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64-511: HEPA ( / ˈ h ɛ p ə / , high efficiency particulate air ) filter, also known as a high efficiency particulate arresting filter, is an efficiency standard of air filters . Filters meeting the HEPA standard must satisfy certain levels of efficiency. Common standards require that a HEPA air filter must remove—from the air that passes through—at least 99.95% ( ISO , European Standard) or 99.97% ( ASME , U.S. DOE ) of particles whose diameter

128-469: A HEPA filter in a vacuum cleaner to be effective, the vacuum cleaner must be designed so that all the air drawn into the machine is expelled through the filter, with none of the air leaking past it. This is often referred to as "Sealed HEPA" or sometimes the more vague "True HEPA". Vacuum cleaners simply labeled "HEPA" may have a HEPA filter, but not all air necessarily passes through it. Finally, vacuum cleaner filters marketed as "HEPA-like" will typically use

192-403: A HEPA filter is working efficiently, the filters should be inspected and changed at least every six months in commercial settings. In residential settings, and depending on the general ambient air quality, these filters can be changed every two to three years. Failing to change a HEPA filter in a timely fashion will result in it putting stress on the machine or system and not removing particles from

256-467: A combination mechanical blower and air purifier unit, which incorporated cellulose-asbestos paper in a deeply-pleated form with spacers between the pleats. It was referred to as an "absolute" air filter and laid the groundwork for further research to come in developing the HEPA filter. The next phase of the HEPA filter was designed in the 1940s and was used in the Manhattan Project to prevent

320-481: A company can achieve effectiveness, for example large production numbers, through inefficient processes if it can afford to use more energy per product, for example if energy prices or labor costs or both are lower than for its competitors. Inefficiency is the absence of efficiency. Kinds of inefficiency include: Productive inefficiency, resource-market inefficiency, and X-inefficiency might be analyzed using data envelopment analysis and similar methods. Efficiency

384-415: A few degrees below 0   °C often stay completely free of ice for long periods ( supercooling ). At these conditions, nucleation of ice is either slow or does not occur at all. However, at lower temperatures nucleation is fast, and ice crystals appear after little or no delay. Nucleation is a common mechanism which generates first-order phase transitions , and it is the start of the process of forming

448-400: A fiber) through a combination of the following three mechanisms: Diffusion predominates below the 0.1 μm diameter particle size, whilst impaction and interception predominate above 0.4 μm. In between, near the most penetrating particle size (MPPS) 0.21 μm, both diffusion and interception are comparatively inefficient. Because this is the weakest point in the filter's performance,

512-488: A filter of a similar construction to HEPA, but without the filtering efficiency. Because of the extra density of a true HEPA filter, HEPA vacuum cleaners require more powerful motors to provide adequate cleaning power. Some newer models claim to be better than the earlier ones with the inclusion of "washable" filters. Generally, washable true HEPA filters are expensive. A high-quality HEPA filter can trap 99.97% of dust particles that are 0.3 microns in diameter. For comparison,

576-498: A growing crystal, thus increasing the number of crystals in the system. So both primary and secondary nucleation increase the number of crystals in the system but their mechanisms are very different, and secondary nucleation relies on crystals already being present. It is typically difficult to experimentally study the nucleation of crystals. The nucleus is microscopic, and thus too small to be directly observed. In large liquid volumes there are typically multiple nucleation events, and it

640-436: A human hair is about 50 to 150 microns in diameter. So, a true HEPA filter is effectively trapping particles several hundred times smaller than the width of a human hair. Some manufacturers claim filter standards such as "HEPA 4," without explaining the meaning behind them. This refers to their Minimum Efficiency Reporting Value (MERV) rating. These ratings are used to rate the ability of an air cleaner filter to remove dust from

704-476: A narrow convoluted pathway through which air passes. When the largest particles are passing through this pathway, the bundles of fibers behave like a kitchen sieve which physically blocks the particles from passing through. However, when smaller particles pass with the air, as the air twists and turns, the smaller particles cannot keep up with the motion of the air and thus they collide with the fibers. The smallest particles have very little inertia and move randomly as

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768-430: A new thermodynamic phase, such as the formation of ice in water below 0   °C, if the system is not evolving with time and nucleation occurs in one step, then the probability that nucleation has not occurred should undergo exponential decay . This is seen for example in the nucleation of ice in supercooled small water droplets. The decay rate of the exponential gives the nucleation rate. Classical nucleation theory

