The Effect of Pre-Filter and Main-Filter Media Matching on the Performance of an Ultra-High-Efficiency Two-Stage Filtration System

Abstract

Ultra-high-efficiency filter media ensure high air cleanliness but need periodic replacement due to pressure drop increase. Adding a pre-filter can extend the filtration system’s service lifetime, yet the pre-filter and main-filter matching lacks systematic study. This research explored how different filter media combinations affected a two-stage ultra-high-efficiency filtration system’s performance. This study focused on five pre-filter grades (F7–F9, H10, H11) and two different types of main-filter media, fiberglass and composite. It was found that the F8 pre-filter media had the best effect on extending the service lifetime of BX and BX + PTFE. The service lifetimes were extended from 44 min and 70 min to 231 min and 326 min, respectively, reaching 5.25 times and 4.65 times the original lifetime. Moreover, the optimal pre-filter grade was less affected by the initial resistance of the main filter. Meanwhile, the BX + PTFE filtration system always showed a longer service lifetime and dust holding capacity compared to the BX filtration system. This study provides guidance for the design and combination of two-stage filters in the context of high-cleanliness applications as well as the improvement of multi-stage filter performance in building ventilation.

1. Introduction

Fine particles (FPs) and ultrafine particles (UFPs) are easy to adsorb pollutants due to their small particle size and large specific surface area, which can rapidly invade the body through multiple pathways and induce oxidative stress and apoptosis, which are serious threats to human health [1,2,3,4,5]. As one of the safest and most effective means to ensure air cleanliness, ultra-high-efficiency filters are widely used [6,7,8,9]. However, the pressure drop of air filters will gradually increase with growing usage time; thus, they need to be replaced periodically [10,11,12,13], which involves multiple costs. And studies have shown that adding a low-efficiency pre-filter before passing through the high-efficiency main filter can reduce the pressure drop and extend the service lifetime of the high-efficiency filter [14,15,16,17]. Su-Gwang Jeong found that in the G4 + F8 + H13 filter system, the pre-filter media can more effectively reduce the external PM 2.5 concentration [18].

At present, there are many studies on ultra-high-efficiency filtration materials at home and abroad, but most of them focus on single filtration performance and influencing factors, such as material structure, pore size, pollutant characteristics, etc. [19,20,21,22,23], and there is less research on the overall performance of two-stage filtration systems and the mutual influence between all levels. In fact, in two-stage or multi-stage filtration systems as a whole, the performance of each level interacts with each other. Studies have shown that the pre-filter media has a protective effect on the main-filter media, but its effect is affected by the application environment and the matching between the pre-filter and the main filter. When 25% of the oil-coated particles are loaded, the addition of G4 pre-filter media actually shortens the service lifetime of the E11 PTFE main filter and the F9 melt-blown main filter [24]. Improper matching may not prolong the service lifetime of the main-filter media, and even in some cases, the pre-filter may change the particle size distribution, increase the proportion of fine particles, and shorten the service lifetime of the main-filter media instead [25].

In current practical applications, the combination of a two-stage or multi-stage ultra-high-efficiency filtration system often relies on empirical configurations and lacks a solid theoretical foundation. In view of this, it is not only of far-reaching academic significance but also of significant economic value to investigate the service lifetime and particle loading characteristics of ultra-high-efficiency two-stage filtration systems under different working conditions. In practice, on the one hand, different pre-filter media have different degrees of influence on a two-stage filtration system [26]. For example, studies have found that compared with G3 and M5 pre-filters, the G4 pre-filter has the best effect on improving the service lifetime of the main filter F9 without causing a rapid increase in pressure drop [27]; on the other hand, the main-filter media of the same grade also present a variety of choices, including glass fibers, chemical fibers, PTFE, composites, etc., whose different composition and manufacturing processes lead to different performance [28,29,30,31]. Tian [32] developed a two-stage filtration test system using G4 and E11 media and bimodal aerosol testing and found that the pre-filter extends the main-filter lifetime, the dust holding capacity of the system, and that the extension is affected by particle size. Tian [25] evaluated the performance of a two-stage indoor air filtration system and found that the pre-filter increased the lifetime of F9 melt-blown and E11 PTFE membrane-coated main-stage filter media but decreased the lifetime of F7 cellulose main-filter media, and that the combination of pre-filter and main-filter types should be carefully considered due to the size distribution of the aerosols. Nevertheless, the systematic study and discussion of the loading characteristics of ultra-high-efficiency two-stage filtration systems, which utilize various combinations of filter media and different main-filter types, have not been thoroughly conducted.

