This article takes an in depth look at HEPA filters and their use.
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A HEPA filter is a high efficiency pleated air filter capable of capturing extremely small particulate matter down to particles that are the size of a micron (µ), or a micrometer, which is 1/1000th of a meter. They are a common part of air purification systems since they strictly comply with the standards set for pollutant removal. The anagram HEPA stands for high efficiency particulate air, which is an efficiency standard that applies to air filtration equipment and systems capable of removing the most minute particulate matter.
For a filter to reach the HEPA standard, it has to be able to remove 99.9% of particles of all sizes down to ones as small as 0.3 micrometers or less. A HEPA rated filter is capable of removing impurities of any kind, including dust mites and particles, pet dander, pollen, smoke, mold spores, and other pollutants that are invisible to the naked eye.
A HEPA system forces air through a fine mesh in order to trap harmful contaminants. The mesh is made of thousands of fine fibers that capture microscopic sized pollutants. Although all HEPA filters must meet the same standards, there are different levels of classifications for HEPA filter efficiency.
When discussing HEPA filters, it is important to distinguish between “True HEPA” filters and “HEPA Like,” “HEPA Type,” or “HEPA Style” filters. For a filter to be classified as a HEPA filter, it has to be capable of removing 99.97% of particles that are the size of 0.3 of a micron. This standard was established by the Environmental Protection Agency (EPA), which uses grades created by the United States military for HEPA filters that begin with “A,” least effective HEPA filter, to “E,” most effective HEPA filter.
The filtration process for HEPA filters is very similar to a strainer, sieve, or web but is far more intricate and capable of catching the most minute forms of airborne particles.
There are four mechanisms to the HEPA filter process: impaction, interception, diffusion, and electrostatic attraction. As with all forms of filters, the first materials to be removed are large and easily trapped. From the early stages of filtration and progressively through finer materials, smaller and smaller particles are trapped, which can graphically be seen in the image below.
When particles that are 1.0 μm in diameter or larger enter a HEPA filter, they make contact with the fibers of the filter and are too big to pass through the fiber barrier. The process of inertial impaction captures the majority of large particles and pollutants such as pollen.
These large particles can quickly saturate a HEPA filter and are the reason that most filtration systems have a prefilter to remove them before the air is passed on to the HEPA filter. Even though the term large refers to 1.0 μm particles, compared to a human hair or grain of sand, they are extremely small, as can be seen in the image below.
After inertial impact, particles that are 0.3 μm or 1 μm move through the first filter onto the interception section. As these tiny particles leave the inertial impact section, they attempt to move around the interception section and follow the air flow. Due to their size and weight, they are too heavy to move through the fibers and get trapped or stuck.
The interception step of a HEPA filter captures particles that are 0.1 μm as they hit the filter’s fibers. As the large particles come in contact with the fibers, they stick or adhere, which is the reason this part of the process is referred to as interception.
In the interception stage, particles collect and saturate the filter, which requires the filter to be replaced. A pre-filter normally protects the inertial impact portion of the filter. This is not possible for the interception section.
At this stage of the filtration process, the particles are so small that they have very little mass and bounce around randomly and move in a zigzag pattern. This particular type of movement is referred to as Brownian movement, which is the random or erratic movement of microscopic particles trapped in a gas. Since the particles are so small and have extremely small mass, they are constantly bumping into each other.
The size of these particles can create the impression that they are too small to be trapped or caught by a HEPA filter. In perfecting HEPA filters, engineers were aware of the Brownian movement and designed the last part of the filter to adapt to it.
In order to capture the smallest particles, which are 0.1 μm in diameter, the fibers of the diffusion part of the filter are placed randomly without patterns or pathways. As the small particles bounce around, they collide with one another and become slower. Their slow motion causes them to stick to the filter’s fibers, which can be seen in the image below.
The smallest particles, including smoke and dust, have an electrostatic charge. The fibers of a HEPA filter have an electrostatic charge. When negative charges and positive charges come in close proximity, they have an electrostatic attraction. In the final stage of a HEPA filter, the smallest particles are pulled and attracted to the fibers where they are held in place.
The image below is a diagram of the complete HEPA Filtration process.
A HEPA filtration system will cause a pressure drop in the airflow since it blocks the flow of air as it filters the air. The amount of pressure drop depends on the type of HEPA filter and the system’s design, with some systems having a prefilter. Other factors that relate to pressure drop are the fan’s power that pulls air into the system. Most HEPA filters can handle air flow at 250 feet per minute (FPM) without too significant of a pressure drop.
