This article takes an in depth look at air filters.
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Air filters are essential devices designed to remove airborne contaminants, including particles, pollutants, and microorganisms, which can pose health risks and harm the environment. In manufacturing settings, they help maintain product and material quality, protect critical equipment, and prevent product loss.
Cleanrooms use a combination of high efficiency particulate air (HEPA) filters, which are approximately 100 times more efficient than a typical furnace filter, with methods for controlling air flow to keep particulate counts within acceptable parameters. Exhaust and stack gasses are filtered and cleaned before releasing them into the atmosphere. Air filters are used in home furnaces, offices, laboratories, clinics, hi-tech microelectronics manufacturing, chip and hard-drive production and the aerospace industry.
Air filters efficiently capture a wide range of contaminants from the air, including dust, dirt, smoke, aerosols, odors, viruses, molds, bacteria, and toxic gases. These pollutants can contribute to and worsen respiratory conditions, skin problems, allergies, and other health issues.
Air filters operate by drawing in contaminated air through their filter medium, which contains fine openings. Particles larger than these openings are captured and retained within the filter, effectively removing them from the air stream. As a result, the air exiting the filter has a significantly lower concentration of particles. Over time, the filtration efficiency improves as the buildup of particles on the filter’s upstream side helps trap additional contaminants. To maintain optimal performance and prevent airflow restriction due to clogging or blinding, the filter medium needs to be replaced or cleaned regularly.
Filter media are the core component of air filters, tasked with trapping unwanted particles and preventing their recirculation in the air. They typically consist of a synthetic fiber or mesh material with fine or microscopic perforations, and are held within a frame that is installed in the air filter system.
Air filter media are generally classified into two types: pleated and non-pleated filters.
Pleated air filters feature folds or pleats that expand the surface area available for filtration. These filters are housed in a frame or cartridge that maintains their pleated structure.
Due to their increased surface area, pleated air filters are highly effective at capturing small and microscopic particles. They offer high filtration efficiency, significantly enhancing indoor air quality by trapping pollutants such as odors, bacteria, pollen, mold spores, and other allergens.
Pleated filters are typically made from pliable materials like paper, polyester, cloth, or cotton. However, the density of these materials can restrict airflow, causing HVAC systems or air filtration equipment to work harder to push air through the filter. This increased workload results in higher energy consumption. Additionally, pleated air filters are generally more expensive than their non-pleated counterparts and may require modifications to HVAC units for proper installation.
Non-pleated air filters have less filtration area but are designed to handle higher air pressures and flow rates. Typically made from woven fiberglass or electrostatic materials, they are more affordable compared to pleated filters. However, they generally have a shorter service life and require more frequent replacement or cleaning. Non-pleated filters are effective primarily in capturing larger particles like dust, debris, and insects, making them less suitable for individuals with respiratory conditions or allergies who need finer particle filtration.
The following materials are commonly used in the construction of air filter media. These materials are often engineered or combined with other substances to enhance the filter's efficiency while minimizing airflow resistance.
Paper is a cost-effective yet less durable material for filter media. It filters out small particles by allowing air to flow through its fine pores. Typically made by compressing woven wood pulp fibers, paper filters are often pleated to enhance their strength and efficiency. These filters are commonly found in automobile air cleaners, furnace filters, and indoor air purifiers.
Foam air filters are made of engineered porous foams made from polyurethane, polyether, polyester, or a combination of those materials. They are characterized by their Pores Per Inch (PPI), which is the number of open pores per linear inch. Foam filters with higher PPI have lower resistance to airflow than those with lower PPI of the same thickness. They also have lower efficiency due to their larger open pores and have less filter material that strangles the particles. Air is progressively purified as it passes through the depth of the foam filter.
Foam air filters are known for their high dirt retention capacity, making them ideal for use as exhaust filters and in automobile intake systems. They effectively capture and hold large particles like dust, dirt, and debris. The efficiency and dirt retention of foam filters are further enhanced by applying filter oil. Additionally, foam air filters are easy to clean, washable, and can be recycled for multiple uses.