Therefore, in this study, an ultra-high-efficiency two-stage filtration test system was constructed to investigate the effects of pre-filter of different filtration grades on the loading performance of ultra-high-efficiency two-stage filtration systems, and the effects of pre-filter on ultra-high-efficiency two-stage filtration systems composed of different types of main filter were further investigated. The aim was to find the best solution to optimize the design of ultra-high-efficiency two-stage filtration systems to prolong the material service lifetime and increase the system dust holding capacity. This not only provides a basis for the design and combination of two-stage filters in the context of high cleanliness applications but also provides a reference for the performance improvement of multi-stage filters in the field of building ventilation.

2. Materials and Methods

2.1. Experimental Design and Materials

2.1.1. Experimental Design

The schematic diagram of the two-stage filtration test system is shown in Figure 1 and the image is shown in Figure 2. It consists of three parts: particle generation unit, filter media clamping unit and test unit. The particle generation system consists of an air compressor, a pressure-reducing valve, a drying tube, a mass flow controller, and an aerosol particle generator. Firstly, the high-pressure air from the air compressor is regulated to constantly low-pressure air by a pressure reducing valve and divided into two streams. Then, the two air streams pass through the drying tube and HEPA filter to form clean and dry compressed air. One of the air streams was regulated by a mass flow controller and then entered a constant flow atomized aerosol generator (Model 3076, Shoreview, MN, USA), which blew the 4% NaCl aqueous solution into a liquid aerosol, which passed through a diffusion dryer to form a solid NaCl aerosol, and the size distribution of the NaCl aerosol particles is shown in Figure 3. Subsequently, the particles enter the corona chamber of the electrostatic neutralizer (Model 1090, PALAS, Karlsruhe, Baden-Württemberg, Germany) and are ionized to produce a high concentration of positive and negative ions, ensuring that the dried aerosol reaches a Boltzmann charge equilibrium distribution when it enters the neutralizer and is fed into the mixing line. In addition, another air stream was adjusted by a mass flow controller and passed into the mixing line to dilute the aerosol in the line and to maintain a constant aerosol concentration inside the experimental line, with a mass concentration of NaCl aerosol of 25 ± 3 mg.m⁻³. Two pressure transducers (Model 166, Alpha Instruments, Acton, MA, USA) were used to record the pressure drop in the pre-filter and the main-filter media, respectively, and the collected data were transferred to a computer via a data acquisition card. The experimental piping was a square pipe with a side length of 10 cm, and the effective test area of both the pre-filter and main-filter media was 100 cm². Each test condition was repeated at least three times, averaged, and plotted in all tables and figures.

2.1.2. Filter Media

According to EN1822-1:1998 and EN779-2012 standards, five kinds of glass fiber filter media with different filtration efficiencies (F7, F8, F9, H10, H11) were selected as pre-filter media, and two kinds of filter media with the same filtration efficiency, including U15 of glass fiber (BX-U15) and U15 composite of glass fiber and PTFE (BX + PTFE-U15), were selected as main-filter media. The glass fiber filter media were from Chongqing Zaisheng Technology Co., Ltd. (Changzhou, China). and the BX + PTFE-U15 was from Changzhou Lianying Environmental Protection Technology Co. (Chongqing, China).

Table 1. Characterization of pre-filter and main-filter media.

Filter Samples	Grade	Filter Media Thickness (μm)	Average Weight (g/m2)	Average Pore Size (μm)	Filtration Efficiency (%)	Initial Resistance (Pa)
Pre-filter	F7	362	77.7	9.3	55	28
	F8	339	75.7	9.2	65	33
	F9	324	74.7	5.8	85	63
	H10	302	74.6	4.1	96	107
	H11	328	78.0	3.9	99	151
Main-filter	U15(BX)	348	76.6	1.8	99.9998	363
	U15(BX + PTFE)	541	138.5	1.1	99.9996	221

lists the main characteristics of the pre-filter media and main-filter media required for the experiment. Figure 4 shows the electron microscope scanning diagram of the pre-filter media after magnification by 1000 times, and Figure 5 shows the electron microscope scanning diagram of the main-filter media after magnification by 1000 times. The initial filtration efficiency and resistance of the filter media were measured by an automatic filter tester (8130A, TSI, Shoreview, MN, USA), and the pore size of the filter media was measured by a capillary flow rate porosimeter (CFP-1100-A, Porous Materials, Inc., Ithaca, NY, USA); the mass of the filter media was measured by an analytical balance (MR104, Mettler Toledo, Zurich, Switzerland); and the thickness of the filter media was measured by a micro-computerized thickness tester (IMT-HK210D, IMT, Dongguan, Guangdong, China) to measure the thickness of the filter media.