832-470: A new thermodynamic phase. In contrast, new phases at continuous phase transitions start to form immediately. Nucleation is often very sensitive to impurities in the system. These impurities may be too small to be seen by the naked eye, but still can control the rate of nucleation. Because of this, it is often important to distinguish between heterogeneous nucleation and homogeneous nucleation. Heterogeneous nucleation occurs at nucleation sites on surfaces in

896-457: A nucleus that may be only of order ten molecules across it is not always clear that we can treat something so small as a volume plus a surface. Also nucleation is an inherently out of thermodynamic equilibrium phenomenon so it is not always obvious that its rate can be estimated using equilibrium properties. However, modern computers are powerful enough to calculate essentially exact nucleation rates for simple models. These have been compared with

960-445: A pre-filter (usually carbon-activated) to extend the usage life of the more expensive HEPA filter. In such setup, the first stage in the filtration process is made up of a pre-filter which removes most of the larger dust, hair , PM10 and pollen particles from the air. The second stage high-quality HEPA filter removes the finer particles that escape from the pre-filter. This is common in air handling units . HEPA filters, as defined by

1024-404: A result of collisions with individual air molecules ( Brownian motion ). Because of their movement, they end up crashing into the fibers. Key factors affecting its functions are fiber diameter, filter thickness, and face velocity, which is the measured air speed at an inlet or outlet of a heating ventilation and air conditioning (HVAC) system. Face velocity is measured in m/s and can be calculated as

1088-527: Is a measurable concept, quantitatively determined by the ratio of useful output to total useful input. Effectiveness is the simpler concept of being able to achieve a desired result, which can be expressed quantitatively but does not usually require more complicated mathematics than addition. Efficiency can often be expressed as a percentage of the result that could ideally be expected, for example if no energy were lost due to friction or other causes, in which case 100% of fuel or other input would be used to produce

1152-533: Is a widely used approximate theory for estimating these rates, and how they vary with variables such as temperature. It correctly predicts that the time you have to wait for nucleation decreases extremely rapidly when supersaturated . It is not just new phases such as liquids and crystals that form via nucleation followed by growth. The self-assembly process that forms objects like the amyloid aggregates associated with Alzheimer's disease also starts with nucleation. Energy consuming self-organising systems such as

1216-419: Is also able to capture some viruses and bacteria which are ≤0.3 μm. A HEPA filter is also able to capture floor dust which contains bacteroidia , clostridia , and bacilli . HEPA was commercialized in the 1950s, and the original term became a registered trademark and later a generic trademark for highly efficient filters. HEPA filters are used in applications that require contamination control , such as

1280-514: Is delayed until the system enters the unstable region where a small perturbation in composition leads to a decrease in energy and, thus, spontaneous growth of the perturbation. This region of a phase diagram is known as the spinodal region and the phase separation process is known as spinodal decomposition and may be governed by the Cahn–Hilliard equation . In many cases, liquids and solutions can be cooled down or concentrated up to conditions where

1344-593: Is derived, defines several classes of filters by their retention at the given most penetrating particle size (MPPS): Efficient Particulate Air filters (EPA), High Efficiency Particulate Air filters (HEPA), and Ultra Low Particulate Air filters (ULPA). The averaged efficiency of the filter is called "overall", and the efficiency at a specific point is called "local": See also the different classes for air filters for comparison. For respirators , MSHA and NIOSH define HEPA as filters blocking ≥ 99.97% of 0.3 micron DOP particles, under 30 CFR 11 and 42 CFR 84. Since

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1408-418: Is difficult to disentangle the effects of nucleation from those of growth of the nucleated phase. These problems can be overcome by working with small droplets. As nucleation is stochastic , many droplets are needed so that statistics for the nucleation events can be obtained. To the right is shown an example set of nucleation data. It is for the nucleation at constant temperature and hence supersaturation of

1472-531: Is equal to 0.3  μm , with the filtration efficiency increasing for particle diameters both less than and greater than 0.3 μm. HEPA filters capture pollen , dirt , dust , moisture , bacteria (0.2–2.0 μm), viruses (0.02–0.3 μm), and submicron liquid aerosol (0.02–0.5 μm). Some microorganisms , for example, Aspergillus niger , Penicillium citrinum , Staphylococcus epidermidis , and Bacillus subtilis are captured by HEPA filters with photocatalytic oxidation (PCO). A HEPA filter

