Does increasing filter thickness definitely reduce resistance?

When selecting a filter, we often hear an empirical saying: a thicker filter has lower resistance and longer life. This statement has a certain origin, but it is not entirely accurate. Thickness is only a structural parameter, and the pressure drop is determined by the characteristics of the filter material, folding structure, filter area, air volume and internal support. For filters of the same thickness, the resistance may vary greatly due to different filter materials and folding methods; increase the thickness but if the folding is too dense or the passage is restricted, the resistance may not decrease but increase.

The following will start from the relationship between thickness and area, folding structure, filter material characteristics, and comparison methods.

Thickness and effective filter area are not equivalent

Increasing the filter depth can theoretically accommodate more pleated filter media, thereby increasing the expansion area. However, the actual effective filtration area also depends on the folding density, partition structure and packaging method. If the folds are too dense, the channels between the filter materials will become narrower and the friction loss will increase when the airflow passes; if the partition design is unreasonable, some filter materials may not be able to effectively participate in filtration.

Therefore, an increase in thickness may only increase the filtering area, but does not mean a certain increase. When judging, you should look at the unfolded area and windward area of ​​the specific model of filter material, rather than just looking at the thickness.

If you are still unclear about the effective area diameter, you can see How should we look at the effective air outlet area of ​​FFU? and How to convert face wind speed and air volume? , first clarify the definition of area, and then compare models with different thicknesses.

Folded structure affects resistance and channeling

The pleated structure determines the flow path of airflow between filter materials. Common structures include:

• Deep pleated structure : The folding depth is large and the filter material unfolds in a large area, but the channel resistance also needs to be checked;

• Dense pleat structure : The folds are denser per unit volume, the area increases, but the channel becomes narrower;

• Partition structure : Use partitions to maintain channels and reduce filter material adhesion, but increase structural thickness and resistance;

• No-partition structure : The structure is more compact, but the filter material support and channel stability need to be checked.

Different structures may have different resistance and dust holding capacity under the same thickness. When comparing, you should look at the resistance curve of the specific model, not sort by thickness.

The characteristics of filter material determine the basis of resistance

The fiber diameter, porosity and weight of the filter material itself determine the basic resistance. Different filter materials may have different resistances under the same area and air volume. Higher efficiency filter media usually have denser fibers and higher resistance.

If you are still unclear about the relationship between filtration level and efficiency, you can read How to choose between H13, H14 and U15? and How do HEPA and ULPA match cleanliness goals? , first understand the relationship between efficiency level and filter material structure.

Resistance data should be compared under the same working conditions

When comparing filters of different thicknesses, the following conditions should be unified:

• Same air volume;

• Same efficiency level;

• Same windward area or installation dimensions;

• Same testing methods and standards;

• The same definition of initial resistance and final resistance.

Only when these conditions are unified can the resistance data be comparable. If the air volume is different, the efficiency is different, or the test caliber is different, direct comparison cannot be made.

If you are not clear about the relationship between rated air volume and operating air volume, you can see How does the rated air volume of the filter match the actual operating air volume? , first confirm whether the working conditions are consistent.

Increased thickness may change installation and airflow conditions

Increasing the filter thickness may also affect the airflow path inside the device. If a thicker filter occupies the inlet or outlet space, it may change the airflow distribution and even increase local resistance. The way the seal is installed may also need adjustment.

When it comes to equipment cavity, please refer to How to plan clean air supply for closed equipment cavities? , first confirm the cavity boundary and airflow path. If the air volume drops after replacement, the fan operating point should be checked instead of directly determining whether the filter is better or worse.

Effects of head-on wind speed and insufficient area

If the filter area is insufficient, the head wind speed will increase and the resistance will increase. At this time, the thickness is only increased but the effective filtration area is not increased, and the resistance may still be high. For details, please refer to How does insufficient filter area affect fan selection? and What will be the impact if the wind speed facing the filter is too high?。

Performance under initial resistance and final resistance

If the increase in thickness does increase the filtration area, the initial resistance may decrease, the dust holding capacity may also increase, and the time to reach the final resistance will be prolonged. However, if the folding is too dense or the filter materials are different, the initial resistance may be higher and the final resistance characteristics may also be different.

When selecting the model, you should check the working points under the initial resistance and final resistance. For details, please refer to What are the initial resistance and final resistance used to judge? . If you only look at one thickness parameter, you cannot judge the performance of the entire service life.

Fan operating point matches the system

After replacing the filter, the fan operating point will change with the resistance. If the resistance of the new filter is lower, the air volume may increase; if the resistance is higher, the air volume may decrease. When selecting the model, you should check the operating point under the target air volume. You can refer to Why should we look at the complete performance curve when selecting FFU? and What does FFU external static pressure mean?。

If the resistance of the new filter decreases but the air volume increases, you need to confirm whether the wind speed at the workstation is too high. Please refer to Is excessive FFU air volume good for cleanliness?。

Project judgment example

One project was to replace the filter with a thicker one, thinking the resistance would be reduced. After installation, it was found that the pressure difference increased and the air volume decreased. The investigation found that the thickness of the new filter increased, but the folds were more dense, the channel became narrower, and the actual resistance was higher. Adjustment method: Change to a model with a more reasonable folding structure and a larger filter area, and check the fan working point. After adjustment, the pressure difference and air volume are restored.

In another project, after replacing the thick filter, the resistance did drop and the air volume increased, but the wind speed at the workstation was too high. Adjustment method: Reduce the fan speed and check the wind speed distribution. After adjustment, the wind speed and energy consumption are within a reasonable range.

These two situations illustrate that increasing the thickness of the filter does not necessarily reduce the resistance. It should be comprehensively judged based on the filter material, folding structure, filter area and fan operating point.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

The relationship between filter thickness and resistance cannot be summarized as 'the thicker, the better' or 'thinner, the worse'. Aili Industrial has the capabilities of filter material, folding structure and packaging in the air filter manufacturing process. Based on the target air volume, installation space and allowable resistance of the project, it can compare the initial resistance, final resistance and dust holding performance of different thicknesses and different folding structure models under the same working conditions, and judge whether the air volume matches after replacement based on the fan working point. Specific filter parameters can be viewed HEPA and ULPA filters , the complete machine package can be viewed FFU clean air supply products and EFU equipment side fan filter unit。