The filtration effect of medium-efficiency bag filters is not fixed but is jointly influenced by multiple factors such as the performance of the filter material itself, external usage conditions, system design and maintenance operations. These factors directly determine the interception efficiency, resistance stability and service life of the filter for 1-5μm particles. Specifically, they can be classified into the following five major categories:
I. Core performance factors of the filter material itself
Filter material is the “core medium” of filtration, and its material, structure and process directly determine the upper limit of the filtration capacity.
I. Core performance factors of the filter material itself
Filter material is the “core medium” of filtration, and its material, structure and process directly determine the upper limit of the filtration capacity.
| Influencing factors | Specific explanation | The influence on the filtration effect |
| Filter material material | The common materials are polyester fiber (polyester), glass fiber and polypropylene fiber. The fineness, temperature resistance and corrosion resistance of the fibers of the three are different. |
Polyester fiber: Uniform fineness (2-5μm), high dust holding capacity, stable filtration efficiency, suitable for most laboratories; Glass fiber: The fibers are finer (1-3μm), with a slightly higher filtration efficiency, but it has a lower dust holding capacity, is prone to cracking, and the fibers tend to fall off over long-term use. Polypropylene fiber: It has strong corrosion resistance, but the fiber fluffiness is low, the resistance rises rapidly, and the filtration stability is relatively poor. |
| Fiber density and pore size | The fiber interweaving density of the filter material determines the pore size, which needs to match the target intercepted particles (1-5μm). |
Too small pore size: The initial filtration efficiency is high, but the resistance rises sharply, it is prone to clogging, and the service life is short. Excessive pore size: Unable to effectively intercept fine particles of 1-3μm, the filtration efficiency is insufficient. |
| Thickness of filter material | The conventional thickness is 0.3-0.8mm, and the thickness is positively correlated with the dust holding capacity and resistance. |
Too thin: Low dust holding capacity, easily penetrated by particles, and requires frequent replacement. Too thick: Excessive resistance increases air conditioning energy consumption, and uneven air flow distribution may lead to local filter failure. |
| Surface treatment process | Some filter materials will undergo treatments such as “electrostatic charging” and “waterproof coating”. |
Electrostatic charging: By electrostatically adsorbing fine particles, the filtration efficiency of 1-3μm particles can be enhanced (for example, F7 grade can increase by 5%-10%), but the static electricity tends to disappear when the humidity is high. Waterproof coating: It prevents the filter material from absorbing moisture and getting moldy (such as in a hot and humid laboratory), but if the coating is too thick, it may clog the pores and reduce efficiency. |
Ii. Factors in the Structural Design of Filters
The structural design of the filter determines whether the airflow can pass through the filter material evenly, avoiding “airflow short circuit” or “local overload”, which directly affects the actual filtration effect.
Number of bags and bag depth
The more bags there are (such as 3 bags, 5 bags, 7 bags), the larger the effective filtration area, the lower the air flow velocity, the easier it is for particles to be intercepted by the filter material, and the lower the resistance and higher the dust holding capacity. If the number of bags is insufficient (such as using a 1-bag type for small air volume), the load on the filter material will be too high and it is prone to rapid clogging. If the bag depth is too shallow (<150mm), the filter bag cannot be fully expanded, the actual filtration area will shrink, and the efficiency will decline.
Frame and sealing design
The filter frame needs to fit closely with the installation frame of the air conditioning unit. If the frame material (such as paper frames which are prone to deformation) or the sealing method (such as the absence of sealing strips) is poor, it will cause the unfiltered “bypass airflow” to directly enter the rear end around the filter material, resulting in a significant reduction in the filtration effect (possibly from 80% to below 50%).
Supporting structure
The supporting keels (such as plastic or metal frames) inside the filter bags should be evenly distributed. If the support is insufficient, the filter bags will be “blown flat” under high air volume, resulting in a reduction in the effective area due to the adhesion of the filter material. If the support is too dense, it will block the airflow and create local dead corners.
Iii. External usage conditions and factors
The actual operating environment of the laboratory (such as air flow, pollution, temperature and humidity, etc.) will dynamically affect the filtration efficiency and stability of the filter.
