The working efficiency (i.e., the ability to capture particles) of non-woven high-efficiency filters (typically referring to HEPA or ULPA levels) is not fixed but is influenced by a combination of multiple factors. These factors can be classified into three major categories: the inherent characteristics of the filter itself, external operating conditions, and the usage and maintenance process. Specifically, they are as follows
I. Inherent Characteristics of the Filter: The core factor determining the basic efficiency
Such factors are the “inherent attributes” of the filter that are determined during the production and manufacturing stage, directly determining the upper limit of its initial filtration efficiency.
1. Filter material performance (the most core factor
Filter materials are the key carriers for capturing particles, and their material, structure and processing technology directly affect the filtration capacity.
Material type: The mainstream filter materials are glass fiber (high efficiency, high temperature resistance, but relatively brittle) and synthetic fibers such as polypropylene (PP) (good toughness, moisture resistance, but poor high-temperature resistance). The fibers of glass fiber filter materials are finer and have a more uniform pore size distribution. Their capture efficiency for particles of 0.3μm and below is usually better than that of ordinary synthetic fibers.
Fiber diameter and density: The finer the fiber, the higher the probability of contact with particles, and the stronger the filtration efficiency (for example, ultrafine glass fibers with a diameter of 0.5-1μm have an efficiency one order of magnitude higher than coarse fibers with a diameter of 5μm or more). The greater the fiber density (the more fibers per unit area), the lower the porosity of the filter material relatively, and the stronger the interception effect on particles, but the resistance will also increase accordingly.
Filter material thickness: Within a reasonable range, increasing the thickness of the filter material can prolong the retention time of particles in the filter material, increase the chances of the action of capture mechanisms such as collision and diffusion, and thereby enhance efficiency (for example, a 10mm thick filter material is usually superior to a 5mm thick filter material of the same material).
2. Structural design
The filter without separators folds the filter material into a “V” or “W” shape through hot melt adhesive or separators. Its structural design directly affects the effective utilization of the filter material and the distribution of air flow.
Folding density and unfolded area: The tighter the folding, the larger the unfolded area of the filter material per unit volume, the lower the flow velocity when the airflow passes through, and the more time the particles have to be captured, resulting in higher efficiency. Conversely, sparse folding will lead to a low utilization rate of filter materials and a decline in efficiency.
Sealing performance: The sealing effect between the filter frame and the filter material, as well as between the frame and the installation frame, is of vital importance. If there is poor sealing (such as weak adhesion of adhesive or deformation of the frame), a “bypass airflow” will be formed – some air passes directly through the gap without passing through the filter material, resulting in a significant drop in the actual filtration efficiency (possibly from 99.99% to below 90%).
Frame material: Usually aluminum alloy or galvanized steel plate. If the flatness of the frame is poor or the dimensional accuracy is insufficient, it will result in an inability to fit closely during installation, indirectly affecting the sealing and efficiency.
Ii. External Operating Conditions: Dynamic factors Affecting Actual Efficiency
Even if the filter itself performs well, if the operating environment does not meet the design requirements, the actual efficiency will significantly deviate from the theoretical value.
1. Airflow parameters
Airflow serves as the “carrier” for particles passing through the filter, and its flow rate, volume and distribution directly affect the capture effect of the filter material.
Face velocity/filter rate: It refers to the speed at which air flows through the effective area of the filter material (unit: m/s or m³/(m² · h)).
The design face velocity of the non-woven high-efficiency filter is usually 0.3-0.5 m/s.
Low wind speed: Although the theoretical capture time is longer, it may lead to uneven airflow distribution, with airflow stagnation in some areas, which in turn reduces the overall efficiency.
Excessively high wind speed: The “thrust” of the airflow on the particles increases, which may re-blow off the already captured tiny particles (i.e., “secondary dust raising”), while shortening the contact time between the particles and the filter material, reducing the effectiveness of mechanisms such as diffusion and interception, and leading to a decrease in efficiency.