Activated carbon filters are designed to eliminate harmful gases such as volatile organic compounds (VOCs), sulfur dioxide, and benzene, as well as allergens, fumes, and unpleasant odors from the air by trapping them on the surface of the activated carbon. These filters are frequently found in air purifiers, range hoods (paired with aluminum filters), bathroom ventilation systems, and microwave ovens.
Typically, activated carbon is the material used for this purpose. It is produced from charcoal through physical or chemical processes that enhance its porosity and surface area.
Aluminum filters consist of several layers of aluminum mesh arranged in alternating patterns to enhance their ability to filter and retain particles. These filters are housed in a sturdy frame and serve as a pre-filter in multi-stage air filtration systems, preventing larger particles from reaching the primary filter. They are also effective at capturing grease and coolant mists.
These filters are known for their strength and long-lasting durability, as they can be washed and reused multiple times. Their performance is reliable in various conditions due to their resistance to corrosion and high temperatures. Common uses include HVAC systems, range hoods, and automotive applications.
Aluminum is a prevalent choice for metal mesh air filters, though alternatives such as stainless steel and galvanized steel are also available.
Fiberglass air filters enhance airflow in HVAC systems and are designed to capture larger particles. However, they are less effective at trapping microscopic particles, which can still pass through the fiberglass medium. As a result, their filtration efficiency is limited, making them less suitable for environments where people have respiratory conditions or allergies, although the air quality remains acceptable for healthy individuals. While these filters are more affordable than pleated alternatives, they need to be replaced more often and require regular maintenance due to their tendency to clog.
Plastic air filters are known for their robust strength, long-lasting durability, and resistance to chemicals. They are typically crafted from materials such as woven HDPE, UHMW-PE, polypropylene, polyester, and PTFE fibers. Additionally, certain plastic filters possess electrostatic features to enhance their performance.
Air filter performance is typically assessed using the following ratings:
The Minimum Efficiency Reporting Value (MERV) is a scale used to assess how effectively air filters capture particles ranging from 0.3 to 10 microns in diameter. This rating helps determine the filter’s ability to maintain good air quality and allows for performance comparisons between different air filters. The MERV rating system was developed by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE).
A higher MERV rating indicates a greater ability of the air filter to trap smaller particles. As the rating increases, the filter can capture finer particles. However, filters with higher MERV ratings often have a denser filter medium, which can reduce airflow, lead to higher energy consumption, and put additional strain on the HVAC system and its components.
The MERV rating scale is detailed in the table below:
| MERV Rating | Air Filter will trap Air Partiles size .3 to 1.0 microns | Air Filter will trap Air Partiles size 1.0 to 3.0 microns | Air Filter will trap Air Partiles size 3 to 10 microns | Filter Type ~ Removes These Particles |
|---|---|---|---|---|
| MERV 1 | < 20% | < 20% | < 20% | Fiberglass & Aluminum Mesh |
| MERV 2 | < 20% | < 20% | < 20% | ~ |
| MERV 3 | < 20% | < 20% | < 20% | Pollen, Dust Mites, Spray Paint, |
| MERV 4 | < 20% | < 20% | < 20% | Carpet Fibres |
| MERV 5 | < 20% | < 20% | 20% - 34% | Cheap Disposable Filters |
| MERV 6 | < 20% | < 20% | 35% - 49% | ~ |
| MERV 7 | < 20% | < 20% | 50% - 69% | Mold Spores, Cooking Dusts, |
| MERV 8 | < 20% | < 20% | 70% - 85% | Hair Spray, Furniture Polish |
| MERV 9 | < 20% | Less than 50% | 85% or Better | Better Home Box Filters |
| MERV 10 | < 20% | 50% to 64% | 85% or Better | ~ |
| MERV 11 | < 20% | 65% to 79% | 85% or Better | Lead Dust, Flour, Auto |
| MERV 12 | < 20% | 80% to 90% | 85% or Better | Fumes, Walking Fumes |
| MERV 13 | Less than 75% | 90% or Better | 90% or Better | Superior Commercial Filters |
| MERV 14 | 75% to 84% | 90% or Better | 90% or Betterr | ~ |
| MERV 15 | 85% - 94% | 95% or Better | 90% or Better | Bacteria, Smoke, Sneezes |
| MERV 16 | 95% or Better | 95% or Better | 90% or Better | |
| *MERV 17 = HEPA 13 | 99.97% | 99% or Better | 99% or Better | HEPA & ULPA |
| *MERV 18 = HEPA 14 | 99.997% | 99% or Better | 99% or Better | ~ |
| *MERV 19 = HEPA 15 | 99.9997% | 99% or Better | 99% or Better | Viruses, Carbon Dust, < 0.3 p |
| *MERV 20 = HEPA 16 | 99.999997% | 99% or Better | 99% or Better | *ASHRAE does not recognize Merv 17-20 |
Filters with MERV ratings exceeding 16 are categorized as HEPA or ULPA filters.