Airflow parameters
Air volume matching degree: Each filter has a rated air volume (such as 500m³/h, 1000m³/h). If the actual air volume exceeds the rated value, the air flow velocity is too fast, the residence time of particles on the filter material surface is short, and the interception efficiency decreases (for example, in F8 grade, when the air volume exceeds the rated value by 20%, the efficiency may drop from 95% to 85%). If the air volume is too low, the dust-holding speed of the filter material will be slow, but it is prone to breed microorganisms (especially in high-humidity environments).
Air flow uniformity: If the air distribution at the air conditioning supply outlet is uneven (such as single-side air supply), it will cause some filter bags to be overloaded and some to be idle, reducing the overall filtration effect.
Characteristics of intake air pollutants
Pollutant concentration: If there is a high concentration of particle contamination in the laboratory (such as sample grinding, dust operation), the filter material will rapidly accumulate dust, the resistance will increase, and the filtration efficiency will first rise and then fall (the initial accumulation of dust can assist in intercepting particles, and the later blockage will cause the airflow to penetrate).
Nature of pollutants: If oily particles (such as aerosols, lubricating oil mists) are present, they will adhere to the surface of the filter material to form an “oil film”, clogging the pore size and making it impossible to clean, resulting in a sudden drop in filtration efficiency. If corrosive gases (such as acid mists and chlorine) are present, they will erode the filter material (for instance, glass fibers are prone to corrosion and breakage), thereby shortening their service life.
Temperature and humidity environment
Temperature: The temperature tolerance range for polyester fiber filter media is typically -10 ℃ to 80℃. If the temperature exceeds the upper limit, the filter media will soften and deform, the pore size will increase, and the filtration efficiency will decline.
Humidity: When the relative humidity is greater than 80%, the filter material is prone to absorbing moisture, and the dust holding capacity decreases (moist particles are likely to clump and clog the pore size). Moreover, the static electricity of the electrostatically charged filter material will disappear, losing its adsorption capacity, resulting in a 10%-20% drop in the filtration efficiency for 1-3μm particles.
Iv. System Compatibility and Pre-processing Factors
The medium-efficiency filter is an intermediate link in the “three-stage filtration” (primary efficiency → medium-efficiency → high-efficiency), and its effect depends on the rationality of the front-end pretreatment and system matching.
The compatibility of the primary filter
The function of the primary filter (such as G3 and G4 grades) is to intercept large particles ≥5μm (such as dust and fibers). If the primary filtration efficiency is insufficient (for example, G2 grade is used to replace G4 grade), a large number of large particles will enter the medium-efficiency filter, causing the medium-efficiency filter material to clog rapidly and the resistance to increase The capacity originally designed to intercept particles of 1-5μm has been “occupied”, and the actual filtration effect has declined.
System sealing performance
If there are gaps in the air ducts and flange interfaces of the air conditioning unit, unfiltered air will directly mix into the air supply system, which is equivalent to “bypassing” the medium-efficiency filter, resulting in excessive particle concentration in the terminal air and failure of the filtration effect.
Fan pressure matching
If the air pressure of the fan is insufficient and cannot overcome the resistance of the medium-efficiency filter (especially when the resistance increases in the later stage of dust holding), it will lead to a decrease in the air supply volume, and the air flow rate will be too low, making it impossible for particles to be effectively intercepted. If the wind pressure is too high, it will cause the filter material to be “blown through”, especially when the frame seal is not good, it is easy to cause air flow short circuit.
V. Maintenance and Operation Factors
Improper installation, replacement and maintenance can prevent a “qualified filter” from functioning as it should and even accelerate its failure.
Installation operation
If the installation is not carried out in the direction of the “airflow arrow” (with the fluffy side of the filter bag facing the airflow in the opposite direction), it will cause the airflow to fail to pass through the filter material evenly, resulting in a sudden increase in resistance, and particles are prone to accumulate at the bottom of the filter bag, making it impossible to effectively intercept.
If the filter bag is pulled too hard during installation, causing damage to the filter material or deformation of the bag opening, local leakage will occur, and unfiltered air will directly pass through the damaged area.
Change the timing
The replacement of medium-efficiency filters should be based on the standard that “the resistance reaches twice the initial resistance” (for example, the initial resistance is 60Pa and the final resistance is 120Pa). If the filter material is replaced too late and completely clogged, it will lead to: ① The airflow penetrating the filter material, resulting in a decrease in filtration efficiency; ② Excessive resistance, fan overload, and insufficient air supply volume. If the filter material is replaced too early, it will cause waste, but it will not affect the filtration effect.
