This article takes an in depth look at HEPA filters and their use.
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A HEPA filter is a specialized air filter designed to trap very tiny particles, as small as one micron (µ), or one-millionth of a meter. These filters are integral to air purifiers because they adhere to strict guidelines for removing pollutants. The acronym HEPA stands for High Efficiency Particulate Air, denoting a standard for air filtration systems that effectively eliminate even the smallest particles.
To qualify as a HEPA filter, it must capture 99.9% of airborne particles as small as 0.3 micrometers. These filters are designed to remove a wide range of contaminants, including dust mites, pet dander, pollen, smoke, mold spores, and other microscopic pollutants that cannot be seen by the naked eye.
A HEPA filtration system directs air through a dense mesh of fine fibers that trap harmful particles. This mesh, composed of thousands of delicate strands, effectively captures even the tiniest pollutants. Although all HEPA filters adhere to a specific standard, there are various classifications that indicate different levels of filtration efficiency.
When talking about HEPA filters, it's crucial to differentiate between “True HEPA” filters and those labeled as “HEPA Like,” “HEPA Type,” or “HEPA Style.” A filter must remove 99.97% of particles as small as 0.3 microns to be considered a True HEPA filter. This standard, set by the Environmental Protection Agency (EPA), uses a grading scale from “A” (least effective) to “E” (most effective) based on military standards.
HEPA filters function similarly to strainers or sieves but with a much more complex design that effectively captures extremely small airborne particles.
HEPA filtration involves four mechanisms: impaction, interception, diffusion, and electrostatic attraction. Initially, larger particles are filtered out, and progressively smaller particles are trapped, a process that can be illustrated with the provided diagram.
When particles with a diameter of 1.0 μm or greater enter a HEPA filter, they collide with the filter's fibers and are too large to pass through. This mechanism, known as inertial impaction, effectively captures most large particles and contaminants like pollen.
To prevent rapid saturation of the HEPA filter by these larger particles, most filtration systems include a prefilter to remove them before the air reaches the HEPA filter. Although 1.0 μm particles are considered large in the context of HEPA filtration, they are still minuscule compared to objects like human hair or grains of sand, as illustrated in the image below.
Following inertial impaction, particles sized 0.3 μm or 1 μm proceed to the interception stage of the filter. As these small particles exit the inertial impaction zone, they try to navigate through the interception area, following the airflow. Due to their weight and size, they are unable to pass through the fibers and become trapped or lodged.
During the interception phase, particles as small as 0.1 μm are captured when they come into contact with the filter’s fibers. The particles adhere to the fibers, which is why this stage is known as interception.
As particles accumulate and fill the filter, it becomes necessary to replace it. Unlike the inertial impaction stage, which is often protected by a pre-filter, the interception stage cannot benefit from such protection.
At this point in the filtration process, particles are so minuscule that they exhibit very little mass and move erratically in a zigzag pattern, known as Brownian motion. This is the random movement of microscopic particles suspended in a gas, where they continuously collide with each other due to their tiny size and low mass.
Despite their small size, which might suggest they could evade capture, HEPA filters are designed to address Brownian motion. Engineers took this into account when developing HEPA filters, ensuring that the final stage of filtration effectively captures these tiny particles.
To capture particles as tiny as 0.1 μm, the fibers in the diffusion section of the filter are arranged randomly, without any specific patterns. As these minuscule particles move erratically, they collide with each other and lose speed. This reduced movement allows them to adhere to the fibers of the filter, as illustrated in the image below.
Very small particles, such as smoke and dust, carry an electrostatic charge. HEPA filter fibers also possess an electrostatic charge. This creates an electrostatic attraction between positively and negatively charged particles. In the final filtration stage, these tiny particles are drawn to and held by the filter fibers due to this attraction.
The diagram below illustrates the entire HEPA filtration process.