Uniformity of air flow distribution: If the air flow entering the filter is unevenly distributed (such as excessively high local wind speed or the presence of vortices), it will cause excessive local load on the filter material and efficiency imbalance – the efficiency decreases in the high wind speed area, and the filter material is not fully utilized in the low wind speed area.
2. Characteristics of the air to be filtered
The types, concentrations and physicochemical properties of pollutants in the air itself will directly affect the capture efficiency and service life of filter materials.
Particle concentration: When the concentration of small particles in the air (especially the “most penetrable particle size” of 0.1-0.3μm) is too high, a “dust layer” will rapidly form on the surface of the filter material. The initial dust layer can assist in capturing particles (i.e., the “bridging effect”, with a slight increase in efficiency); However, if the concentration is too high for a long time, it will cause the dust layer to be too thick, which not only increases the airflow resistance but also may lead to the penetration of particles due to the uneven internal pressure of the dust layer, resulting in a decrease in efficiency.
Particle properties
Particle size: The capture efficiency of HEPA filters for 0.3μm particles is the standard (for example, H13 grade is ≥99.97%), but for smaller particles (<0.1μm), it mainly relies on the “diffusion effect”, and for larger particles (>1μm), it mainly relies on “interception and inertial collision”. The efficiency of these two types of particle sizes is usually higher.
Shape: Irregular and angular particles are more easily intercepted than spherical particles.
Charge: Filter materials with static electricity (such as “electrostatically charged filter materials”) have a higher efficiency in capturing charged particles. However, if the air humidity is high or there are oily pollutants, the static electricity on the filter materials may be neutralized, resulting in a decrease in efficiency.
Air humidity and temperature
Humidity: High humidity (such as relative humidity > 80%) can cause the filter material fibers to absorb moisture, leading to adhesion between fibers and changes in pore size. At the same time, it may cause the dust layer to clump, affecting the air flow passage and capture efficiency. If the humidity approaches the dew point, condensation water may also form, damaging the filter material (for example, glass fiber filter materials are prone to breakage after absorbing moisture).
Temperature: When the temperature exceeds the tolerance range of the filter material (for example, glass fiber filter materials usually tolerate 80-120℃, and PP filter materials can tolerate below 60℃), the filter material will soften, deform or even burn, resulting in complete failure of efficiency.
3. Types of pollutants
In addition to solid particles, special pollutants in the air can affect efficiency by “damaging filter materials” or “clogging pores” :
Oily pollutants: such as cooking fumes, oil mists, and oil droplets formed by the condensation of organic vapors. Oily substances will adhere to the surface of the filter material fibers, clog the pores, and at the same time neutralize the charge of the electrostatic filter material, resulting in a rapid decline in efficiency (i.e., “oil mist failure”). The tolerance of ordinary HEPA filters to oily particles is much lower than that to solid particles, and special “oil-resistant HEPA” (such as H13-O grade) is required.
Chemically corrosive gases: such as chlorine, sulfur dioxide, ammonia, etc. These gases will undergo chemical reactions with the filter material fibers (such as the silicon-oxygen bonds in glass fibers) or the frame material, leading to the degradation of the filter material and the corrosion of the frame, thereby damaging the structural integrity and reducing efficiency.
Iii. Usage and Maintenance Process: Key Factors Determining Efficiency stability
The “postnatal” usage and maintenance methods of the filter directly affect the rate of its efficiency decline and service life.
The effectiveness of the pre-filtering system
The non-woven high-efficiency filter belongs to “end filtration”. Usually, a primary filter (G3-G4 grades) and a medium-efficiency filter (F5-F9 grades) are equipped in front of it to intercept large particles of dust (such as dust and hair ≥5μm). If the pre-filter fails (such as not being replaced in time or having insufficient filtration efficiency), a large number of large particles will directly impact the high-efficiency filter material, causing rapid clogging of the filter material, fiber wear, and premature decline in efficiency.
