Dust Loading Performance of a Novel Submicro-Fiber Composite Filter Medium for Engine

Airborne dust can cause engine wear and contribute to engine gas emission. This study developed a novel submicro-fiber filter medium to provide protection to engines against dust. The wet-laid submicro-fiber medium was prepared by a dual-layer paper machine, and its dust loading performance was compared with other filter media during laboratory and field tests. During the laboratory tests, the dust holding capacity of the wet-laid submicro-fiber medium was 48% and 10% higher than that of the standard heavy-duty medium and electrospun submicro-fiber medium, respectively. During the field tests, the pressure drop of the wet-laid submicro-fiber filter was 45% lower than that of the standard heavy-duty filter after 10,000 km of operation. It was found that there were two crucial ways to design a better filter medium for protection against dust. Firstly, the surface loading rather than the depth loading was preferred for dust filtration. The submicro-fiber layer kept large amounts of dust particles from penetrating into the depth of filter medium. Secondly, particles were captured preferably by fibers rather than pores. The unique fibrous structure of the wet-laid submicro-fiber medium made more particle deposition take place on fibers via interception and inertial impaction.


Introduction
Arid and semi-arid areas, which cover approximately one-third of land surfaces, are a major source of mineral dust [1]. If meteorological conditions are favorable, the dust is mobilized and further emitted into the atmosphere. Besides natural processes, a significant amount of dust contaminants are generated by anthropogenic activities, such as farming, industrial manufacturing and mining operations [2]. For motor vehicles, dust suspended in the air is hazardous and can degrade vehicle performance because the combustion engine of the vehicle is exposed to large quantities of air. Airborne dust (quartz) is highly abrasive and is the most common cause of high wear of critical parts, such as piston rings and cylinder walls in engines [3]. Studies have shown that engine life is dependent upon the cleanliness of the air taken in [4]. Engine wear is produced by particles in the size range of 1-40 µm, where the most harmful particles are in the range of 1-20 µm [5]. Dust particles may also significantly contribute to the total engine emissions, including crankcase emissions [6]. According to Schilling [7], 30% of contaminants entering the engine can pass out of the exhaust.
Air filtration is applied to provide protection against the effects of contaminated air. The effects of the engine intake air filter can be both positive and negative [8]. The positive side is to prevent the decrease in engine performance, however, placing an air filter in the path of air coming into the This dual-channel headbox had been patented by the authors [31]. In the dual-channel headbox, fluids in both channels were stabilized by a step diffuser and then became a steady laminar flow. These two channels were separated by a plate so that the submicro-fiber slurry did not mix with the microfiber slurry. Before entering the water drainage section, the slurries in the two channels converged at the end of the plate and were kept separated without mixing due to the steady laminar flow. Water from the submicro-fiber slurry drained first, and then the submicro-fiber deposited on the forming screen. The forming screen was a multilayer wire of PET and nylon fiber strands with a top layer pore size of 100 µm. After the submicro-fiber layer formed, microfibers started to deposit on the submicro-fiber and the dual-layer paper sheet was formed. The crucial part of the drainage section was to keep the water drainage process steady and prevent the dual-layer structure from being damaged. To achieve this goal, the submicro-fiber layer was put underneath the microfiber layer because the submicro-fiber deposition on the forming screen can slow down the initial waterdraining rate and thus, make the water drainage steadier.
The wet paper sheet was then transferred to the drying and coating section, as shown in Figure  2. The dried paper from the first drying section was transferred to the gap of two resin-coating rolls. The lower roll brought 20 wt.% waterborne acrylic polymer (PR-36, Wuwei Co., Ltd., Guangzhou, China) from the resin tank to saturate the paper. This polymer can enhance the strength of the paper, especially the weak submicro-fiber layer. The second drying section was used to dry the coated paper. A major problem that existed during the drying of the coated porous material was that the resin could migrate from the inner part of paper to the surface when heated [32] and gradually clog the surface pores. Pore clogging can remarkably reduce the filtration performance and cancel out the advantages of the submicro-fiber. To mitigate binder migration, the temperature of each dryer in the second drying section was accurately controlled. The cylinder surface temperatures of dryer 4, 5, and 6 were 75 ± 5 °C, 105 ± 5 °C, and 125 ± 5 °C, respectively. The resin content in the final dried paper was 20%. After drying and winding, the preparation of the submicro-fiber medium was complete.
Step This dual-channel headbox had been patented by the authors [31]. In the dual-channel headbox, fluids in both channels were stabilized by a step diffuser and then became a steady laminar flow. These two channels were separated by a plate so that the submicro-fiber slurry did not mix with the microfiber slurry. Before entering the water drainage section, the slurries in the two channels converged at the end of the plate and were kept separated without mixing due to the steady laminar flow. Water from the submicro-fiber slurry drained first, and then the submicro-fiber deposited on the forming screen. The forming screen was a multilayer wire of PET and nylon fiber strands with a top layer pore size of 100 µm. After the submicro-fiber layer formed, microfibers started to deposit on the submicro-fiber and the dual-layer paper sheet was formed. The crucial part of the drainage section was to keep the water drainage process steady and prevent the dual-layer structure from being damaged. To achieve this goal, the submicro-fiber layer was put underneath the microfiber layer because the submicro-fiber deposition on the forming screen can slow down the initial water-draining rate and thus, make the water drainage steadier.
The wet paper sheet was then transferred to the drying and coating section, as shown in Figure 2. The dried paper from the first drying section was transferred to the gap of two resin-coating rolls. The lower roll brought 20 wt.% waterborne acrylic polymer (PR-36, Wuwei Co., Ltd., Guangzhou, China) from the resin tank to saturate the paper. This polymer can enhance the strength of the paper, especially the weak submicro-fiber layer. The second drying section was used to dry the coated paper. A major problem that existed during the drying of the coated porous material was that the resin could migrate from the inner part of paper to the surface when heated [32] and gradually clog the surface pores. Pore clogging can remarkably reduce the filtration performance and cancel out the advantages of the submicro-fiber. To mitigate binder migration, the temperature of each dryer in the second drying section was accurately controlled. The cylinder surface temperatures of dryer 4, 5, and 6 were Materials 2018, 11, 2038 4 of 17 75 ± 5 • C, 105 ± 5 • C, and 125 ± 5 • C, respectively. The resin content in the final dried paper was 20%. After drying and winding, the preparation of the submicro-fiber medium was complete. The running speed of the pilot-scale paper machine was 5 m/min. The basic weight of the filter paper was 120 ± 3 g/m 2 . The full width of the final paper sheet was 0.7 m after cutting 0.05 m on both edges. Filter paper samples for this study were taken 1 h after stable operation of the paper machine was achieved.