A HEPA filtration system inevitably causes a pressure drop in airflow because it obstructs air movement while filtering. The extent of this pressure drop varies based on the HEPA filter type and system design, including the presence of a prefilter. The fan’s power also influences the pressure drop. Typically, HEPA filters can manage airflow up to 250 feet per minute (FPM) with minimal pressure drop.
HEPA filters feature densely packed fibers to capture even the smallest particles. This dense arrangement results in a considerable pressure drop as it resists air flow. As the filter becomes clogged with particles, the pressure drop increases. To mitigate this, HEPA filters should be inspected regularly, based on their usage and airflow conditions.
HEPA stands for High Efficiency Particulate Air and signifies a filter capable of removing 99.9% of airborne particles. This designation, approved by agencies like the EPA, EN, and IEST, includes various levels of filter efficiency.
Over the past two decades, HEPA filters have gained importance for both residential and specialized industrial uses. Various agencies have developed classification and rating systems to guide manufacturers and users. One such system is the Minimum Efficiency Reporting Value (MERV), which rates HEPA filters between 17 and 20, indicating their effectiveness and recommended applications.
A HEPA filter’s rating depends on its ability to capture different particle sizes. The MERV scale ranks filters based on their efficiency, with HEPA filters scoring between 17 and 20, as shown in the chart below. This rating signifies that HEPA filters can capture particles such as viruses, carbon dust, sea salt, smoke, and bacteria.
HEPA filters with a MERV rating of 17 to 20 are suitable for environments like cleanrooms, pharmaceutical manufacturing, and sensitive electronics. A rating in this range confirms the filter’s true HEPA status and its ability to trap particles smaller than 0.3 μm. HEPA filters exceed the minimum efficiency thresholds indicated by the MERV system.
| Minimum Efficiency Reporting Value (MERV) Scale | |||||
|---|---|---|---|---|---|
| MERV Std 52.2 Spot Efficiency | Intended Dust Spot Efficiency Std 52. (2) | Average Arrestance | Particle Size Ranges | Typical Size Applications | Typical Filter Type |
| 1-4 | <20% | 60 To 80% | >10.0 μm | Residential/Minimum Light Commercial/Minimum Minimum Equipment Protection |
Permanent/Self Charging(Passive) Washable/Metal,Foam Synthetics Disposable Panel Fiberglass/Syntheics |
| 5-8 | <20% To 60% | <80% To 95% | >3.0-10.0μm | Industrial VVorkplaces Commercial Better/Residential Paint Booth/Finishing | Pleated Filter Extended Surface Filters Media Panel Filters |
| 9-12 | 40% To 85% | >90% To 98% | 1.0-3.0μm | Superior/Residential Better/lndustrial Workplaces Better/Commercial Buildings Care Superior/Commercial Buildings | Non-Supported/Pocket Filter/Rigid Box Rigid Cell/Cartridge V-Cells |
| 13-16 | 70% To 98% | >95% To 99% | Smoke Removal General Surgery Hospitals & Health | Rigid Cell/Cartridge Rigid Box Non-Supported/Pocket Filter V-Cells | |
| MERV Std 52.2 | Efficiency | Typical Applications | Typical Filter Type | ||
| 17-20 | 99.97%-99.9999% | Hospital Surgery Suites Cleanrooms Hazardous Biological Contaminants Nuclear Material |
HEPA ULPA |
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In 1998, the European Standards (EN for Europäische Norm) established the first classification standards for HEPA filters with the introduction of EN 1822. This standard introduced the Most Penetrating Particle Size (MPPS) evaluation, which measures the size of particles most likely to bypass a filter.