2.2. Loading Test Procedure

In practical applications, it is common for pre-filter media to be replaced more frequently than the main-filter media. This observation was illustrated in our two-stage filtration tests, during which the pre-filter samples underwent uniform replacement three times. The complete procedure for the two-stage filtration tests is outlined in detail, as depicted in Figure 6.

Step 1: (a) Install new pre-filter and main-filter samples; (b) introduce NaCl aerosol particles until the total resistance reaches 600 Pa; (c) remove the pre-filter sample. Document the parameters: the loading duration in this step, T₁; the resistance of the main-filter media at the conclusion, P_2,1; and the dust holding capacity of the pre-filter media, m_1,1.

Step 2: (a) Substitute the old pre-filter sample with a fresh one, while keeping the main filter intact; (b) continue to load particles until the total resistance levels at 600 Pa; (c) remove the pre-filter sample again. Note the parameters: the loading duration in this step, T₂; the main-filter media’s resistance at the end, P_2,2; and the dust holding capacity for the pre-filter media, m_1,2.

Step 3: Execute Step 2 once more. Record the parameters: the loading duration in this step, T₃; the main-filter media’s resistance at the end, P_2,3; and the dust holding capacity of the pre-filter media, m_1,3.

Step 4: (a) No additional pre-filter is utilized, maintaining only the main filter; (b) continue loading particles until the resistance of the main filter reaches 600 Pa; (c) extract the main-filter sample. Note the parameters: the loading duration in this step, T₄; and the dust holding capacity of the main-filter sample, m₂.

2.3. Evaluation Parameters of Dust Holding Performance of Two-Stage Filtration Systems

In order to make the comparison between the single-stage and two-stage filtration systems more convenient, Tian [32] put forward some customized experimental parameters (T*, T**, and P**). These parameters were employed to assess the influence of the pre-filter on the dust holding capacity and lifetime of not only the main filter but also the two-stage filtration systems. The details are presented as follows:

$$T^{\ast }={\displaystyle \frac{\left(T_{1–3}\right)}{{\mathrm{T}}_{\mathit{ref}}^{‘}}}$$

(1)

T_1–3 indicates the moment when the initial three phases of the dust holding performance assessment for the two-stage filtration system are finalized, at which point the resistance of the main-filter media reaches P_2,3 after these first three phases. T′_ref signifies the moment the resistance in the single-stage system’s main-filter media achieves P_2,3. Thus, T* indicates how the pre-filter media’s performance during the initial three phases impacts the operational lifespan of the main-filter media. A higher value of T* is associated with an extended lifespan for the main-filter media. Nevertheless, the final phase, which lacks the protective function of the pre-filter, does occur in real-world scenarios. To explore the influence of pre-filter media throughout the entire two-stage filtration system, including Step 4 where pre-filter protection ceases, T** has been defined.

$$T^{\ast \ast }={\displaystyle \frac{T_{total}}{T_{ref}}}$$

(2)

$$T_{total}=T_1+T_2+T_3+T_4$$

(3)

$$M_{total}=m_{\mathrm{1,1}}+m_{\mathrm{1,2}}+m_{\mathrm{1,3}}+m_2$$

(4)

Here, T_total represents the completion time for the dust holding performance test of the entire two-stage filtration system, while M_total denotes the overall dust holding capacity of the same system, and T_ref indicates when the dust holding performance test for the single-stage main filter ends. T** illustrates the impact of the pre-filter on prolonging the service lifetime of the main-filter media at the end of the complete experimental timeline. Several factors affect T**, such as the type of filter media used, the surface flow rate, and the replacement schedule for the pre-filter media. In practice, the pre-filter can be exchanged multiple times, with each instance likely resulting in a further increase of the T** value. However, increasing the frequency of pre-filter changes may also lead to an escalation in the initial resistance of the main filter, which gradually diminishes the pre-filter’s lifespan and its protective efficiency. Therefore, replacing pre-filters too frequently may prove unnecessary. To determine an optimal replacement interval, P** is introduced.

$$P^{\ast \ast }=\left({\displaystyle \frac{P_{\mathrm{2,3}}-P_0}{P_{final}-P_0}}\right)$$

(5)

P** is defined as the proportion of the resistance increment of the main-filter media in the initial three steps to the resistance increment of the main filter over the whole process of the two-stage filtration system. When the value of P** is low, it implies that the pre-filter media has the potential to be substituted more frequently. Nevertheless, various aspects, such as the expenses related to the pre-filter media, the cost of labor, and other relevant factors, also need to be taken into account.