Loading Performance Evaluation in Laboratory-Filter Media Specifications
The loading performance of the wet-laid submicro-fiber composite medium was compared to the two other filter media. The standard heavy-duty filter medium (H&V Inc., East Walpole, MA, USA) is currently used in many inlet filters of engineering machinery. The electrospun composite medium (Fibrway Co., Ltd., Guangzhou, China) is known for its outstanding performance in many studies [19,20]. The submicro-fiber in the electrospun medium was made from nylon-66 (Elmarco, Liberec, Czech Republic). The basic properties of these filter media are described in Table 1. The micro-structure of these filter media was observed by scanning electron microscope (Phenom TM , Phenom-World B.V. Inc., Eindhoven, The Netherland). The pore size was measured by capillary flow porometer (model: CFP-1100A, Porous Materials Inc., Ithaca, NY, USA).

Loading Test Setup in Laboratory
The loading test rig (MFP 3000, Palas GmbH, Karlsruhe, Germany) was used to test the dust holding capacity according to the ISO 5011 [36] standard, as shown in Figure 3.
The particles used in the test were ISO 12103-1 [38] A2 fine dust (Powder Technology Inc., Arden Hills, MN, USA). The volume mean diameter of A2 fine dust was approximately 10 µm, which was suitable to evaluate the filter medium for the engine. When the loading test started, dry A2 dust was  The running speed of the pilot-scale paper machine was 5 m/min. The basic weight of the filter paper was 120 ± 3 g/m 2 . The full width of the final paper sheet was 0.7 m after cutting 0.05 m on both edges. Filter paper samples for this study were taken 1 h after stable operation of the paper machine was achieved.

Loading Performance Evaluation in Laboratory-Filter Media Specifications
The loading performance of the wet-laid submicro-fiber composite medium was compared to the two other filter media. The standard heavy-duty filter medium (H&V Inc., East Walpole, MA, USA) is currently used in many inlet filters of engineering machinery. The electrospun composite medium (Fibrway Co., Ltd., Guangzhou, China) is known for its outstanding performance in many studies [19,20]. The submicro-fiber in the electrospun medium was made from nylon-66 (Elmarco, Liberec, Czech Republic). The basic properties of these filter media are described in Table 1. The micro-structure of these filter media was observed by scanning electron microscope (Phenom TM , Phenom-World B.V. Inc., Eindhoven, The Netherland). The pore size was measured by capillary flow porometer (model: CFP-1100A, Porous Materials Inc., Ithaca, NY, USA).