The MPPS system, like the MERV system, rates filters based on their effectiveness, from those with 85% efficiency to those on par with HEPA filters. The goal of the EN classification was to standardize filter evaluations, but this has not been fully achieved, as different systems have been developed by the United States, the International Organization for Standardization (ISO), and the Institute of Environmental Science and Technology (IEST).
| EN 1822 Classification of Filters | ||||
|---|---|---|---|---|
| Overall Value | Local Value | |||
| Filter Classes According EN 1822 | Efficiency | Penetration | Efficiency | Penetration |
| E10 | ≥85% | <15% | - | - |
| E11 | ≥95% | ≤5% | - | - |
| ≥99% | ≤1% | - | - | |
| E12 | ≥99.5% | ≤0.5% | - | - |
| ≥99.90% | ≤0.1% | - | - | |
| H13 | ≥99.95% | ≤0.05% | ≥99.75% | ≤0.25% |
| ≥99.99% | ≤0.01% | ≥99.95% | ≤0.05% | |
| H14 | ≥99.995% | ≤0.005% | ≥99.975% | ≤0.025% |
| ≥99.999% | ≤0.001% | ≥99.995% | ≤0.005% | |
| U15 | ≥99.9995% | ≤0.0005% | ≥99.9975% | ≤0.0025% |
| ≥99.9999% | ≤0.0001% | ≥99.9995% | ≤0.0005% | |
| U16 | ≥99.99995% | ≤0.00005% | ≥99.99975% | ≤0.00025% |
| ≥99.99999% | ≤0.00001% | ≥99.99995% | ≤0.00005% | |
| U17 | ≥99.999995 | ≤0.000005% | ≥99.9999% | ≤0.0001% |
The Institute of Environmental Science and Technology (IEST) has a classification system dedicated specifically to HEPA filters, following the standards set by Military Standard (MIL-STD) 282. HEPA filters were initially developed during World War II by the Atomic Energy Commission to protect scientists from radioactive dust. The military and commission's testing methods laid the foundation for HEPA filter standards.
In the IEST system, Type A HEPA filters are the least efficient and are suitable for general use in homes, stores, and public areas. In contrast, Type E filters are the most efficient, designed for use in cleanrooms, electronics manufacturing, and pharmaceutical environments.
| IEST HEPA Filter Types | ||||||
|---|---|---|---|---|---|---|
| Filter Type | Penetration Test | Scan Test (See Notes) | Comments | Minimum Efficiency Rating | ||
| Method | Aerosol | Method | Aerosol | |||
| A | MIL-STD 282 | Thermal DOP | None | None | 99.97 % * At 0.3 μm | |
| B | MIL-STD 282 | Thermal DOP | None | None | Two Flow Leak Test | 99.97 % * At 0.3 μm |
| C | MIL-STD 282 | Thermal DOP | Photometer | Polydisperse DOP | 99.99% At 0.3 μm | |
| D | MIL-STD 282 | Thermal DOP | Photometer | Polydisperse DOP | 99.999% At 0.3 μm | |
| E | MIL-STD 51477 or MIL-STD F51068 | Thermal DOP | Photometer | Polydisperse DOP | Two Flow Leak Test | 99.97 % At 0.3 μm |
| F | IES-RP CC007 | Open | Photometer | Open | 99.999% At 0.1 To 0.2 μm | |
ISO 29463 is widely recognized as the global standard for HEPA filters. Developed in conjunction with EN 1822, the ISO classification system employs a similar method for filter evaluation. ISO 29463 is structured into five parts, each corresponding to different testing methods and materials. The classification starts with ISO 15 E, which is not a HEPA filter, and extends to ISO 75 U. Filters that comply with EPA standards for HEPA filtration start at ISO 30 E.