2. Installation quality
Improper installation is a common “human factor” that leads to a decline in actual efficiency:
Poor frame fit: If the flatness of the installation frame is poor and its size does not match that of the filter, it will cause a gap between the filter and the frame, resulting in bypass airflow.
Physical damage during installation: If excessive force is applied during the installation process, it may cause the filter material to fold and deform, the frame to crack, or damage the sealant between the filter material and the frame, directly reducing efficiency.
Incorrect airflow direction: The filter has a clear “air inlet surface” and “air outlet surface” (usually marked with arrows). If installed in reverse, the airflow cannot pass through the folded structure of the filter material along the designed path, which will lead to a sudden increase in resistance and a significant drop in efficiency.
3. Replacement Cycle and Maintenance
Over-use: After the filter has been in use for a period of time, the dust layer on the surface of the filter material will gradually thicken. When the resistance reaches the design upper limit (usually 2 to 2.5 times the initial resistance), even if there is no obvious damage, the efficiency will significantly decrease due to “particle penetration” and “airflow bypass”, and it must be replaced in a timely manner.
Improper maintenance: For instance, using high-pressure air to blow off the surface of the filter (which may blow off particles or damage fibers on the filter material), or not cleaning the accumulated dust in the installation frame during replacement (causing the new filter to be re-contaminated), will all affect efficiency.
Iv. Conclusion: The Interrelationships among Various factors
The working efficiency of the non-woven high-efficiency filter is the comprehensive result of “inherent characteristics + operating conditions + maintenance management”
The filter material and structural design are the “foundation”, determining the upper limit of the initial efficiency.
Airflow parameters and pollutant characteristics are “dynamic variables” that determine the efficiency performance in actual operation.
Pre-filtering and installation and maintenance are the “guarantees”, which determine the stability of efficiency and service life.
Therefore, to ensure that the filter always maintains high efficiency, comprehensive control should be exercised from three aspects: “selection (high-quality filter materials and structures), use (matching operating parameters), and maintenance (standardized installation and regular replacement)”.
I. Inherent Characteristics of the Filter: The core factor determining the basic efficiency
Such factors are the “inherent attributes” of the filter that are determined during the production and manufacturing stage, directly determining the upper limit of its initial filtration efficiency.
1. Filter material performance (the most core factor
Filter materials are the key carriers for capturing particles, and their material, structure and processing technology directly affect the filtration capacity.
Material type: The mainstream filter materials are glass fiber (high efficiency, high temperature resistance, but relatively brittle) and synthetic fibers such as polypropylene (PP) (good toughness, moisture resistance, but poor high-temperature resistance). The fibers of glass fiber filter materials are finer and have a more uniform pore size distribution. Their capture efficiency for particles of 0.3μm and below is usually better than that of ordinary synthetic fibers.
Fiber diameter and density: The finer the fiber, the higher the probability of contact with particles, and the stronger the filtration efficiency (for example, ultrafine glass fibers with a diameter of 0.5-1μm have an efficiency one order of magnitude higher than coarse fibers with a diameter of 5μm or more). The greater the fiber density (the more fibers per unit area), the lower the porosity of the filter material relatively, and the stronger the interception effect on particles, but the resistance will also increase accordingly.
Filter material thickness: Within a reasonable range, increasing the thickness of the filter material can prolong the retention time of particles in the filter material, increase the chances of the action of capture mechanisms such as collision and diffusion, and thereby enhance efficiency (for example, a 10mm thick filter material is usually superior to a 5mm thick filter material of the same material).
2. Structural design
The filter without separators folds the filter material into a “V” or “W” shape through hot melt adhesive or separators. Its structural design directly affects the effective utilization of the filter material and the distribution of air flow.