Arrestance is a measure used for air filters with lower MERV ratings to gauge their efficiency. It reflects the filter's capability to capture synthetic dust from the air. A higher arrestance value indicates that the filter is more effective at removing larger particles, such as dust, hair, lint, and dirt.
Dust holding capacity measures the average amount of dust an air filter can accumulate when progressively loaded with synthetic dust under controlled testing conditions.
Arrestance and dust holding capacity are metrics used for air filters with MERV ratings between 1 and 4. These parameters are outlined in the ANSI/ASHRAE Standard 52.2.
Dust spot efficiency measures an air filter's effectiveness at removing airborne dust from the air stream. This is assessed through a staining test that calculates efficiency based on the flow rates and opacity of the air before and after filtration. This metric is specified in the 2009 ANSI/ASHRAE Standard 52.1, which is an earlier version of the ANSI/ASHRAE Standard 52.2.
The Most Penetrating Particle Size (MPPS) refers to the particle size that is most likely to bypass the air filter and pass through it with the greatest ease.
The following are the various types of air filter equipment:
Activated carbon air filters excel at eliminating gaseous pollutants, fumes, vapors, and odors from the air. They function by adsorbing these gaseous molecules onto the surface of the activated carbon. To achieve effective removal, the molecules must have sufficient time to interact with the carbon matrix.
As standalone filters, they are not capable of capturing solid particles, so they are often used alongside HEPA or electrostatic filters to enhance overall air quality. The optimal replacement frequency for activated carbon filters is not well-defined, as they do not exhibit visible signs of saturation.
Air compressor filters, or airline filters, are installed in condensed airlines, which are used to remove water, solid particulates, oil, and other contaminants in a multi-stage filtration process. They prevent these contaminants and protect the internal components of the air compressor unit, ensuring the unit is in top condition.
Baghouse filters are designed to capture particulates and air pollutants, such as soot, ash dust, smoke, and toxic gases, from flue gases emitted by industrial processes like boilers, burners, and furnaces before these gases are released into the environment. The filters consist of multiple layers of pleated materials, including polymers, fabrics, or glass fibers, which are arranged in a frame and contained within a vessel. The captured particles accumulate in a hopper located at the bottom of the vessel. Baghouse filters are known for their high airflow capacity.
Cabin air filters are specialized HVAC filters used in vehicles. Typically constructed from multi-layered paper filter media, these filters ensure that the air entering the passenger compartment is clean and free from contaminants. They also help protect the vehicle’s air conditioning system by filtering particulates from the air before it reaches the air conditioner. Cabin air filters generally have different replacement intervals compared to engine air filters.
Car exhaust filters are installed directly in the exhaust pipes of vehicles to capture fine, harmful particles from engine emissions, thereby reducing atmospheric pollution. However, these filters are not designed to capture toxic gases such as carbon monoxide and nitrogen dioxide.
A Diesel Particulate Filter (DPF) is a specialized exhaust filter designed for diesel engines. It features filter media made from ceramic material arranged in a honeycomb structure, which captures soot, ash, and other particulates.
DPFs are equipped with an oxidative catalytic converter that transforms the carbon in the collected soot into carbon dioxide through passive or active regeneration processes. These regeneration methods can occur automatically without driver intervention, making DPFs self-cleaning to some extent. Nonetheless, regular maintenance is still required to ensure optimal performance.