| ISO Filter Standards | ||
|---|---|---|
| Filter Class | Overall Efficiency (%) | Local Or Leak |
| Penetration (%) | ||
| ISO 15 E | ≥95 | NA |
| ISO 20 E | ≥99 | NA |
| ISO 25 E | ≥99.5 | NA |
| ISO 30 E | ≥99.90 | |
| ISO 35 H | ≥99.95 | ≤0.25 |
| ISO 40 H⁴ | ≥99.99 | ≤0.05 |
| ISO 45 H⁴ | ≥99.995 | ≤0.025 |
| ISO 50 U | ≥99.999 | ≤0.005 |
| ISO 55 U | ≥99.9995 | ≤0.0025 |
| ISO 60 U | ≥99.9999 | ≤0.0005 |
| ISO 65 U | ≥99.99995 | ≤0.00025 |
| ISO 70 U | ≥99.99999 | ≤0.0001 |
| ISO 75 U | ≥99.999995 | ≤0.0001 |
| Table 1 - ISO FIlter Classes | ||
While the International Organization for Standardization (ISO) is typically the leading authority on manufacturing and industrial standards, HEPA filters have various classification systems developed by different countries. The classification methods mentioned above are among the most widely accepted and can guide you in selecting a HEPA filter.
Originally designed to protect against toxic dust, HEPA filters have evolved with advancements in technology and precision manufacturing. Today, they are essential across many industries and for public use, offering protection from asthma and allergies.
ULPA filters are very closely related to HEPA filters but are more efficient. To be classified as a ULPA filter, a filter must be able to remove 99.999% of contaminants with diameters of 0.12 µm or larger. As with HEPA filters, ULPA filters are composed of tangled and randomly arranged fibers that attract particles as they pass through the filter. There is a long list of particulate matter that ULPA filters can trap. The only type of microscopic matter that a ULPA filter cannot remove are viruses.
Duct and fan HEPA filter units are employed in clean rooms and laboratories to eliminate harmful airborne particles. These units are designed to filter recirculated air in both turbulent and unidirectional airflow clean rooms. By creating positive pressure within the room, they help minimize contamination from ceiling bypasses. These self-contained units offer a versatile solution for effectively removing contaminants from the environment.
At first glance, a HEPA filter appears similar to other filters, featuring interwoven fibers and pleated structures. However, the true effectiveness of a HEPA filter lies in its fiber composition, typically consisting of randomly arranged fiberglass fibers.
The effectiveness of a HEPA filter stems from the random arrangement of these fibers. This design leverages Brownian Motion, ensuring that even the tiniest particles are trapped within the intricate mesh of fibers and cannot escape.
HEPA filters are made from polyester, polypropylene, or fiberglass fibers that are tightly interlaced with diameters of less than one micron. The fibers are twisted, turned, scattered, and randomly placed in different directions to create a mesh maze without a straight true path.
The spaces between the fibers in a HEPA filter are smaller than half a micron, allowing the filter to capture particles smaller than 0.3 microns. The image below, taken from a microscopic view of HEPA filter fibers, clearly shows the irregular and non-uniform arrangement of the fibers.
The frame of a HEPA filter can be made from a variety of materials. For ones being used for industrial and manufacturing operations, the frames are normally made of tough, resilient, and durable materials such as carbon steel, aluminum, stainless steel, or galvanized steel. The size of the frame has to be carefully planned since its resistance to the airstream can increase the pressure drop.
Choosing the right adhesive for HEPA filter construction is crucial to ensure it does not impact the filter's performance. The adhesive must stay in place without migrating into the fiber material. Common adhesives for HEPA filters include polyurethane, silicone, and ceramic.
Polyurethane is often used in HEPA filters due to its suitability for the filtering process. It securely bonds the fiber material within metal frames and cures either at room temperature or through accelerated heat curing.
Silicone adhesives offer flexibility, temperature resistance, and come in various hardness levels, transparencies, and viscosities. They withstand shock, vibrations, heat, and corrosion and can also provide electrical insulation.
Ceramic adhesives form a strong bond between filter materials and metal frames, such as stainless steel or aluminum. They are ideal for HEPA filters and can secure various internal components.
Gaskets play a critical role in the performance of a HEPA filter and can be made from die cut urethane rubber and closed cell sponge rubber. The choice of rubber as a gasketing material is more economical and easier to install than liquid silicone systems.