Loading Test Setup in Laboratory
The loading test rig (MFP 3000, Palas GmbH, Karlsruhe, Germany) was used to test the dust holding capacity according to the ISO 5011 [36] standard, as shown in Figure 3.
The particles used in the test were ISO 12103-1 [38] A2 fine dust (Powder Technology Inc., Arden Hills, MN, USA). The volume mean diameter of A2 fine dust was approximately 10 µm, which was suitable to evaluate the filter medium for the engine. When the loading test started, dry A2 dust was dispersed by compressed air (0.6 MPa), and then neutralized to achieve Boltzman charge equilibrium by an ion stream from a high-voltage neutralizer. Make-up air was mixed with particles in the mixing chamber to achieve a final particle mass concentration of 1.0 mg/m 3 . The filter medium clamped in the filter holder with a test area of 100 cm 2 was challenged by the dust particles under a constant flow rate controlled by a mass flow controller. The testing face velocity was 11.1 cm/s, which was close to the operating velocity in real applications [39]. The test stopped after the pressure drop reached 2 kPa. During the test, the temperature was 21 • C and the relative humidity (RH) was 50 ± 10%. For every sample, the loading test was repeated 5 times. dispersed by compressed air (0.6 MPa), and then neutralized to achieve Boltzman charge equilibrium by an ion stream from a high-voltage neutralizer. Make-up air was mixed with particles in the mixing chamber to achieve a final particle mass concentration of 1.0 mg/m 3 . The filter medium clamped in the filter holder with a test area of 100 cm 2 was challenged by the dust particles under a constant flow rate controlled by a mass flow controller. The testing face velocity was 11.1 cm/s, which was close to the operating velocity in real applications [39]. The test stopped after the pressure drop reached 2 kPa. During the test, the temperature was 21 °C and the relative humidity (RH) was 50 ± 10%. For every sample, the loading test was repeated 5 times.
where S is the test area of filter medium. Because the gravimetric efficiency for all filter media was close to 100%, as shown in Table 1, it was considered that all particles were deposited on the filter media. Dust mass deposit per unit area, Mi, at loading time of ti can be calculated by Equation (2).
where ttotal is the total loading time (min). The curves of pressure drop versus dust mass deposit per unit area were used to analyze the loading performance of the different media. Two parameters obtained from the pressure drop curves were considered for discussion: Dust holding capacity and energy consumption. Dust holding capacity was defined as the dust mass deposit per unit area when the pressure drop reached 2 kPa. The energy consumption of different filter media under a fixed amount of dust mass deposit can be compared by calculation methods described in Eurovent 4/21 [9].
The loading performance of the wet-laid submicro-fiber composite medium and two commercial filter media was evaluated in the laboratory. Because the wet-laid submicro-fiber composite medium The filter medium was weighed when it was clean (M 0 ) and fully loaded (M 1 ). The final dust mass deposit, M final , was calculated by Equation (1).
where S is the test area of filter medium. Because the gravimetric efficiency for all filter media was close to 100%, as shown in Table 1, it was considered that all particles were deposited on the filter media. Dust mass deposit per unit area, M i , at loading time of t i can be calculated by Equation (2).
where t total is the total loading time (min). The curves of pressure drop versus dust mass deposit per unit area were used to analyze the loading performance of the different media. Two parameters obtained from the pressure drop curves were considered for discussion: Dust holding capacity and energy consumption. Dust holding capacity was defined as the dust mass deposit per unit area when the pressure drop reached 2 kPa. The energy consumption of different filter media under a fixed amount of dust mass deposit can be compared by calculation methods described in Eurovent 4/21 [9].
The loading performance of the wet-laid submicro-fiber composite medium and two commercial filter media was evaluated in the laboratory. Because the wet-laid submicro-fiber composite medium was a dual-layer structure, loading tests using the microfiber or submicro-fiber layer at the inlet were conducted to analyze the difference in filtration performance, as shown in Figure 4.
Materials 2018, 11, x FOR PEER REVIEW 6 of 17 was a dual-layer structure, loading tests using the microfiber or submicro-fiber layer at the inlet were conducted to analyze the difference in filtration performance, as shown in Figure 4.

Loading Performance Evaluation in Field Test
The loading performance of the wet-laid submicro-fiber composite medium and the standard heavy-duty filter medium were evaluated in the field test. Because one sample of the electrospun composite medium was not big enough to make a full-size filter, no field test was conducted for the electrospun medium. During the field test, two heavy-duty trucks (Tianlong 375, Dongfeng Motor Corp., Wuhan, China) with close mileage were used. The original equipment manufacturer (OEM) air filter (Model: 3048u, Shanghai Fleetguard Filter Co., Ltd., Shanghai, China) for this truck was made by the standard heavy-duty filter medium. In comparison, a filter based on identical filter geometry was made using the wet-laid submicro-fiber medium, as shown in Figure 5. The two trucks equipped with the OEM filter and submicro-fiber filter operated at the same coal mine (Lyuliang Coal Mine, Shanxi, China) under the same tasks. The surrounding region of this coal mine was semi-arid, and many roads are unpaved and dusty, as shown in Figure 6, which was a harsh environment for the air filter. The nominal flow rate of air intake filter in the truck was 1500 m 3 /h. After running 10,000 km, these two filters were taken out of the trucks and put into an ISO 5011 [36] test rig to conduct the pressure drop measurement under an airflow rate ranging from 750 to 1800 m 3 /h. The pressure drop results were used to evaluate the loading performance of the wet-laid submicro-fiber medium and the standard heavy-duty filter medium.

Loading Performance Evaluation in Field Test
The loading performance of the wet-laid submicro-fiber composite medium and the standard heavy-duty filter medium were evaluated in the field test. Because one sample of the electrospun composite medium was not big enough to make a full-size filter, no field test was conducted for the electrospun medium. During the field test, two heavy-duty trucks (Tianlong 375, Dongfeng Motor Corp., Wuhan, China) with close mileage were used. The original equipment manufacturer (OEM) air filter (Model: 3048u, Shanghai Fleetguard Filter Co., Ltd., Shanghai, China) for this truck was made by the standard heavy-duty filter medium. In comparison, a filter based on identical filter geometry was made using the wet-laid submicro-fiber medium, as shown in Figure 5. was a dual-layer structure, loading tests using the microfiber or submicro-fiber layer at the inlet were conducted to analyze the difference in filtration performance, as shown in Figure 4.