Folding density and unfolded area: The tighter the folding, the larger the unfolded area of the filter material per unit volume, the lower the flow velocity when the airflow passes through, and the more time the particles have to be captured, resulting in higher efficiency. Conversely, sparse folding will lead to a low utilization rate of filter materials and a decline in efficiency.
Sealing performance: The sealing effect between the filter frame and the filter material, as well as between the frame and the installation frame, is of vital importance. If there is poor sealing (such as weak adhesion of adhesive or deformation of the frame), a “bypass airflow” will be formed – some air passes directly through the gap without passing through the filter material, resulting in a significant drop in the actual filtration efficiency (possibly from 99.99% to below 90%).
Frame material: Usually aluminum alloy or galvanized steel plate. If the flatness of the frame is poor or the dimensional accuracy is insufficient, it will result in an inability to fit closely during installation, indirectly affecting the sealing and efficiency.
Ii. External Operating Conditions: Dynamic factors Affecting Actual Efficiency
Even if the filter itself performs well, if the operating environment does not meet the design requirements, the actual efficiency will significantly deviate from the theoretical value.
1. Airflow parameters
Airflow serves as the “carrier” for particles passing through the filter, and its flow rate, volume and distribution directly affect the capture effect of the filter material.
Face velocity/filter rate: It refers to the speed at which air flows through the effective area of the filter material (unit: m/s or m³/(m² · h)).
The design face velocity of the non-woven high-efficiency filter is usually 0.3-0.5 m/s.
Low wind speed: Although the theoretical capture time is longer, it may lead to uneven airflow distribution, with airflow stagnation in some areas, which in turn reduces the overall efficiency.
Excessively high wind speed: The “thrust” of the airflow on the particles increases, which may re-blow off the already captured tiny particles (i.e., “secondary dust raising”), while shortening the contact time between the particles and the filter material, reducing the effectiveness of mechanisms such as diffusion and interception, and leading to a decrease in efficiency.
Uniformity of air flow distribution: If the air flow entering the filter is unevenly distributed (such as excessively high local wind speed or the presence of vortices), it will cause excessive local load on the filter material and efficiency imbalance – the efficiency decreases in the high wind speed area, and the filter material is not fully utilized in the low wind speed area.
2. Characteristics of the air to be filtered
The types, concentrations and physicochemical properties of pollutants in the air itself will directly affect the capture efficiency and service life of filter materials.
Particle concentration: When the concentration of small particles in the air (especially the “most penetrable particle size” of 0.1-0.3μm) is too high, a “dust layer” will rapidly form on the surface of the filter material. The initial dust layer can assist in capturing particles (i.e., the “bridging effect”, with a slight increase in efficiency); However, if the concentration is too high for a long time, it will cause the dust layer to be too thick, which not only increases the airflow resistance but also may lead to the penetration of particles due to the uneven internal pressure of the dust layer, resulting in a decrease in efficiency.
Particle properties
Particle size: The capture efficiency of HEPA filters for 0.3μm particles is the standard (for example, H13 grade is ≥99.97%), but for smaller particles (<0.1μm), it mainly relies on the “diffusion effect”, and for larger particles (>1μm), it mainly relies on “interception and inertial collision”. The efficiency of these two types of particle sizes is usually higher.
Shape: Irregular and angular particles are more easily intercepted than spherical particles.
Charge: Filter materials with static electricity (such as “electrostatically charged filter materials”) have a higher efficiency in capturing charged particles. However, if the air humidity is high or there are oily pollutants, the static electricity on the filter materials may be neutralized, resulting in a decrease in efficiency.
Air humidity and temperature
Humidity: High humidity (such as relative humidity > 80%) can cause the filter material fibers to absorb moisture, leading to adhesion between fibers and changes in pore size. At the same time, it may cause the dust layer to clump, affecting the air flow passage and capture efficiency. If the humidity approaches the dew point, condensation water may also form, damaging the filter material (for example, glass fiber filter materials are prone to breakage after absorbing moisture).