Maintain cleanliness
For reusable polyester fiber filter media, if they are not cleaned properly (such as being rinsed with a high-pressure water gun, causing the filter media fibers to fall off, or being installed directly without drying, leading to mold), the structure of the filter media will be damaged, the pore size will increase, and the filtration efficiency will permanently decline.
Summary: Priority ranking of key influencing factors
To ensure the filtration effect of medium-efficiency bag filters, the influencing factors should be controlled in accordance with the following priority:
Core premise: Select the appropriate filter material (for example, polyester fiber, F7-F8 grade is preferred in the laboratory) and a reasonable structure (sufficient bag number, sealed frame);
System foundation: Ensure that the primary filter is compatible, the system has good sealing performance, and the air volume/air pressure is matched.
Operation assurance: Control the concentration of pollutants and temperature and humidity in the laboratory to prevent oily/corrosive contamination.
Key operation points: Standardized installation, timely replacement according to resistance, and correct cleaning and maintenance.
Only by comprehensively controlling the above factors can the medium-efficiency bag filter continuously and stably play its core role of “pretreatment + particle interception”, ensuring the air quality in the laboratory.
The structural design of the filter determines whether the airflow can pass through the filter material evenly, avoiding “airflow short circuit” or “local overload”, which directly affects the actual filtration effect.
Number of bags and bag depth
The more bags there are (such as 3 bags, 5 bags, 7 bags), the larger the effective filtration area, the lower the air flow velocity, the easier it is for particles to be intercepted by the filter material, and the lower the resistance and higher the dust holding capacity. If the number of bags is insufficient (such as using a 1-bag type for small air volume), the load on the filter material will be too high and it is prone to rapid clogging. If the bag depth is too shallow (<150mm), the filter bag cannot be fully expanded, the actual filtration area will shrink, and the efficiency will decline.
Frame and sealing design
The filter frame needs to fit closely with the installation frame of the air conditioning unit. If the frame material (such as paper frames which are prone to deformation) or the sealing method (such as the absence of sealing strips) is poor, it will cause the unfiltered “bypass airflow” to directly enter the rear end around the filter material, resulting in a significant reduction in the filtration effect (possibly from 80% to below 50%).
Supporting structure
The supporting keels (such as plastic or metal frames) inside the filter bags should be evenly distributed. If the support is insufficient, the filter bags will be “blown flat” under high air volume, resulting in a reduction in the effective area due to the adhesion of the filter material. If the support is too dense, it will block the airflow and create local dead corners.
Iii. External usage conditions and factors
The actual operating environment of the laboratory (such as air flow, pollution, temperature and humidity, etc.) will dynamically affect the filtration efficiency and stability of the filter.
Airflow parameters
Air volume matching degree: Each filter has a rated air volume (such as 500m³/h, 1000m³/h). If the actual air volume exceeds the rated value, the air flow velocity is too fast, the residence time of particles on the filter material surface is short, and the interception efficiency decreases (for example, in F8 grade, when the air volume exceeds the rated value by 20%, the efficiency may drop from 95% to 85%). If the air volume is too low, the dust-holding speed of the filter material will be slow, but it is prone to breed microorganisms (especially in high-humidity environments).
Air flow uniformity: If the air distribution at the air conditioning supply outlet is uneven (such as single-side air supply), it will cause some filter bags to be overloaded and some to be idle, reducing the overall filtration effect.
Characteristics of intake air pollutants
Pollutant concentration: If there is a high concentration of particle contamination in the laboratory (such as sample grinding, dust operation), the filter material will rapidly accumulate dust, the resistance will increase, and the filtration efficiency will first rise and then fall (the initial accumulation of dust can assist in intercepting particles, and the later blockage will cause the airflow to penetrate).
Nature of pollutants: If oily particles (such as aerosols, lubricating oil mists) are present, they will adhere to the surface of the filter material to form an “oil film”, clogging the pore size and making it impossible to clean, resulting in a sudden drop in filtration efficiency. If corrosive gases (such as acid mists and chlorine) are present, they will erode the filter material (for instance, glass fibers are prone to corrosion and breakage), thereby shortening their service life.