Loading Performance Evaluation in Field Test
The loading performance of the wet-laid submicro-fiber composite medium and the standard heavy-duty filter medium were evaluated in the field test. Because one sample of the electrospun composite medium was not big enough to make a full-size filter, no field test was conducted for the electrospun medium. During the field test, two heavy-duty trucks (Tianlong 375, Dongfeng Motor Corp., Wuhan, China) with close mileage were used. The original equipment manufacturer (OEM) air filter (Model: 3048u, Shanghai Fleetguard Filter Co., Ltd., Shanghai, China) for this truck was made by the standard heavy-duty filter medium. In comparison, a filter based on identical filter geometry was made using the wet-laid submicro-fiber medium, as shown in Figure 5. The two trucks equipped with the OEM filter and submicro-fiber filter operated at the same coal mine (Lyuliang Coal Mine, Shanxi, China) under the same tasks. The surrounding region of this coal mine was semi-arid, and many roads are unpaved and dusty, as shown in Figure 6, which was a harsh environment for the air filter. The nominal flow rate of air intake filter in the truck was 1500 m 3 /h. After running 10,000 km, these two filters were taken out of the trucks and put into an ISO 5011 [36] test rig to conduct the pressure drop measurement under an airflow rate ranging from 750 to 1800 m 3 /h. The pressure drop results were used to evaluate the loading performance of the wet-laid submicro-fiber medium and the standard heavy-duty filter medium. The two trucks equipped with the OEM filter and submicro-fiber filter operated at the same coal mine (Lyuliang Coal Mine, Shanxi, China) under the same tasks. The surrounding region of this coal mine was semi-arid, and many roads are unpaved and dusty, as shown in Figure 6, which was a harsh environment for the air filter. The nominal flow rate of air intake filter in the truck was 1500 m 3 /h. After running 10,000 km, these two filters were taken out of the trucks and put into an ISO 5011 [36] test rig to conduct the pressure drop measurement under an airflow rate ranging from 750 to 1800 m 3 /h. The pressure drop results were used to evaluate the loading performance of the wet-laid submicro-fiber medium and the standard heavy-duty filter medium.

Properties and Structural Analysis of Wet-Laid Submicro-Fiber Medium
The properties of the wet-laid submicro-fiber medium are in Table 1. The wet-laid submicrofiber medium had a close basic weight and gravimetric efficiency compared to the other two media. The initial pressure drop of the wet-laid submicro-fiber medium was 29% lower than that of the standard heavy-duty medium, but 44% higher than that of the electrospun medium. Although the initial pressure drop can indicate initial performance, it did not represent the overall performance of the loading, which is discussed in the following sections. The bending stiffness in the machine direction, which indicated the ability of sustaining the pleat structure in the operation, was very close for the three different filter media. Figure 7 shows scanning electron microscopy (SEM) images of the wet-laid submicro-fiber medium. The microfiber and submicro-fiber layer were bonded together to create a dual-layer structure. Due to the intrinsic stiffness of glass, the glass wool fiber in the submicro-fiber layer was relatively straight and stiff, thus sustaining a highly porous fiber network. Few pores were clogged by resin in either the submicro-fiber or microfiber layer, which indicated that the temperature profile of the second drying section was an effective strategy to mitigate binder migration.

Loading Performance of Wet-laid Submicro-Fiber Medium in Laboratory
When a dual-layer filter medium was used for filtration, it was important to determine which media was the inlet. Several researchers have studied the loading difference of the microfiber and submicro-fiber layers. Leung and Hung [40], Leung et al. [41], and Tang et al. [42] predicted that

Properties and Structural Analysis of Wet-Laid Submicro-Fiber Medium
The properties of the wet-laid submicro-fiber medium are in Table 1. The wet-laid submicro-fiber medium had a close basic weight and gravimetric efficiency compared to the other two media. The initial pressure drop of the wet-laid submicro-fiber medium was 29% lower than that of the standard heavy-duty medium, but 44% higher than that of the electrospun medium. Although the initial pressure drop can indicate initial performance, it did not represent the overall performance of the loading, which is discussed in the following sections. The bending stiffness in the machine direction, which indicated the ability of sustaining the pleat structure in the operation, was very close for the three different filter media. Figure 7 shows scanning electron microscopy (SEM) images of the wet-laid submicro-fiber medium. The microfiber and submicro-fiber layer were bonded together to create a dual-layer structure. Due to the intrinsic stiffness of glass, the glass wool fiber in the submicro-fiber layer was relatively straight and stiff, thus sustaining a highly porous fiber network. Few pores were clogged by resin in either the submicro-fiber or microfiber layer, which indicated that the temperature profile of the second drying section was an effective strategy to mitigate binder migration.

Properties and Structural Analysis of Wet-Laid Submicro-Fiber Medium
The properties of the wet-laid submicro-fiber medium are in Table 1. The wet-laid submicrofiber medium had a close basic weight and gravimetric efficiency compared to the other two media. The initial pressure drop of the wet-laid submicro-fiber medium was 29% lower than that of the standard heavy-duty medium, but 44% higher than that of the electrospun medium. Although the initial pressure drop can indicate initial performance, it did not represent the overall performance of the loading, which is discussed in the following sections. The bending stiffness in the machine direction, which indicated the ability of sustaining the pleat structure in the operation, was very close for the three different filter media. Figure 7 shows scanning electron microscopy (SEM) images of the wet-laid submicro-fiber medium. The microfiber and submicro-fiber layer were bonded together to create a dual-layer structure. Due to the intrinsic stiffness of glass, the glass wool fiber in the submicro-fiber layer was relatively straight and stiff, thus sustaining a highly porous fiber network. Few pores were clogged by resin in either the submicro-fiber or microfiber layer, which indicated that the temperature profile of the second drying section was an effective strategy to mitigate binder migration.