3. Results and Discussion

3.1. Influence of Pre-Filter Grade on the Dust Holding Performance of Two-Stage Filtration Systems

In the present research, BX-U15 served as the main-filter media. On the other hand, F7, F8, F9, H10, and H11 were chosen as the pre-filter media. The aim was to assess how effectively these pre-filter media could prolong the total lifetime of the main filter within the two-stage filtration system. Figure 7a illustrates the resistance variation curves of each filter media during the single-stage dust holding experiments. Figure 7b–f shows the resistance variation curves of the main filter in the two-stage filtration system under the protection of F7, F8, F9, H10, and H11 pre-filter media, respectively. From Figure 7b–f, it can be observed that, in different two-stage filtration system configurations, the resistance of the main filter increases from 366 Pa to 394 Pa, 434 Pa, and 485 Pa (corresponding to three replacement cycles of the pre-filter media) when F7 is used as the pre-filter. The resistance growth values are 28 Pa, 40 Pa, and 51 Pa, respectively, with the third cycle’s resistance increase being 1.82 times that of the first cycle. When the pre-filter is F8, the resistance growth values are 21 Pa, 26 Pa, and 32 Pa, and the resistance increase in the third cycle is 1.52 times that of the first cycle. When the pre-filter is F9, the resistance growth values are 7 Pa, 7 Pa, and 6 Pa, respectively. When the pre-filter is H10, the resistance growth values are 3 Pa, 2 Pa, and 2 Pa, respectively. Finally, when the pre-filter is H11, the resistance growth values are all 1 Pa. As the filtration grade of the pre-filter increases, its filtration efficiency also increases, leading to more aerosols being intercepted by the pre-filter media. Consequently, the resistance increases in the main filter decreases as the pre-filter grade progresses from F7 to H11. Additionally, analysis of the single-stage dust holding resistance curves of the filter media reveals that, as the dust holding capacity of the main-filter media increases, the internal pore structure of the filter media becomes blocked, resulting in a higher rate of resistance growth. In a two-stage filtration system, when the pre-filter is F7, F8, or F9, the resistance growth of the main filter increases as the number of particles intercepted by the main filter rises during the first three stages when the pre-filter is present. In contrast, when the pre-filter is H10 or H11, the resistance increases of the main filter are lower because the majority of particles are effectively intercepted by the pre-filter media.

From Figure 8a, it can be observed that when the pre-filter media grades are F7, F8, F9, H10, and H11, the corresponding overall lifetimes of the two-stage system are 197 min, 231 min, 193 min, 146 min, and 112 min, respectively. In comparison to the 44 min of the single-stage main-filter media, the overall lifetime of the two-stage system is improved by 4.48, 5.25, 4.39, 3.32, and 2.55 times, respectively.

From the comparison in Figure 8b, it can be seen that when the pre-filter media grade is increased from F7 to F8, both the overall lifetime and dust holding capacity of the two-stage filtration system increase. However, when the pre-filter media grade is increased from F8 to H11, both the overall lifetime and dust holding capacity begin to decrease. When the pre-filter media grade is F8, the two-stage filtration system achieves the highest overall lifetime (231 min) and dust holding capacity (18.86 g/m²), which are 5.25 and 5.11 times greater, respectively, than the values of 44 min and 3.69 g/m² when using the main filter alone.

This is because in the two-stage filtration system, when pre-filter media grade is F7 or F8, the resistance of the main-filter media is significantly larger than that of the pre-filter media. Increasing the pre-filter media grade reduces the number of particles reaching the main-filter media, slowing the increase in resistance and thus extending the overall lifetime of the system in terms of dust holding capacity. However, when the pre-filter media grade is increased to F9, the resistance of the pre-filter media becomes a larger proportion of the total resistance of the two-stage filtration system. At this point, the increase in resistance from the pre-filter media dominates the dust holding process, leading to a decrease in both the overall lifetime and dust holding capacity of the system. Therefore, an optimal pre-filter media grade, F8, exists that provides the best balance between system lifetime and dust holding capacity.

