final resistance together, several things can be calculated.

Fan margin : How much extra static pressure the fan needs to provide from the initial resistance to the final resistance. This difference should be checked during the selection stage to ensure that the fan can work throughout the entire operating range.

The available pressure difference range of the filter element : the final resistance minus the initial resistance is the resistance growth space of the filter element from new to old. If this range is large, it means that the filter element can accumulate more dust before it needs to be processed; if the range is small, it may be replaced more frequently.

Estimation of replacement cycle : Combining the on-site pollution load and the actual pressure difference rising rate, we can roughly estimate how long it takes to go from the initial resistance to the final resistance. This estimate can be used for maintenance planning, but the actual cycle is affected by working conditions and needs to be dynamically adjusted.

involving For HEPA/ULPA air filter products of different grades, the verification methods for these two values ​​are the same, and the specific criteria are implemented according to the project documents.

When judging replacement, don’t just look at the final resistance.

This is where things can easily go wrong.

Damage, leakage, moisture and process changes may trigger maintenance when the pressure difference does not reach the upper limit. For example, if a leak is found during on-site leak inspection, even if the pressure difference is normal, it still needs to be dealt with. For specific investigation directions, please refer to the relevant instructions for sudden changes in pressure difference.

The judgment criteria for filter elements in different locations may be different. The maintenance requirements of key process areas and general areas are different, and they cannot be uniformly treated according to a final resistance value.

Records must be complete. The reasons and results of every adjustment, every abnormality, and every replacement should be recorded. This way, subsequent analysis can distinguish between normal aging and abnormal events.

Project judgment example

In a certain system, the fan is selected based on the initial resistance. After running for a period of time, the terminal air volume is insufficient.

The investigation found that the pressure difference of the filter element was close to the final resistance, and the fan selection only considered the initial resistance condition and did not reserve a static pressure margin from the initial resistance to the final resistance. This is not a problem with the filter element, nor is it a fan failure, but a miscalculation during the selection stage.

The correct approach is to check the initial resistance, final resistance and fan curve together during the selection stage to confirm that the fan can maintain the target air supply volume during the entire service life of the filter element. This is much less troublesome than adjusting the speed or changing the fan afterwards.

Selection and Acceptance Checklist

About Aili Industrial’s resistance accounting cooperation

The matching of initial resistance and final resistance is essentially an account between the filter element and the fan. Calculating this account clearly during the selection stage will save a lot of trouble during operation.

Aili Industrial's manufacturing system covers filters, motors, fans, control systems and complete machine assembly, and can provide ULPA ultra-high efficiency filter and supporting air supply equipment. filter with When the FFU fan filter unit and EFU are matched in the same system, the initial resistance, final resistance of the filter element and the fan operating point can be calculated together at the planning stage to reduce air volume deviations after installation.

Providing design air volume, available static pressure and expected final resistance conditions when enquiring, can help confirm the appropriate configuration. Specific performance is subject to the testing data of the corresponding model.