Loading Performance of Wet-laid Submicro-Fiber Medium in Laboratory
When a dual-layer filter medium was used for filtration, it was important to determine which media was the inlet. Several researchers have studied the loading difference of the microfiber and submicro-fiber layers. Leung and Hung [40], Leung et al. [41], and Tang et al. [42] predicted that

Loading Performance of Wet-laid Submicro-Fiber Medium in Laboratory
When a dual-layer filter medium was used for filtration, it was important to determine which media was the inlet. Several researchers have studied the loading difference of the microfiber and submicro-fiber layers. Leung and Hung [40], Leung et al. [41], and Tang et al. [42] predicted that because the surface loading was taking place on the submicro-fiber layer, the pressure drop across the submicro-fiber filter could rise at a much faster rate than the microfiber filter. It was also hypothesized that the microfiber layer could be placed upstream in a dual-layer filter to help collect parts of the particles, thus suppressing the increased rate of pressure drop. However, their studies focused on fine particles (e.g., PM 2.5 ), and it must be noted that the loading behavior of coarse particles was very different. Figure 8 shows the loading results of the wet-laid submicro-fiber medium with the microfiber or submicro-fiber layer as the inlet. because the surface loading was taking place on the submicro-fiber layer, the pressure drop across the submicro-fiber filter could rise at a much faster rate than the microfiber filter. It was also hypothesized that the microfiber layer could be placed upstream in a dual-layer filter to help collect parts of the particles, thus suppressing the increased rate of pressure drop. However, their studies focused on fine particles (e.g., PM2.5), and it must be noted that the loading behavior of coarse particles was very different. Figure 8 shows the loading results of the wet-laid submicro-fiber medium with the microfiber or submicro-fiber layer as the inlet. The loading results demonstrated that when the submicro-fiber layer was placed as the inlet, the pressure drop increased at a lower rate. When the microfiber layer was the inlet, the dust holding capacity was 84 ± 2 g/m 2 . When the submicro-fiber layer was the inlet, the dust holding capacity was 95 ± 1 g/m 2 , which was 13% higher than that of the microfiber layer as the inlet. This meant that when the filter medium was challenged by dust particles, the lifetime was 13% longer if the submicro-fiber layer was placed in the upstream. This conclusion was contradictory to the conclusion of Leung et al. [41], which indicated that the loading behavior of coarse particles was different from that of fine particles. The pressure drop of the loaded filter medium can be considered as the sum of the pressure drop across the loaded filter medium and the pressure drop of the dust cake, as shown in Figure 9.  The loading results demonstrated that when the submicro-fiber layer was placed as the inlet, the pressure drop increased at a lower rate. When the microfiber layer was the inlet, the dust holding capacity was 84 ± 2 g/m 2 . When the submicro-fiber layer was the inlet, the dust holding capacity was 95 ± 1 g/m 2 , which was 13% higher than that of the microfiber layer as the inlet. This meant that when the filter medium was challenged by dust particles, the lifetime was 13% longer if the submicro-fiber layer was placed in the upstream. This conclusion was contradictory to the conclusion of Leung et al. [41], which indicated that the loading behavior of coarse particles was different from that of fine particles.
The pressure drop of the loaded filter medium can be considered as the sum of the pressure drop across the loaded filter medium and the pressure drop of the dust cake, as shown in Figure 9.
According to Endo et al. [43,44], the pressure drop of dust cake can be calculated by Equation (3). It can be seen from Equation (3) that the pressure drop of dust cake was related to the properties of fluid and dust, and independent of filter medium. The latter part of the two curves in Figure 8 showed a similar increasing rate, which demonstrated that the pressure drop of the dust cake had no dependency on the filter medium. Both theoretical model and experimental results indicated that it was the particles' deposition pattern inside the filter medium that determined the pressure drop evolution in the loading. Therefore, the interaction between the particles and the fibrous structure in the early loading stage was the key to analyzing the results in Figure 8. where ∆P cake is pressure drop across the dust cake; µ is gas viscosity; u s is face velocity; H is cake height; ε is cake porosity; κ is dynamic shape factor of A2 dust; d vg is geometric mean of volume equivalent diameter; σ g is geometric standard deviation.

Figure 8.
Loading results of wet-laid submicro-fiber medium with the microfiber or submicro-fiber layer as inlet.
The pressure drop of the loaded filter medium can be considered as the sum of the pressure drop across the loaded filter medium and the pressure drop of the dust cake, as shown in Figure 9.  To illustrate the loading behavior of the particles, SEM images of dust deposition on the filter medium were obtained when the pressure drop increased 10, 50, and 100 Pa, respectively, as shown in Figures 10 and 11. When the microfiber layer was placed as the inlet, dust particles penetrated the depth of the filter medium. The deposited particles bridged the pores and gradually clogged the pores. The dust particles captured inside the filter medium reduced the porosity and led to a higher pressure drop. When the submicro-fiber layer was placed as the inlet, less dust particles penetrated the depth of the filter medium and surface loading occurred rapidly. Based on the results in Figure 8, the filter medium clogged by dust particles was more difficult for air to permeate than the dust cake. Therefore, it was preferable for surface loading to happen earlier for coarse particles, while in the studies of Leung et al. [41] and Tang et al. [42], surface loading should take place later when fine particles were tested. For this reason, the submicro-fiber layer was placed as the inlet in the following discussions. of fluid and dust, and independent of filter medium. The latter part of the two curves in Figure 8 showed a similar increasing rate, which demonstrated that the pressure drop of the dust cake had no dependency on the filter medium. Both theoretical model and experimental results indicated that it was the particles' deposition pattern inside the filter medium that determined the pressure drop evolution in the loading. Therefore, the interaction between the particles and the fibrous structure in the early loading stage was the key to analyzing the results in Figure 8. (1 ) 180 exp(4 ln ) where ΔPcake is pressure drop across the dust cake; μ is gas viscosity; us is face velocity; H is cake height; ε is cake porosity; κ is dynamic shape factor of A2 dust; dvg is geometric mean of volume equivalent diameter; σg is geometric standard deviation.
To illustrate the loading behavior of the particles, SEM images of dust deposition on the filter medium were obtained when the pressure drop increased 10, 50, and 100 Pa, respectively, as shown in Figures 10 and 11. When the microfiber layer was placed as the inlet, dust particles penetrated the depth of the filter medium. The deposited particles bridged the pores and gradually clogged the pores. The dust particles captured inside the filter medium reduced the porosity and led to a higher pressure drop. When the submicro-fiber layer was placed as the inlet, less dust particles penetrated the depth of the filter medium and surface loading occurred rapidly. Based on the results in Figure  8, the filter medium clogged by dust particles was more difficult for air to permeate than the dust cake. Therefore, it was preferable for surface loading to happen earlier for coarse particles, while in the studies of Leung et al. [41] and Tang et al. [42], surface loading should take place later when fine particles were tested. For this reason, the submicro-fiber layer was placed as the inlet in the following discussions.