Figure 9 illustrates the evaluation parameters of the two-stage filtration system, as specified in Equations (1)–(5). For each of the five different grades of pre-filter media, the values of both T* and T** are above 1. This indicates that all five types of pre-filter media offer protection to the main filter and are capable of prolonging the service lifetime of the U15 main filter. As depicted in Figure 9, when combined with the pre-filter media F7, F8, F9, H10, and H11, respectively, the T* values of the two-stage filtration system rose from 6.96 to 45.33 as the grade of the pre-filter media increased from F7 to H11.

This is because, as the pre-filter grade increases, the filtration efficiency of the pre-filter media improves, resulting in fewer particles reaching the main filter. This leads to a reduced growth in the resistance of the U15 main filter, yielding the highest T* values upon completion of the first three steps of the dust holding experiment. Similarly, the pre-filter grade H11 has the lowest P** value of 1.76%, while F7 has a P** value of 49.33%, as the resistance of the main filter protected by the H11 pre-filter increases the slowest during the first three loading steps. However, as the pre-filter grade increases from F7 to F8 to F9 to H10 to H11, the T_1–3 values of the two-stage filtration system increase from 174 to 202, then begin to decrease to 153, 104, and 68, respectively. This is because, with higher grades of pre-filter protection, the resistance of the main filter increases more slowly. As a result, T_1–3 increases when the pre-filter grade is increased from F7 to F8, but when the pre-filter grade is raised from F8 to H11, the resistance increase of the pre-filter itself becomes more significant, causing the T_1–3 values to decrease.

T_1–3, T* and P** were used to evaluate the dust holding performance of the two-stage filtration system during the first three steps of loading, and the whole loading process also included the fourth step, i.e., the loading of the main-filter media was continued after removing the pre-filter media. T** was used to evaluate the dust holding performance of the two-stage filtration system after the whole four-step loading process, and the values of T** for the systems under the pre-filter of F7, F8, F9, H10, and H11 were 4.48, 5.25, 4.39, 3.32, and 2.46, respectively. For the same two-stage loading case, it can be noticed that the value of T** is smaller than the value of T*, which is due to the fact that the pre-filter is no longer replaced in the fourth loading step, but the main filter is loaded separately, thus reducing the protection effect of the pre-filter on the main filter in the first three steps. Since the final stage of loading without replacing the pre-filter always exists in practical applications, T** can be better used to evaluate the protection effect of the pre-filter on the whole two-stage filtration system compared to T*, and within the range of the main-filter grade and material in this study, when the pre-filter grade is F8, it is the best protection for the two-stage filtration system as a whole.

3.2. Influence of Main-Filter Type on the Dust Holding Performance of Two-Stage Filtration Systems

This section examines the impact of different main-filter types on the dust holding capacity and service lifetime of the two-stage filtration system. As shown in Figure 10a, the BX + PTFE main filter exhibits a lower initial resistance compared to the BX main filter, leading to a higher dust holding capacity. Figure 10b–f displays the resistance curves of the main filters in the two-stage filtration system for two different main-filter types (F7, F8, F9, H10, H11 + U15). The lifetimes of the BX and BX + PTFE main filters are 198 min and 301 min, respectively, when the pre-filter is F7; 231 min and 326 min, respectively, when the pre-filter is F8; 193 min and 272 min, respectively, when the pre-filter is F9; 146 min and 144 min, respectively, when the pre-filter is H10; and 112 min and 228 min, respectively, when the pre-filter is H11. Under the condition of the same pre-filter grade, because the number of particles arriving at the main filter through the pre-filter is basically the same, the resistance growth rate of the main filter at each step of loading is basically the same as that of the main filter when it carries out the single-stage dust holding. The BX + PTFE main filter has a lower initial resistance, so the two-stage filtration system shows a longer service lifetime when the main filter is BX + PTFE.

From Figure 11, it is observed that when the main-filter type is the BX + PTFE filter, increasing the pre-filter grade from F7 to H11 results in an overall service lifetime increase from 301 min to 326 min, followed by a decrease to 272 min, 236 min, and 215 min. The longest service lifetime is achieved when the pre-filter grade is F8. As discussed in Section 3.1,when the pre-filter grade is increased from F7 to H11, the optimal pre-filter grade for enhancing the service lifetime of the BX two-stage filtration system is F8. Compared to the BX main filter, although the BX + PTFE main filter has a lower initial resistance, the rate of resistance growth during the dust holding process is similar, meaning that the optimal pre-filter grade for the BX + PTFE two-stage filtration system remains F8.

