Air Purification Filter Knowledge (Basic Principles)
2026-02-02
Air Purification Filter Knowledge (Basic Principles)
1. Filter Media
The core function of filter media is to effectively capture dust particles without creating excessive resistance to airflow. The randomly interwoven fibers form countless barriers to trap particles, while the wide spaces between fibers allow airflow to pass through smoothly.
2. Service Life
As more dust accumulates on the filter media, resistance increases. When resistance exceeds the design limit, the filter reaches the end of its service life. Excessive resistance can also cause captured dust to become airborne, resulting in secondary contamination, which also requires filter replacement.
3. Electrostatic Effect
If the filter media or dust particles carry a static charge, filtration efficiency is significantly improved. Electrostatic forces alter the trajectory of dust particles, causing them to collide with filter fibers and enhancing adhesion.
4. Filtration Efficiency
The definition of dust "load" varies, leading to different calculated and measured efficiency values. In practical applications, dust load can be expressed by total weight, particle count, concentration of specific particle sizes, or total dust concentration. Indirect measurement methods include light transmittance (colorimetric method) and fluorescence intensity (fluorescence method). Efficiency values can be instantaneous or weighted averages over the filter's lifespan.
Different test methods yield different efficiency results for the same filter. Filtration efficiency cannot be meaningfully discussed without specifying the test method.
5. Filter Resistance
(1) Resistance Variation
Filters create resistance to airflow, which increases as dust accumulates. The filter is replaced when resistance reaches a predetermined threshold. The resistance of a new filter is called "initial resistance," while the resistance at replacement is called "final resistance."
(2) Impact and Recommended Final Resistance Values
Impact: Final resistance selection directly affects filter lifespan, system airflow stability, and energy consumption. In most cases, final resistance is 2–4 times the initial resistance.
Recommended Values:
Efficiency Class Recommended Final Resistance (Pa)
G3 (Coarse) 100 ~ 200
G4 (Medium-Coarse) 150 ~ 250
F5 ~ F6 (Medium) 250 ~ 300
F7 ~ F8 (High-Medium) 300 ~ 400
F9 ~ H11 (Sub-High Efficiency) 400 ~ 450
High & Ultra-High Efficiency 400 ~ 600
Notes: Resistance increases more rapidly as filters become dirtier. Excessively high final resistance does not significantly extend lifespan but reduces system airflow, so it should be avoided. For low-efficiency filters (below G4) using coarse fibers, high resistance can dislodge accumulated dust, reducing efficiency to zero, requiring strict final resistance limits. Each filter section must include resistance monitoring instruments; final resistance must be measured instrumentally, not estimated.
6. Dust Holding Capacity
Dust holding capacity is the mass of test dust a filter can hold under specific conditions, defined as:
a. Standard test wind tunnel and associated equipment;
b. Standard "road dust" with larger particles than ambient air;
c. Test methods and calculation procedures agreed between the client and laboratory or specified by standards;
d. Termination conditions agreed between the client and laboratory.
Dust holding capacity does not directly correlate with real-world dust retention, so isolated values have limited practical significance.
7. Respirable Particulate Matter
Large dust particles are trapped in the nasal cavity, while smaller particles may penetrate the trachea and lungs. These particles are engulfed and digested by macrophages, with remaining bacteria and viruses eliminated by white blood cells.
Nasal hair, mucus, and mucosal membranes filter out most particles larger than 10 μm, leaving "respirable particulate matter" (PM10, particles ≤10 μm). Removing particles larger than 5 μm yields PM5. Total Suspended Particulates (TSP) refers to all airborne dust particles.
Health Effects of Respirable Particulates
TSP Concentration (mg/m³) PM10 Concentration (mg/m³) Health Effect
>0.29 >0.20 Threshold for immune function changes; increased respiratory disease prevalence
0.21 0.15 Maximum allowable 24-hour average concentration for residential areas
<0.16 <0.11 Sub-threshold level; no significant immune effects in children or increased respiratory symptoms
8. Activated Carbon Media
Activated carbon media includes granular carbon, fibrous carbon, and powdered carbon. Fibrous activated carbon is made from carbonaceous organic fibers, offering small pore size (<50 Å), high adsorption capacity, rapid adsorption, and regeneration. Common fiber substrates include phenolic resin, plant fibers, polyacrylonitrile, and pitch.
9. Adsorption Performance
Adsorption Capacity: The maximum amount of contaminants adsorbed per unit mass of carbon. Capacity varies by carbon type, target gas, temperature, and background concentration.
Residence Time: The duration air remains in the carbon bed. Longer residence time improves adsorption efficiency, requiring sufficient bed depth and low filtration velocity.
Service Life: New activated carbon has high adsorption efficiency, which decreases over time. The filter is replaced when downstream contaminant concentrations approach allowable limits; this period is called "effective protection time."
Selectivity: Gases with high molecular weight, high boiling points, and volatile organic compounds are preferentially adsorbed in physical processes. Chemically impregnated carbon can remove challenging gases or enhance selectivity for specific contaminants.
10. Activated Carbon Filter Selection
Key factors affecting adsorption efficiency and lifespan include contaminant type and concentration, residence time, air temperature, and humidity.
Selection depends on contaminant properties, concentration, and airflow rate, determining filter configuration and carbon type.
Activated carbon filters should be installed with pre-filters and post-filters of at least F7 efficiency. Pre-filters prevent dust clogging, while post-filters contain carbon dust emissions.