Loading Performance Comparison Between Wet-laid Submicro-Fiber Medium and Standard Heavy-duty Medium in the Laboratory and Field
The loading results of the wet-laid submicro-fiber medium and the standard heavy-duty medium are shown in Figure 12. The pressure drop of the standard heavy-duty medium increased faster than that of the wet-laid submicro-fiber medium. When the pressure drop reached 2 kPa, the dust mass deposit of the standard heavy-duty medium was 64 g/m 2 , which meant that the dust holding capacity of the wet-laid submicro-fiber medium was 48% higher than that of the standard heavy-duty medium.

Loading Performance Comparison Between Wet-laid Submicro-Fiber Medium and Standard Heavy-duty Medium in the Laboratory and Field
The loading results of the wet-laid submicro-fiber medium and the standard heavy-duty medium are shown in Figure 12. The pressure drop of the standard heavy-duty medium increased faster than that of the wet-laid submicro-fiber medium. When the pressure drop reached 2 kPa, the dust mass deposit of the standard heavy-duty medium was 64 g/m 2 , which meant that the dust holding capacity of the wet-laid submicro-fiber medium was 48% higher than that of the standard heavy-duty medium.
The energy consumption of air filter media was proportional to average pressure drop in loading process [9]. Δp(Mi) was the pressure drop when dust mass deposit was Mi. The average pressure drop in a complete loading test can be calculated by Equation (4). The polynomial of fourth order Δp(m) = anMi 4 + bnMi 3 + cnMi 2 + dnMi + ΔpI and parameters an, bn, cn and dn were used to curve fit the pressure drop data as a function of loading time. ΔpI is the initial pressure drop of filter media.
Under a dust mass deposit of 64 ± 1 g/m 2 , the estimated energy consumption of the wet-laid submicro-fiber medium was 36% lower than that of standard heavy-duty medium based on calculation of average pressure drop via Equation (4). The laboratory results showed that the wetlaid submicro-fiber medium had a longer lifetime and lower energy consumption than the standard heavy-duty medium when challenged by dust particles. The SEM images of the clean standard heavy-duty medium and the loaded medium when the pressure drop increased 10, 50, and 100 Pa were obtained, respectively, as shown in Figure 13. The dust particles penetrated into the depth of the fibrous structure. Surface loading took place later than the wet-laid submicro-fiber medium compared to Figure 11. The dust loading behavior was similar to that of the microfiber layer in Figure 10. The results further confirmed that depth filtration was not suitable for coarse particle filtration. The energy consumption of air filter media was proportional to average pressure drop in loading process [9]. ∆p(M i ) was the pressure drop when dust mass deposit was M i . The average pressure drop in a complete loading test can be calculated by Equation (4). The polynomial of fourth order ∆p(m) = a n M i 4 + b n M i 3 + c n M i 2 + d n M i + ∆p I and parameters a n , b n , c n and d n were used to curve fit the pressure drop data as a function of loading time. ∆p I is the initial pressure drop of filter media.
Under a dust mass deposit of 64 ± 1 g/m 2 , the estimated energy consumption of the wet-laid submicro-fiber medium was 36% lower than that of standard heavy-duty medium based on calculation of average pressure drop via Equation (4). The laboratory results showed that the wet-laid submicro-fiber medium had a longer lifetime and lower energy consumption than the standard heavy-duty medium when challenged by dust particles.
The SEM images of the clean standard heavy-duty medium and the loaded medium when the pressure drop increased 10, 50, and 100 Pa were obtained, respectively, as shown in Figure 13. The dust particles penetrated into the depth of the fibrous structure. Surface loading took place later than the wet-laid submicro-fiber medium compared to Figure 11. The dust loading behavior was similar to that of the microfiber layer in Figure 10. The results further confirmed that depth filtration was not suitable for coarse particle filtration.  Figure 14 shows the pressure drop of the standard heavy-duty filter and the wet-laid submicrofiber filter under different flow rates after 10,000 km of operation. Under the nominal flow rate of the air filter, 1500 m 3 /h, the pressure drop of the standard heavy-duty filter was higher than 2 kPa and the filter service life ended. The pressure drop of the used wet-laid submicro-fiber filter was approximately 45% lower than that of the standard heavy-duty filter, which demonstrated the advantage of the novel filter medium in the field test. During the laboratory tests, when the pressure drop of the standard heavy-duty medium reached 2 kPa, the pressure drop of the wet-laid submicrofiber medium under the same dust mass deposit was 38% lower than that of the standard heavy-duty medium, which was close to the field test results. The field test results showed that the wet-laid submicro-fiber medium had a longer lifetime when challenged by dust particles.  Figure 14 shows the pressure drop of the standard heavy-duty filter and the wet-laid submicro-fiber filter under different flow rates after 10,000 km of operation. Under the nominal flow rate of the air filter, 1500 m 3 /h, the pressure drop of the standard heavy-duty filter was higher than 2 kPa and the filter service life ended. The pressure drop of the used wet-laid submicro-fiber filter was approximately 45% lower than that of the standard heavy-duty filter, which demonstrated the advantage of the novel filter medium in the field test. During the laboratory tests, when the pressure drop of the standard heavy-duty medium reached 2 kPa, the pressure drop of the wet-laid submicro-fiber medium under the same dust mass deposit was 38% lower than that of the standard heavy-duty medium, which was close to the field test results. The field test results showed that the wet-laid submicro-fiber medium had a longer lifetime when challenged by dust particles.