From Figure 12, it can be observed that for the BX + PTFE main filter, the T* and T** values are greater than one for all five grades of pre-filter media, indicating that each of the pre-filter media serves a protective function for the BX + PTFE main filter. When the main-filter type is BX, T* increases from 6.96, 11.22, 25.5, 34.67 to 45.33 as the pre-filter grade increases from F7 to H11; when the main-filter type is BX + PTFE, T* increases from 9.37, 13.5, 26.38, 56.67 to 146 as the pre-filter grade increases from F7 to H11. The T* value of the BX + PTFE main filter was consistently higher than that of the BX main filter when protected by the same pre-filter media, as the BX + PTFE main filter exhibited lower initial resistance and a longer dust holding time before the system reached the termination resistance of 600 Pa. T**, which is used to evaluate the dust holding performance of the two-stage filtration system after the entire four-step loading process, increased from 4.3 to 4.66 for the BX + PTFE main filter as the pre-filter grade was increased from F7 to H11, and then decreased to 3.89, 3.37, and 3.07. For both the BX main filter and the BX + PTFE main filter, the overall system service lifetime was longest when the pre-filter grade was F8. Consequently, the initial resistance of the main filter has less impact on the optimal pre-filter grade, and F8 is recommended as the optimal pre-filter grade for all main filters exhibiting similar resistance growth rates. It is important to note that, under the protection of the optimal pre-filter F8, although the T** of the BX + PTFE main filter (4.66) is lower than that of the BX main filter (5.25), the BX + PTFE system still exhibits the longest service life of 326 min when considering the overall system performance. This is because, during the single-stage main-filter dust holding experiments, the BX + PTFE main filter demonstrates a longer service lifetime. Since T** is the ratio of the system service lifetime to the service lifetime of the single-stage main filter, the T** value of the BX + PTFE system is lower. Therefore, in practical applications, when selecting the appropriate main-filter type, one should not solely rely on the T** parameter.

4. Conclusions

In this study, a series of experiments were conducted to investigate the effect of pre-filter media grade on the dust holding characteristics of a two-stage filtration system, as well as the impact of the main-filter media type on these characteristics. Comparative analyses were performed using T_1–3, T*, T**, and P** values. The results revealed that, for the BX-U15 main filter, although increasing the pre-filter grade from F7 to H11 reduced the resistance increase of the main-filter media during the first three loading steps, the overall resistance of the two-stage filtration system began to rise when the pre-filter grade exceeded F9, due to the higher resistance of the pre-filter. This increase in resistance diminished the system’s lifetime extension benefits. The pre-filter media with a grade of F8 was found to have the most significant effect on improving the main filter’s lifetime. Therefore, the optimal pre-filter media must not only have high filtration efficiency to reduce the number of particles reaching the main filter but also maintain low resistance during the dust holding process to provide adequate protection. Selecting F8 as the optimal pre-filter grade in two-stage systems has notable economic perks. It is cheaper initially than higher grades, like H10 or H11. It extends main-filter lifetime, cutting replacement costs. Its low resistance saves energy, reducing electricity bills. Less frequent replacements mean less system downtime. Overall, F8 balances filtration performance and costs well.

The effect of different main-filter types on the dust holding performance of the two-stage filtration system was then investigated. It was found that the BX + PTFE two-stage filtration system exhibited a longer service lifetime compared to the BX system across different pre-filter grades, which was attributed to the lower initial resistance of the BX + PTFE main filter compared to the BX main filter. Additionally, the optimal pre-filter grade was found to be primarily related to the resistance growth rate. For a two-stage filtration system, if the main filter exhibits the same resistance growth rate, the pre-filter grade can be selected as F8. However, the greater thickness of the composite filter media may increase the complexity and cost of the filter in the process of cutting, fixing and other processing. Furthermore, space constraints on practical applications may prevent the installation of thicker filters. Therefore,- for industry professionals, it is recommended to use F8 as the pre-filter in an ultra-high-efficiency two-stage filtration system to achieve the best balance between filtration performance, cost savings and energy consumption; while when selecting the main filter, BX + PTFE should be considered to extend its service lifetime, while also considering potential additional processing costs and space limitations.