The fibers on a HEPA filter are tightly packed together to be able to catch the most minute particles. Pressure drop is the measurement of resistance to air passing through a filter. With fibers packed so tightly, a HEPA filter creates a significant amount of pressure drop. As it captures more particulate matter, it becomes more saturated, which creates more pressure drop. To avoid this problem, HEPA filters should be checked frequently depending on their location and the type of air flow.
The anagram HEPA refers to a set of filters that can remove 99.9% of particulate matter from an air stream. It is a classification that has been given to HEPA qualifying filters by the EPA, the EN, IEST, and other organizations. Within the HEPA classification are different levels of filters that are capable of filtering an airstream with greater or less efficiency.
In the last twenty years, HEPA filters have become more important and essential for homeowners and sensitive industrial applications. For this reason, several agencies and organizations have developed classification and rating methods to assist manufacturers and the public regarding their use and performance. One of the classification systems is the Minimum Efficiency Reporting Value (MERV) system which gives a rating of 17 to 20 to HEPA filters and suggestions regarding where they can best be used.
The rating of a HEPA filter is determined by the size of particulate matter it can capture. The MERV rating rates all filters regarding their process and effectiveness. On its rating scale, HEPA filters are rated at the top, with 17 to 20 and can be seen on the chart below. This level of rating indicates that a filter is capable of removing viruses, carbon dust, sea salt, smoke, and bacteria.
HEPA filters with a MERV rating of 17 to 20 are used for cleanrooms, pharmaceutical manufacturing, and sensitive electronics. If a filter is rated at 17 to 20, it is an indication that it is a true HEPA filter and able to remove particulate matter that has a size of fewer than 0.3 μm. As the MERV rating of a HEPA filter indicates, HEPA filters are not minimum efficient and far surpass the MERV rating system.
| 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, European Standards (EN for Europäische Norm) developed the first set of standards for a filter classification system for HEPA filters based on the filters filtration process. It was labeled EN 1822 and introduced the evaluation system Most Penetrating Particle Size (MPPS), which refers to the size of particles that can easily pass through a filter.
The MPPS system is similar to the MERV system in that it classifies all filters from ones with an 85% effectiveness up to ones with ratings comparable to HEPA filters. The EN classification aims to bring standardization to the filter classification process. Unfortunately, that has not been the case since the United States, the International Organization for Standardization (ISO), and the Institute of Environmental Science and Technology (IEST) have each developed classification systems.
| 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 IEST focuses their classification system exclusively on HEPA filters and follows the testing standards as outlined in Military Standard (MIL-STD) 282. The initial HEPA filters were developed during World War II by the Atomic Energy Commission to remove and capture radioactive dust particles to protect the respiratory system scientists. It was from the testing methods designed by the military or the commission that the standards for HEPA filters were established.
In the IEST classification method, Type A HEPA filters are the least effective of the HEPA filters and are designed to be used in homes, stores, and other public places. The Type E classification is the most efficient and effective HEPA filters used in cleanrooms, electronics production, and pharmaceutical labs.
| 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 | |
Many organizations have accepted ISO 29463 as the global standard for HEPA filters. The ISO classification system was developed in coordination with EN 1822 and has a similar classification method. ISO 29463 is divided into five parts according to the testing method and material being tested. The system begins with ISO 15 E, which is not a HEPA type filter, and continues to ISO 75 U. Filters that meet the EPA standards for HEPA filters begin at ISO 30 E.
| 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 | ||
The International Organization for Standards has been globally accepted as the primary organization for manufacturing and industrial standards for practices, quality, and processes. In the case of HEPA filters, this is not the case since several countries have developed their own classifications and scales. The few classification methods described above are the most accepted and popular ways for determining the performance of a HEPA filter and should serve as guidelines for purchasing a HEPA filter.
Although HEPA filters were initially introduced as protection against toxic and harmful dust, as technology has advanced and more precision production methods have developed, HEPA filters have become a necessity for a wide variety of industries. Additionally, they have become important for use by the public as protection from asthma attacks and various 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 filters units are used with clean rooms and laboratories to remove harmful airborne particles. They are designed to remove particles from recirculated air of turbulent and unidirectional ventilated clean rooms. Duct and fan HEPA filter units create positive room pressure to reduce contamination from ceiling bypasses. They are self contained units that provide a flexible solution for the removal of contaminants.