Loading Performance Comparison Between Wet-Laid Submicro-Fiber Medium and Electrospun Composite Medium in Laboratory
The loading results of the wet-laid submicro-fiber medium and an electrospun composite medium are shown in Figure 15. The initial pressure drop of the electrospun medium was lower, but the pressure drop increased faster than the wet-laid submicro-fiber medium. When the pressure drop reached 2 kPa, the dust mass deposit of the electrospun medium was 86 ± 1 g/m 2 , which meant that the dust holding capacity of the wet-laid submicro-fiber medium was 10% higher than that of the electrospun medium. Under a dust mass deposit of 86 g/m 2 , the estimated energy consumption of the wet-laid submicro-fiber medium was 10% lower than that of the electrospun medium based on calculation of average pressure drop via Equation (4). The laboratory results showed that the wetlaid submicro-fiber medium had a longer lifetime and lower energy consumption than the electrospun medium when challenged by dust particles.
The SEM images of the clean electrospun composite medium and loaded filter medium when the pressure drop increased 10, 50, and 100 Pa were obtained, respectively, as shown in Figure 16. The electrospun medium was believed to be a typical surface loading material. When the pressure drop increased 10 kPa, the electrospun medium had already shown evident signs of surface loading. Although surface loading was preferred in the removal of coarse particles, the process of surface loading would affect the loading performance.

Loading Performance Comparison Between Wet-Laid Submicro-Fiber Medium and Electrospun Composite Medium in Laboratory
The loading results of the wet-laid submicro-fiber medium and an electrospun composite medium are shown in Figure 15. The initial pressure drop of the electrospun medium was lower, but the pressure drop increased faster than the wet-laid submicro-fiber medium. When the pressure drop reached 2 kPa, the dust mass deposit of the electrospun medium was 86 ± 1 g/m 2 , which meant that the dust holding capacity of the wet-laid submicro-fiber medium was 10% higher than that of the electrospun medium. Under a dust mass deposit of 86 g/m 2 , the estimated energy consumption of the wet-laid submicro-fiber medium was 10% lower than that of the electrospun medium based on calculation of average pressure drop via Equation (4). The laboratory results showed that the wet-laid submicro-fiber medium had a longer lifetime and lower energy consumption than the electrospun medium when challenged by dust particles.