Engine air filters are typically rectangular and pleated, designed to trap particulates from the air before it enters the engine. By preventing solid particles from entering the engine, these filters help avoid wear and damage to internal components, reduce fuel consumption, and maintain engine efficiency.
Engine air filters generally need to be replaced every 15,000 to 30,000 miles, depending on the vehicle type and driving conditions. A buildup of debris in the filter can restrict airflow, impair acceleration, and lead to increased emissions of harmful gases.
Exhaust filters are installed in ventilation systems to filter the air from a closed space before releasing it to the environment.
Exhaust hood filters, also known as grease filters, are used in kitchen hoods to capture grease, oil, smoke, and odors generated during cooking. Typically constructed from metal filter media, these filters are installed above stoves, grills, and fryers to maintain air quality in the kitchen.
HEPA stands for High-Efficiency Particulate Air. To qualify as a true HEPA filter, as specified by MIL-STD-282, the filter must capture at least 99.97% of airborne particles that are 0.3 microns in diameter. According to European and ISO standards, specifically ISO 29463 and EN 1822, the required efficiency is 99.95%. Filters with efficiencies ranging from 85% to 99.95% are classified as EPA filters.
HEPA filters typically use media composed of multiple layers of borosilicate glass fibers or polypropylene fibers arranged in a random, web-like pattern to enhance particulate capture. These filters employ a combination of mechanisms to trap particulates, including:
HEPA filters are crucial for protecting user health as they efficiently capture microorganisms, allergens, odors, irritants, and smoke that could potentially cause illness. They are extensively utilized in settings such as hospitals, clinics, cleanrooms, and chemical manufacturing facilities.
HVAC filters prevent particulates (e.g., dust, dirt, debris) and other contaminants in the air from entering the internal components of the HVAC system. These solid particles can damage and deteriorate the efficiency of the HVAC system. HVAC filters also improve the circulating air quality inside a room or facility. AC filters and furnace filters are types of HVAC filters and basically have the same design and construction.
Given that HVAC filters rely on proper airflow for effective operation, it is advisable to use furnace filters with a MERV rating below 13 for residential settings to ensure optimal performance and prevent airflow restrictions.
In an ionizer air filter, air first passes through a pre-filter before reaching the ionizer, which charges airborne particles electrically. These charged particles are then attracted to and captured by plates with an opposite charge. This type of filter does not use traditional filter media. Ionizer air filters are often called electrostatic air filters.
Ionizer air filters typically have a MERV rating between 4 and 5. They are cost-effective and have a long lifespan but require regular maintenance to ensure proper airflow. However, they may not be ideal for individuals with respiratory issues, as the electric field they generate can produce ozone, which may exacerbate asthma and other lung conditions.
Ultra-low particulate air (ULPA) filters can remove even smaller particles in the air, down to 0.12 microns in diameter with at least 99.99% efficiency. ULPA and HEPA filters have the same working mechanism; however, ULPA filters have lower filter medium porosity. Hence, their greater filter media density reduces airflow; this results in higher energy consumption than HEPA filters. Moreover, they have a shorter service life and are more expensive.
UV air filters use intense short-wave ultraviolet light to destroy the DNA of bacteria, viruses, molds, and other pathogens, effectively neutralizing them. However, when used alone, they cannot capture solid particulates or gaseous pollutants. To enhance overall air quality, UV filters are often combined with HEPA filters, similar to how activated carbon filters are used.
Washable electrostatic air filters rely on an electrostatic filter media, which is typically made from woven polypropylene or polyester. Airborne particles encounter friction as it passes through the filter media. They eventually become charged and are attracted to the inner layers of the filter media. The filter media holds the particles by static electricity until it is washed to remove those particles. Its electrostatic property remains constant over time and is not affected by washing.
Washable electrostatic air filters typically have a MERV rating between 6 and 8. They are a safe choice as they do not produce ozone. These filters are cost-effective, durable, and offer a long service life.
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