To prevent mold release, the material is processed by splitting or skiving the top layer. Gaskets are then precisely cut from sheets or rolls to ensure they fit the required shape.
Common gasket shapes include strip, one-piece, and interlocking designs, with interlocking being the most cost-effective and easiest to install. Gaskets are affixed to the frame using solvent-activated or pressure-sensitive adhesives. The joints are sealed with RTV materials compatible with closed-cell rubbers.
The most commonly used gasket type is die-cut, which is attached to the outer edge of the frame and pressed against a flat surface. All HEPA filter gaskets are resistant to oil and ozone.
In HEPA filters, separators are positioned between the pleats or folds of the filter material. These separators, made from aluminum, glass fiber strings, or hotmelt, keep the pleats open. This design enhances particulate matter capture and reduces airflow resistance.
The description provided outlines the basic elements of HEPA filter design. However, each manufacturer may use proprietary methods and variations in their production processes, which can differ from this general overview.
With rapid technological advancements, air filtration systems have become crucial for building management and industrial operations. In technical and craft industries where air quality is vital for worker safety, HEPA filters are essential for removing contaminants and maintaining air purity.
HEPA filters are particularly critical in clean room environments, where even minor contaminants can disrupt processes. Clean rooms are classified based on the particle count and size per volume of air, with particles as small as 0.1 μm being measured. HEPA filters must meet and often exceed these stringent requirements to ensure clean room standards are maintained.
Biosafety cabinets are designed to protect workers handling hazardous materials by using vertical laminar airflow to create a barrier against airborne particles and microorganisms. HEPA filters are employed to clean the air as it is recirculated back into the workspace and released into the environment.
Biosafety cabinets are categorized into three classes: I, II, and III. Classes II and III provide comprehensive protection for workers, the environment, and products, while Class I cabinets offer basic protection primarily for the environment and personnel. When used correctly, biosafety cabinets significantly reduce the risk of contamination, disease, and the spread of hazardous materials.
The working conditions that require the most enhanced methods of contamination removal are cleanrooms, which work with a wide variety of substances, products, and materials that can be potentially damaged by unfiltered and unclean air. The term clean room covers a wide array of work areas constructed and designed to create a perfect and ideal set of working conditions.
The clean room industry consists of specialized engineers and designers who excel in creating environments that are sealed and meticulously cleansed to capture and remove even the tiniest particles instantly.
Cleanroom classifications are based on the quantity and size of particulate matter present in the room. The fewer particles present per cubic foot, the higher the cleanroom's classification. The filtration system, which includes HEPA filters installed in ceilings, walls, or cabinets, plays a crucial role in maintaining these stringent conditions.
The image below illustrates how every component of a cleanroom is thoroughly inspected to ensure that the environment is maintained at the highest level of cleanliness and particle-free conditions.
HEPA filters are crucial in hospitals to prevent cross-contamination and the spread of infectious diseases. In the context of COVID-19, these filters are essential for safeguarding both patients and healthcare workers.
In operating rooms, maintaining clean and clear air is critical. HEPA filters are employed to eliminate airborne contaminants that could pose risks during surgical procedures when patients' organs are exposed.
Newborns, with their vulnerable immune systems, are at risk from airborne diseases. HEPA filters in incubators, especially in neonatal intensive care units, remove bacteria, viruses, and other infectious agents to ensure a sterile environment.
Warming beds in forced air systems help regulate patient temperature. HEPA filters in these systems capture dangerous particulate matter, ensuring it does not come into contact with the patient.
Laboratories, similar to clean rooms, handle hazardous pathogens and require HEPA filters to meet legal standards. These filters are essential for removing microbes and contaminants, preventing their spread beyond the testing environment.
Typically, warehouses are used for short-term storage before items are shipped. However, some goods may remain in a warehouse for extended periods. Despite the large space, prolonged storage can lead to stagnant air filled with harmful particles, dust, and dirty surfaces.