1536-583: Is just as important as the quality of that process. This saying popular in business, however, obscures the more common sense of "effectiveness", which would/should produce the following mnemonic: "Efficiency is doing things right; effectiveness is getting things done". This makes it clear that effectiveness, for example large production numbers, can also be achieved through inefficient processes if, for example, workers are willing or used to working longer hours or with greater physical effort than in other companies or countries or if they can be forced to do so. Similarly,

1600-437: Is often measured as the ratio of useful output to total input, which can be expressed with the mathematical formula r = P / C , where P is the amount of useful output ("product") produced per the amount C ("cost") of resources consumed. This may correspond to a percentage if products and consumables are quantified in compatible units, and if consumables are transformed into products via a conservative process. For example, in

1664-449: Is technology that uses air filters, such as HEPA filters, to remove pollutants from the air either indoors or in vehicles. Pollutants include smoke, viruses, powders , etc., and can originate either outside or inside. HVAC is used to provide environmental comfort and in polluted cities to maintain health. Modern airliners use HEPA filters to reduce the spread of airborne pathogens in recirculated air. Critics have expressed concern about

1728-497: Is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture . Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears. For example, if a volume of water is cooled (at atmospheric pressure ) significantly below 0   °C, it will tend to freeze into ice , but volumes of water cooled only

1792-441: Is the very first nucleus of that phase to form, or because the nucleus forms far from any pre-existing piece of the new phase. Particularly in the study of crystallisation, secondary nucleation can be important. This is the formation of nuclei of a new crystal directly caused by pre-existing crystals. For example, if the crystals are in a solution and the system is subject to shearing forces, small crystal nuclei could be sheared off

1856-525: The microtubules in cells also show nucleation and growth. Heterogeneous nucleation, nucleation with the nucleus at a surface, is much more common than homogeneous nucleation. For example, in the nucleation of ice from supercooled water droplets, purifying the water to remove all or almost all impurities results in water droplets that freeze below around −35   °C, whereas water that contains impurities may freeze at −5   °C or warmer. This observation that heterogeneous nucleation can occur when

1920-562: The "most penetrating size"—the most difficult and concerning. It was commercialized in the 1950s, and the original term became a registered trademark and later a generic trademark for highly efficient filters. Over the decades filters have evolved to satisfy the higher and higher demands for air quality in various high technology industries, such as aerospace, pharmaceutical industry, hospitals, health care, nuclear fuels, nuclear power, and integrated circuit fabrication. Efficiency In more mathematical or scientific terms, it signifies

1984-503: The Earth's limited resources in a sustainable manner while minimising impacts on the environment. It allows us to create more with less and to deliver greater value with less input." Writer Deborah Stone notes that efficiency is "not a goal in itself. It is not something we want for its own sake, but rather because it helps us attain more of the things we value." Efficiency is very often confused with effectiveness . In general, efficiency

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2048-679: The HEPA specifications use the retention of particles near this size (0.3 μm) to classify the filter. However it is possible for particles smaller than the MPPS to not have filtering efficiency greater than that of the MPPS. This is due to the fact that these particles can act as nucleation sites for mostly condensation and form particles near the MPPS. HEPA filters are designed to arrest very fine particles effectively, but they do not filter out gasses and odor molecules. Circumstances requiring filtration of volatile organic compounds , chemical vapors, or cigarette , pet or flatulence odors call for

2112-415: The HEPA standard and may not have been tested in independent laboratories. Although such filters may come reasonably close to HEPA standards, others fall significantly short. In general terms (and allowing for some variation depending on factors such as the air-flow rate, the physical properties of the particles being filtered, as well as engineering details of the entire filtration-system design and not just

2176-564: The United States Department of Energy (DOE) standard adopted by most American industries, remove at least 99.97% of aerosols 0.3 micrometers (μm) in diameter. The filter's minimal resistance to airflow, or pressure drop , is usually specified around 300 pascals (0.044 psi) at its nominal volumetric flow rate . The specification used in the European Union : European Standard EN 1822-1:2019, from which ISO 29463

2240-401: The air as it passes through the filter. MERV is a standard used to measure the overall efficiency of a filter. The MERV scale ranges from 1 to 16, and measures a filter's ability to remove particles from 10 to 0.3 micrometer in size. Filters with higher ratings not only remove more particles from the air, but they also remove smaller particles. Heating, ventilation, and air conditioning (HVAC)