In the initial examination of a HEPA filter, it looks very much like any type of filter with interwoven fibers and pleating. The essence and strength of a HEPA filter are in examining its fibers and materials, which are generally randomly woven fiberglass fibers.
The strength of a HEPA filter is found in the scattered and random nature of how the fibers are placed. The principle of the Brownian Movement is at the heart of a HEPA filter. The construction is such that even the most minute particle will be unable to find its way out of the tangled mesh.
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 openings between the fibers are smaller than a half micron, which is why HEPA filters can catch particles smaller than 0.3 microns. The image below is taken from a microscopic examination of the fibers of a HEPA filter. What can be clearly seen is the lack of uniformity in the placement 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.
The selection of the proper adhesive for the making of a HEPA filter includes ones that are safe to use and will not interfere with the function of the filter. For the longevity of the filter, they must remain in place and not wick up into the fiber material. The three most common types of adhesives used to make HEPA filters are polyurethane, silicone, and ceramic.
Polyurethane is widely used in constructing HEPA filters since it is perfectly suited for the filtering process. It can be used to securely hold the fiber material in a metal frame and cures at room temperature or can have heat accelerated curing.
Silicone adhesives have flexibility and temperature resistance and come in different hardness, transparency, and viscosities. They are resistant to shock, vibrations, heat, and corrosion and can serve as electrical insulation.
Ceramic adhesives create a tight bond between the filter material and stainless steel or aluminum frames. It is perfect for HEPA filters and can bond a variety of internal components to the filter.
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.
The material is free of mold release by splitting or skiving the top layer. To ensure the proper shape, gaskets are die cut from sheets or rolls.
Typical gasket shapes are strip, one piece, and interlocking designs. Of the three, interlocking is the most cost effective and easy to install. Gaskets are connected to the frame using solvent activated or pressure sensitive adhesives. Joints in the gasket are sealed with RTV materials that are compatible with closed cell rubbers.
The most widely used type of gasket is die cut, which is attached to the outer edge of the frame and pressed against its flat surface. All HEPA filter gaskets are oil and ozone resistant.
In HEPA filters, separators are placed between the pleats or folds in the filter material. They are made of aluminum, glass fiber strings, or hotmelt. Separators open the pleats in the filtering material to allow for greater particulate matter capturing and less resistance to airflow.
The above description of the design of a HEPA filter is a general outline of the basic elements that can go into making a HEPA filter. Each manufacturer has their proprietary methods for producing their products, which may vary from this very general description.
In recent years, and with the rapid advancement of technology, air filtration systems have become an essential part of building management and industrial operations. In technical and craft industries, where air quality is important for worker safety, HEPA filters have become a must to ensure the proper removal of contaminants and the purity of the air.
The most critical use of HEPA filters is found in clean room applications where any form of contaminant can modify or damage a process. Clean rooms are classified in accordance with the number and size of particles present per volume of air with sizes being as small as 0.1 μm. It is the responsibility of HEPA filters to meet and exceed those requirements.
Biosafety cabinets are designed for the protection of workers who work with hazardous materials. They have a vertical laminar airflow that creates a barrier for airborne particles and microorganisms. HEPA filters are used to clean air returning to the work area and out into the environment.
There are three levels of Biosafety cabinets: Classes I, II, and III. Classes II and III offer protection for workers, the environment, and products. Class I cabinets are very basic and protect the environment and personnel. When implemented properly, biosafety cabinets effectively reduce contamination, diseases, and the spread of harmful 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 is a specialized group of engineers and designers who are proficient and experts in the creation of rooms that are sealed and cleansed to the point that the most minute element is trapped, caught, and removed instantly.
Grades of cleanrooms are dependent on the number and sizes of particulate matter in the room. The smaller the number of particles and the fewer per cubic foot, the higher is the classification of a clean room. An essential part of ensuring the proper environment is the filtration system, which relies on the performance of HEPA filters that are placed in the ceiling, walls, or cabinets.
As can be seen in the image below, each element of a cleanroom is closely inspected to guarantee that the created conditions are contaminated and particle free to the highest possible degree.
As with clean rooms, HEPA filters are a necessity for hospitals to prevent cross contamination between patients and the spread of infectious and dangerous diseases. In the age of COVID-19, HEPA filters have become an essential part of preventing the spread of the disease and protecting hospital personnel and patients.