Temperature and humidity environment
Temperature: The temperature tolerance range for polyester fiber filter media is typically -10 ℃ to 80℃. If the temperature exceeds the upper limit, the filter media will soften and deform, the pore size will increase, and the filtration efficiency will decline.
Humidity: When the relative humidity is greater than 80%, the filter material is prone to absorbing moisture, and the dust holding capacity decreases (moist particles are likely to clump and clog the pore size). Moreover, the static electricity of the electrostatically charged filter material will disappear, losing its adsorption capacity, resulting in a 10%-20% drop in the filtration efficiency for 1-3μm particles.
Iv. System Compatibility and Pre-processing Factors
The medium-efficiency filter is an intermediate link in the “three-stage filtration” (primary efficiency → medium-efficiency → high-efficiency), and its effect depends on the rationality of the front-end pretreatment and system matching.
The compatibility of the primary filter
The function of the primary filter (such as G3 and G4 grades) is to intercept large particles ≥5μm (such as dust and fibers). If the primary filtration efficiency is insufficient (for example, G2 grade is used to replace G4 grade), a large number of large particles will enter the medium-efficiency filter, causing the medium-efficiency filter material to clog rapidly and the resistance to increase The capacity originally designed to intercept particles of 1-5μm has been “occupied”, and the actual filtration effect has declined.
System sealing performance
If there are gaps in the air ducts and flange interfaces of the air conditioning unit, unfiltered air will directly mix into the air supply system, which is equivalent to “bypassing” the medium-efficiency filter, resulting in excessive particle concentration in the terminal air and failure of the filtration effect.
Fan pressure matching
If the air pressure of the fan is insufficient and cannot overcome the resistance of the medium-efficiency filter (especially when the resistance increases in the later stage of dust holding), it will lead to a decrease in the air supply volume, and the air flow rate will be too low, making it impossible for particles to be effectively intercepted. If the wind pressure is too high, it will cause the filter material to be “blown through”, especially when the frame seal is not good, it is easy to cause air flow short circuit.
V. Maintenance and Operation Factors
Improper installation, replacement and maintenance can prevent a “qualified filter” from functioning as it should and even accelerate its failure.
Installation operation
If the installation is not carried out in the direction of the “airflow arrow” (with the fluffy side of the filter bag facing the airflow in the opposite direction), it will cause the airflow to fail to pass through the filter material evenly, resulting in a sudden increase in resistance, and particles are prone to accumulate at the bottom of the filter bag, making it impossible to effectively intercept.
If the filter bag is pulled too hard during installation, causing damage to the filter material or deformation of the bag opening, local leakage will occur, and unfiltered air will directly pass through the damaged area.
Change the timing
The replacement of medium-efficiency filters should be based on the standard that “the resistance reaches twice the initial resistance” (for example, the initial resistance is 60Pa and the final resistance is 120Pa). If the filter material is replaced too late and completely clogged, it will lead to: ① The airflow penetrating the filter material, resulting in a decrease in filtration efficiency; ② Excessive resistance, fan overload, and insufficient air supply volume. If the filter material is replaced too early, it will cause waste, but it will not affect the filtration effect.
Maintain cleanliness
For reusable polyester fiber filter media, if they are not cleaned properly (such as being rinsed with a high-pressure water gun, causing the filter media fibers to fall off, or being installed directly without drying, leading to mold), the structure of the filter media will be damaged, the pore size will increase, and the filtration efficiency will permanently decline.
Summary: Priority ranking of key influencing factors
To ensure the filtration effect of medium-efficiency bag filters, the influencing factors should be controlled in accordance with the following priority:
Core premise: Select the appropriate filter material (for example, polyester fiber, F7-F8 grade is preferred in the laboratory) and a reasonable structure (sufficient bag number, sealed frame);
System foundation: Ensure that the primary filter is compatible, the system has good sealing performance, and the air volume/air pressure is matched.
Operation assurance: Control the concentration of pollutants and temperature and humidity in the laboratory to prevent oily/corrosive contamination.
Key operation points: Standardized installation, timely replacement according to resistance, and correct cleaning and maintenance.
Only by comprehensively controlling the above factors can the medium-efficiency bag filter continuously and stably play its core role of “pretreatment + particle interception”, ensuring the air quality in the laboratory.