Loading Performance Comparison Between Wet-Laid Submicro-Fiber Medium and Electrospun Composite Medium in Laboratory
The loading results of the wet-laid submicro-fiber medium and an electrospun composite medium are shown in Figure 15. The initial pressure drop of the electrospun medium was lower, but the pressure drop increased faster than the wet-laid submicro-fiber medium. When the pressure drop reached 2 kPa, the dust mass deposit of the electrospun medium was 86 ± 1 g/m 2 , which meant that the dust holding capacity of the wet-laid submicro-fiber medium was 10% higher than that of the electrospun medium. Under a dust mass deposit of 86 g/m 2 , the estimated energy consumption of the wet-laid submicro-fiber medium was 10% lower than that of the electrospun medium based on calculation of average pressure drop via Equation (4). The laboratory results showed that the wetlaid submicro-fiber medium had a longer lifetime and lower energy consumption than the electrospun medium when challenged by dust particles.
The SEM images of the clean electrospun composite medium and loaded filter medium when the pressure drop increased 10, 50, and 100 Pa were obtained, respectively, as shown in Figure 16. The electrospun medium was believed to be a typical surface loading material. When the pressure drop increased 10 kPa, the electrospun medium had already shown evident signs of surface loading. Although surface loading was preferred in the removal of coarse particles, the process of surface loading would affect the loading performance.  The SEM images of the clean electrospun composite medium and loaded filter medium when the pressure drop increased 10, 50, and 100 Pa were obtained, respectively, as shown in Figure 16. The electrospun medium was believed to be a typical surface loading material. When the pressure drop increased 10 kPa, the electrospun medium had already shown evident signs of surface loading. Although surface loading was preferred in the removal of coarse particles, the process of surface loading would affect the loading performance. The governing mechanisms of capturing coarse particles were interception, inertial impaction, and sieving [14]. Particles were captured by fibers due to interception and inertial impaction, while particles were arrested at the pores (smaller than the particle diameter) due to sieving. Figure 11 shows that the dust particles were largely captured by submicro-fibers of the wet-laid medium, while particles were largely arrested by pores of the electrospun medium as shown in Figure 16. Figure 17 was the pore size distribution of electrospun media and wet-laid submicro-fiber media. The peak pore size of electrospun media and wet-laid submicro-fiber media was 3.3 μm and 8.8 μm, respectively. The pore size distribution of electrospun media was narrower than that of wet-laid submicro-fiber media. Since the volume mean diameter of A2 fine dust was approximately 10 µm [38], more dust particles were captured by pores instead of fibers for electrospun media than wetlaid submicro-fiber media. When the pores were blocked by larger particles, the airflow path through the fibers and particles was restricted. Airflow velocity in the restricted pores rapidly increased and thus the pressure drop increased. The unique fibrous structure of the wet-laid submicro-fiber The governing mechanisms of capturing coarse particles were interception, inertial impaction, and sieving [14]. Particles were captured by fibers due to interception and inertial impaction, while particles were arrested at the pores (smaller than the particle diameter) due to sieving. Figure 11 shows that the dust particles were largely captured by submicro-fibers of the wet-laid medium, while particles were largely arrested by pores of the electrospun medium as shown in Figure 16. Figure 17 was the pore size distribution of electrospun media and wet-laid submicro-fiber media. The peak pore size of electrospun media and wet-laid submicro-fiber media was 3.3 µm and 8.8 µm, respectively. The pore size distribution of electrospun media was narrower than that of wet-laid submicro-fiber media. Since the volume mean diameter of A2 fine dust was approximately 10 µm [38], more dust particles were captured by pores instead of fibers for electrospun media than wet-laid submicro-fiber media. When the pores were blocked by larger particles, the airflow path through the fibers and particles was restricted. Airflow velocity in the restricted pores rapidly increased and thus the pressure drop increased. The unique fibrous structure of the wet-laid submicro-fiber medium made more particle deposition take place on fibers via interception and inertial impaction rather than sieving, which led to a porous structure with a larger airflow path. Therefore, the wet-laid submicro-fiber medium showed a better loading performance against coarse particles than the electrospun medium. The current study is limited to room temperature and controlled RH (50 ± 10%). In the future, the effect of excess RH and temperature on dust loading performance of wet-laid submicro-fiber media should be studied.

Conclusions
In this study, a novel wet-laid submicro-fiber composite medium was prepared by a dual-layer paper machine, and its loading performance and dust deposition behavior was compared with a standard heavy-duty medium and electrospun medium in laboratory and field tests. It was found that: (1) The submicro-fiber layer, laminated by dual-channel headbox technology and reinforced by acrylic resin, was highly porous without pore clogging. The loading results demonstrated that when the submicro-fiber layer was placed as the inlet, the pressure drop increased at a slower rate than that when the microfiber layer was the inlet. It was concluded that it was preferable for surface loading to happen earlier for dust particles.
(2) During the laboratory test, the dust holding capacity of the wet-laid submicro-fiber medium was 48% higher than that of the standard heavy-duty medium, and the estimated energy consumption was 36% lower. In the field test, the pressure drop of the wet-laid submicro-fiber filter was approximately 45% lower than that of the standard heavy-duty filter after 10,000 km of operation. This showed that the wet-laid submicro-fiber medium had a longer lifetime and lower energy consumption than the standard heavy-duty medium when challenged by dust particles.
(3) The dust holding capacity of the wet-laid submicro-fiber medium was 10% higher than that of the electrospun medium, and the estimated energy consumption was 10% lower. The key point of submicro-fiber medium design was that the particles were preferable to be captured by fibers rather than pores. The fibrous structure of the wet-laid submicro-fiber medium made more particle deposition take place on fibers via interception and inertial impaction. Thus, in this work, the submicro-fiber medium showed a lower pressure drop than the electrospun medium in the loading process.  The current study is limited to room temperature and controlled RH (50 ± 10%). In the future, the effect of excess RH and temperature on dust loading performance of wet-laid submicro-fiber media should be studied.

Conclusions
In this study, a novel wet-laid submicro-fiber composite medium was prepared by a dual-layer paper machine, and its loading performance and dust deposition behavior was compared with a standard heavy-duty medium and electrospun medium in laboratory and field tests. It was found that: (1) The submicro-fiber layer, laminated by dual-channel headbox technology and reinforced by acrylic resin, was highly porous without pore clogging. The loading results demonstrated that when the submicro-fiber layer was placed as the inlet, the pressure drop increased at a slower rate than that when the microfiber layer was the inlet. It was concluded that it was preferable for surface loading to happen earlier for dust particles.
(2) During the laboratory test, the dust holding capacity of the wet-laid submicro-fiber medium was 48% higher than that of the standard heavy-duty medium, and the estimated energy consumption was 36% lower. In the field test, the pressure drop of the wet-laid submicro-fiber filter was approximately 45% lower than that of the standard heavy-duty filter after 10,000 km of operation. This showed that the wet-laid submicro-fiber medium had a longer lifetime and lower energy consumption than the standard heavy-duty medium when challenged by dust particles.
(3) The dust holding capacity of the wet-laid submicro-fiber medium was 10% higher than that of the electrospun medium, and the estimated energy consumption was 10% lower. The key point of submicro-fiber medium design was that the particles were preferable to be captured by fibers rather than pores. The fibrous structure of the wet-laid submicro-fiber medium made more particle deposition take place on fibers via interception and inertial impaction. Thus, in this work, the submicro-fiber medium showed a lower pressure drop than the electrospun medium in the loading process.

Conflicts of Interest:
The authors declare no conflict of interest.