The common idea of a warehouse is where items are stored for a short time before being shipped. In some situations, items can be stored in a warehouse for long periods of time. Although the warehouse space is very large, when goods sit for a long time, they can create stagnant air with harmful airborne particles, dust, and dirty unclean surfaces.
The conditions created by long storage periods are ideal for the use of HEPA filters, which can remove harmful particles and clean and freshen the air.
One of the dangers of weather conditions is the build up of mold. Much like bacteria, mold cannot be seen but can be extremely dangerous and harmful. The use of air scrubbers in conjunction with HEPA filters can clean the air of mold and save the public from mold contamination.
In harsh and hazardous weather, water and moisture enter buildings, homes, manufacturing facilities, and production operations, leading to mold growth. In those conditions, the first steps are to dry out the areas and make them habitable. Although drying is appropriate, it is not sufficient to catch the growth of airborne mold that needs to be removed through a HEPA filtration system.
It is possible for COVID-19 particles to remain in the air for hours after exposure. The EPA, Centers for Disease Control (CDC), and other agencies recommend using air purifiers equipped with HEPA filters to remove all contaminants from the air including COVID-19. The selection of the air purifier depends on the size of the area where it will be used and should be able to accommodate the amount of airflow.
The pharmaceutical industry has similar requirements as hospitals regarding the control of contaminants and infectious or dangerous substances. Quality assurance is a necessity in the production of drugs, which is closely monitored by the Food and Drug Administration (FDA). The FDA requires the pharmaceutical industry to use HEPA filters with an efficiency rating of H13, H14, or U15 due to the nature of the materials that the industry handles.
Additionally, to meet FDA standards, all HEPA filter installations must be tested for leaks, efficiency, and reliability. The nature of the industry and the potential dangers require that air processing equipment be regularly checked and approved.
The use for HEPA filters listed above are some of the most critical areas for their use but do not include the total number of ways that HEPA filters are used. HEPA filters can be found in aerospace manufacturing, electronics production, fertilizer blending and mixing, cement production, and other industries where dust, chemicals, and dangerous materials are fabricated.
Over the last twenty years, HEPA filters have radically risen to meet the growing demand for cleaner air. This trend will likely continue for several years to come.
All manufacturers of HEPA filters test their filters for efficiency, integrity, and performance. These tests are part of the production process to ensure the quality of the final product. As all inclusive and demanding as the tests are, HEPA filters must be tested a second time after installation.
Prior to performing any evaluation of a HEPA filter, it is important to examine the flow rate through the filter. The ISO has a set of standards regarding checking HEPA filters on site for leaks to verify the airflow volume. Although the specifications are clearly delineated, they do allow for flexibility between the vendor and customer.
Cold and hot generated aerosols and microspheres are used to test the installation of HEPA filters in clean rooms with cold and hot generated aerosols formed from oil type liquids. The test aerosols concentration is set upstream of the filter with concentrations of 10µg/l and 100µg/l with lower concentrations recommended to prevent blockage or bleed through.
A probe is used to scan the system for leaks. The size of the probe determines its effectiveness, with large probes being less effective than smaller ones. Additionally, the velocity of the scan is another factor that affects its results.
HEPA filters have space between the filter and its housing with the gasket situated to the rear. To locate leaks between the frame and housing, a probe is inserted to scan the area. Particles from a gasket leak will spread and fill the space, and the scanner will encounter a large concentration of particles that are a distance from the actual leak.
The image below is a visual representation of a gasket leak.
A probe is used to scan the complete face of a HEPA filter using overlapping strokes that move slowly across the filter at a controlled and even rate. The examination process may require several passes to pinpoint the location of a leak. For greater accuracy, a baffle plate can be placed over the face to avoid any confusion between gasket leaks and face leaks. The sample probe passes a fraction of an inch off the surface of the filter to ensure there are no leaks.
Several procedures are outlined by manufacturers and the IEST regarding methods for repairing leaks. The IEST does have a set of recommendations to avoid the creation of blockage or restrictions on the filter. As with all repairs, once a leak has been repaired, the system should be tested again to ensure proper patching and airflow.
The importance of HEPA filter processes requires that testing and examination be completed to ensure the performance of a HEPA filter system. These precautions can guarantee proper performance and are part of the installation process for manufacturers.
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