Temperature: When the temperature exceeds the tolerance range of the filter material (for example, glass fiber filter materials usually tolerate 80-120℃, and PP filter materials can tolerate below 60℃), the filter material will soften, deform or even burn, resulting in complete failure of efficiency.
3. Types of pollutants
In addition to solid particles, special pollutants in the air can affect efficiency by “damaging filter materials” or “clogging pores” :
Oily pollutants: such as cooking fumes, oil mists, and oil droplets formed by the condensation of organic vapors. Oily substances will adhere to the surface of the filter material fibers, clog the pores, and at the same time neutralize the charge of the electrostatic filter material, resulting in a rapid decline in efficiency (i.e., “oil mist failure”). The tolerance of ordinary HEPA filters to oily particles is much lower than that to solid particles, and special “oil-resistant HEPA” (such as H13-O grade) is required.
Chemically corrosive gases: such as chlorine, sulfur dioxide, ammonia, etc. These gases will undergo chemical reactions with the filter material fibers (such as the silicon-oxygen bonds in glass fibers) or the frame material, leading to the degradation of the filter material and the corrosion of the frame, thereby damaging the structural integrity and reducing efficiency.
Iii. Usage and Maintenance Process: Key Factors Determining Efficiency stability
The “postnatal” usage and maintenance methods of the filter directly affect the rate of its efficiency decline and service life.
The effectiveness of the pre-filtering system
The non-woven high-efficiency filter belongs to “end filtration”. Usually, a primary filter (G3-G4 grades) and a medium-efficiency filter (F5-F9 grades) are equipped in front of it to intercept large particles of dust (such as dust and hair ≥5μm). If the pre-filter fails (such as not being replaced in time or having insufficient filtration efficiency), a large number of large particles will directly impact the high-efficiency filter material, causing rapid clogging of the filter material, fiber wear, and premature decline in efficiency.
2. Installation quality
Improper installation is a common “human factor” that leads to a decline in actual efficiency:
Poor frame fit: If the flatness of the installation frame is poor and its size does not match that of the filter, it will cause a gap between the filter and the frame, resulting in bypass airflow.
Physical damage during installation: If excessive force is applied during the installation process, it may cause the filter material to fold and deform, the frame to crack, or damage the sealant between the filter material and the frame, directly reducing efficiency.
Incorrect airflow direction: The filter has a clear “air inlet surface” and “air outlet surface” (usually marked with arrows). If installed in reverse, the airflow cannot pass through the folded structure of the filter material along the designed path, which will lead to a sudden increase in resistance and a significant drop in efficiency.
3. Replacement Cycle and Maintenance
Over-use: After the filter has been in use for a period of time, the dust layer on the surface of the filter material will gradually thicken. When the resistance reaches the design upper limit (usually 2 to 2.5 times the initial resistance), even if there is no obvious damage, the efficiency will significantly decrease due to “particle penetration” and “airflow bypass”, and it must be replaced in a timely manner.
Improper maintenance: For instance, using high-pressure air to blow off the surface of the filter (which may blow off particles or damage fibers on the filter material), or not cleaning the accumulated dust in the installation frame during replacement (causing the new filter to be re-contaminated), will all affect efficiency.
Iv. Conclusion: The Interrelationships among Various factors
The working efficiency of the non-woven high-efficiency filter is the comprehensive result of “inherent characteristics + operating conditions + maintenance management”
The filter material and structural design are the “foundation”, determining the upper limit of the initial efficiency.
Airflow parameters and pollutant characteristics are “dynamic variables” that determine the efficiency performance in actual operation.
Pre-filtering and installation and maintenance are the “guarantees”, which determine the stability of efficiency and service life.
Therefore, to ensure that the filter always maintains high efficiency, comprehensive control should be exercised from three aspects: “selection (high-quality filter materials and structures), use (matching operating parameters), and maintenance (standardized installation and regular replacement)”.