1. Filter Media
The core function of filter media is to effectively capture dust particles without creating excessive resistance to airflow. The randomly interwoven fibers form countless barriers to trap particles, while the wide spaces between fibers allow airflow to pass through smoothly.
2. Service Life
As more dust accumulates on the filter media, resistance increases. When resistance exceeds the design limit, the filter reaches the end of its service life. Excessive resistance can also cause captured dust to become airborne, resulting in secondary contamination, which also requires filter replacement.
3. Electrostatic Effect
If the filter media or dust particles carry a static charge, filtration efficiency is significantly improved. Electrostatic forces alter the trajectory of dust particles, causing them to collide with filter fibers and enhancing adhesion.
4. Filtration Efficiency
The definition of dust "load" varies, leading to different calculated and measured efficiency values. In practical applications, dust load can be expressed by total weight, particle count, concentration of specific particle sizes, or total dust concentration. Indirect measurement methods include light transmittance (colorimetric method) and fluorescence intensity (fluorescence method). Efficiency values can be instantaneous or weighted averages over the filter's lifespan.
Different test methods yield different efficiency results for the same filter. Filtration efficiency cannot be meaningfully discussed without specifying the test method.
5. Filter Resistance
(1) Resistance Variation
Filters create resistance to airflow, which increases as dust accumulates. The filter is replaced when resistance reaches a predetermined threshold. The resistance of a new filter is called "initial resistance," while the resistance at replacement is called "final resistance."
(2) Impact and Recommended Final Resistance Values
Impact: Final resistance selection directly affects filter lifespan, system airflow stability, and energy consumption. In most cases, final resistance is 2–4 times the initial resistance.
Recommended Values:
Efficiency Class Recommended Final Resistance (Pa)
G3 (Coarse) 100 ~ 200
G4 (Medium-Coarse) 150 ~ 250
F5 ~ F6 (Medium) 250 ~ 300
F7 ~ F8 (High-Medium) 300 ~ 400
F9 ~ H11 (Sub-High Efficiency) 400 ~ 450
High & Ultra-High Efficiency 400 ~ 600
Notes: Resistance increases more rapidly as filters become dirtier. Excessively high final resistance does not significantly extend lifespan but reduces system airflow, so it should be avoided. For low-efficiency filters (below G4) using coarse fibers, high resistance can dislodge accumulated dust, reducing efficiency to zero, requiring strict final resistance limits. Each filter section must include resistance monitoring instruments; final resistance must be measured instrumentally, not estimated.
6. Dust Holding Capacity
Dust holding capacity is the mass of test dust a filter can hold under specific conditions, defined as:
a. Standard test wind tunnel and associated equipment;
b. Standard "road dust" with larger particles than ambient air;
c. Test methods and calculation procedures agreed between the client and laboratory or specified by standards;
d. Termination conditions agreed between the client and laboratory.
Dust holding capacity does not directly correlate with real-world dust retention, so isolated values have limited practical significance.
7. Respirable Particulate Matter
Large dust particles are trapped in the nasal cavity, while smaller particles may penetrate the trachea and lungs. These particles are engulfed and digested by macrophages, with remaining bacteria and viruses eliminated by white blood cells.
Nasal hair, mucus, and mucosal membranes filter out most particles larger than 10 μm, leaving "respirable particulate matter" (PM10, particles ≤10 μm). Removing particles larger than 5 μm yields PM5. Total Suspended Particulates (TSP) refers to all airborne dust particles.
Health Effects of Respirable Particulates
TSP Concentration (mg/m³) PM10 Concentration (mg/m³) Health Effect
>0.29 >0.20 Threshold for immune function changes; increased respiratory disease prevalence
0.21 0.15 Maximum allowable 24-hour average concentration for residential areas
<0.16 <0.11 Sub-threshold level; no significant immune effects in children or increased respiratory symptoms
8. Activated Carbon Media
Activated carbon media includes granular carbon, fibrous carbon, and powdered carbon. Fibrous activated carbon is made from carbonaceous organic fibers, offering small pore size (<50 Å), high adsorption capacity, rapid adsorption, and regeneration. Common fiber substrates include phenolic resin, plant fibers, polyacrylonitrile, and pitch.
9. Adsorption Performance
Adsorption Capacity: The maximum amount of contaminants adsorbed per unit mass of carbon. Capacity varies by carbon type, target gas, temperature, and background concentration.
Residence Time: The duration air remains in the carbon bed. Longer residence time improves adsorption efficiency, requiring sufficient bed depth and low filtration velocity.
Service Life: New activated carbon has high adsorption efficiency, which decreases over time. The filter is replaced when downstream contaminant concentrations approach allowable limits; this period is called "effective protection time."
Selectivity: Gases with high molecular weight, high boiling points, and volatile organic compounds are preferentially adsorbed in physical processes. Chemically impregnated carbon can remove challenging gases or enhance selectivity for specific contaminants.
10. Activated Carbon Filter Selection
Key factors affecting adsorption efficiency and lifespan include contaminant type and concentration, residence time, air temperature, and humidity.
Selection depends on contaminant properties, concentration, and airflow rate, determining filter configuration and carbon type.
Activated carbon filters should be installed with pre-filters and post-filters of at least F7 efficiency. Pre-filters prevent dust clogging, while post-filters contain carbon dust emissions.
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