In such conditions, HEPA filters are highly effective. They help remove harmful particles and improve air quality by cleaning and refreshing the air in warehouses.
Weather conditions can lead to mold buildup, which, although invisible, is highly dangerous and harmful. Using air scrubbers along with HEPA filters helps remove mold from the air, protecting public health from mold contamination.
In severe weather, water and moisture can infiltrate buildings, homes, and manufacturing facilities, fostering mold growth. While drying out these areas is essential, it is not enough to address airborne mold. HEPA filtration systems are necessary to capture and remove mold spores from the air effectively.
COVID-19 particles can remain airborne for hours following exposure. Agencies such as the EPA and the CDC recommend using air purifiers equipped with HEPA filters to remove contaminants, including COVID-19, from the air. When choosing an air purifier, it is important to consider the size of the area it will serve and ensure it is capable of handling the required airflow.
The pharmaceutical industry shares similar requirements with hospitals for controlling contaminants and hazardous substances. Quality assurance in drug production is closely regulated by the Food and Drug Administration (FDA), which mandates the use of HEPA filters with efficiency ratings of H13, H14, or U15 due to the sensitive nature of the materials handled.
Moreover, to comply with FDA standards, all HEPA filter installations must be tested for leaks, efficiency, and reliability. The industry’s stringent demands necessitate regular inspections and approvals of air processing equipment to ensure safety and efficacy.
The applications of HEPA filters outlined above are among the most critical, but they represent only a portion of their uses. HEPA filters are also employed in industries such as aerospace manufacturing, electronics production, fertilizer blending, cement production, and other sectors where dust, chemicals, and hazardous materials are present.
In the past two decades, HEPA filters have significantly evolved to meet the increasing demand for cleaner air, and this trend is expected to continue well into the future.
Manufacturers of HEPA filters rigorously test their products for efficiency, integrity, and performance as part of the production process to ensure high quality. However, HEPA filters must also undergo a secondary round of testing after installation to confirm their effectiveness in the operational environment.
Before assessing a HEPA filter, it is crucial to evaluate the flow rate through it. The ISO provides standards for on-site testing of HEPA filters to check for leaks and verify airflow volume. These standards are detailed but offer some flexibility between vendors and customers.
To test HEPA filters in clean rooms, cold and hot generated aerosols, including microspheres, are used. These test aerosols are produced from oil-based liquids. The concentration of test aerosols is set upstream of the filter at levels of 10µg/l and 100µg/l, with lower concentrations recommended to avoid blockage or bleed-through.
Leak detection is performed using a probe, with smaller probes generally offering better accuracy than larger ones. The speed of scanning also impacts the effectiveness of leak detection.
HEPA filters are designed with a gap between the filter and its housing, with the gasket positioned at the rear. To detect leaks between the frame and housing, a probe scans the area. If there is a gasket leak, particles will spread and fill the surrounding space, causing the scanner to detect a higher concentration of particles away from the actual leak.
The image below illustrates a gasket leak.
To scan the entire face of a HEPA filter for leaks, a probe is used to make overlapping strokes across the filter at a controlled, consistent speed. This process may require multiple passes to accurately locate any leaks. For increased precision, a baffle plate can be placed over the filter face to distinguish between gasket and face leaks. The probe operates at a fraction of an inch from the filter surface to ensure all potential leaks are detected.
Manufacturers and the IEST provide several procedures for repairing leaks in HEPA filters. The IEST offers guidelines to prevent creating blockages or restrictions during the repair process. After any repair is made, the system must be retested to verify that the leak has been properly patched and that airflow remains unobstructed.
Ensuring the proper performance of a HEPA filter system is crucial, and thorough testing and examination are integral to this process. These steps are essential for confirming the effectiveness and reliability of the HEPA filter system as part of the installation and maintenance procedures.
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