2304-473: The air properly. Additionally, depending on the gasketing materials chosen in the design of the system, a clogged HEPA filter can result in extensive bypassing of airflow around the filter. HEPA filters are critical in the prevention of the spread of airborne bacterial and viral organisms and, therefore, infection . Typically, medical use HEPA filtration systems also incorporate high-energy ultraviolet light units or panels with anti-microbial coating to kill off

2368-454: The analysis of the energy conversion efficiency of heat engines in thermodynamics , the product P may be the amount of useful work output, while the consumable C is the amount of high-temperature heat input. Due to the conservation of energy , P can never be greater than C , and so the efficiency r is never greater than 100% (and in fact must be even less at finite temperatures). Nucleation In thermodynamics , nucleation

2432-617: The classical theory, for example for the case of nucleation of the crystal phase in the model of hard spheres. This is a model of perfectly hard spheres in thermal motion, and is a simple model of some colloids . For the crystallization of hard spheres the classical theory is a very reasonable approximate theory. So for the simple models we can study, classical nucleation theory works quite well, but we do not know if it works equally well for (say) complex molecules crystallising out of solution. Phase-transition processes can also be explained in terms of spinodal decomposition , where phase separation

2496-414: The concentration of dissolved chemicals in the water increases. Thus small droplets of water, as found in clouds, may remain liquid far below 0   °C. An example of experimental data on the freezing of small water droplets is shown at the right. The plot shows the fraction of a large set of water droplets, that are still liquid water, i.e., have not yet frozen, as a function of temperature. Note that

2560-489: The crystal phase in small droplets of supercooled liquid tin; this is the work of Pound and La Mer. Nucleation occurs in different droplets at different times, hence the fraction is not a simple step function that drops sharply from one to zero at one particular time. The red curve is a fit of a Gompertz function to the data. This is a simplified version of the model Pound and La Mer used to model their data. The model assumes that nucleation occurs due to impurity particles in

2624-399: The desired result. In some cases efficiency can be indirectly quantified with a non-percentage value, e.g. specific impulse . A common but confusing way of distinguishing between efficiency and effectiveness is the saying "Efficiency is doing things right, while effectiveness is doing the right things". This saying indirectly emphasizes that the selection of objectives of a production process

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2688-399: The effectiveness and state of repair of air filtering systems, since they think that much of the air in an airplane cabin is recirculated. Almost all of the air in a pressurized aircraft is, in fact, brought in from the outside, circulated through the cabin and then exhausted through outflow valves in the rear of the aircraft. About 40 percent of the cabin's air goes through a HEPA filter and

2752-444: The energy barrier for nucleation. The time until the appearance of the first crystal is also called primary nucleation time, to distinguish it from secondary nucleation times. Primary here refers to the first nucleus to form, while secondary nuclei are crystal nuclei produced from a preexisting crystal. Primary nucleation describes the transition to a new phase that does not rely on the new phase already being present, either because it

2816-443: The existing theories including the classical nucleation theory explain well the steady nucleation state when the crystal nucleation rate is not time dependent, the initial non-steady state transient nucleation, and even more mysterious incubation period, require more attention of the scientific community. Chemical ordering of the undercooling liquid prior to crystal nucleation was suggested to be responsible for that feature by reducing

2880-438: The filter-media properties), HEPA filters experience the most difficulty in capturing particles in the size range of 0.15 to 0.2 μm. HEPA filtration works by mechanical means, unlike ionic and ozone treatment technologies, which use negative ions and ozone gas respectively. So, the likelihood of potential triggering of pulmonary side-effects such as asthma and allergies is much lower with HEPA purifiers. To ensure that

2944-527: The level of performance that uses the least amount of inputs to achieve the highest amount of output. It often specifically comprises the capability of a specific application of effort to produce a specific outcome with a minimum amount or quantity of waste, expense, or unnecessary effort. Efficiency refers to very different inputs and outputs in different fields and industries. In 2019, the European Commission said: "Resource efficiency means using

3008-554: The liquid or solution is significantly less thermodynamically stable than the crystal, but where no crystals will form for minutes, hours, weeks or longer; this process is called supercooling . Nucleation of the crystal is then being prevented by a substantial barrier. This has consequences, for example cold high altitude clouds may contain large numbers of small liquid water droplets that are far below 0   °C. In small volumes, such as in small droplets, only one nucleation event may be needed for crystallisation. In these small volumes,

3072-400: The liquid tin droplets, and it makes the simplifying assumption that all impurity particles produce nucleation at the same rate. It also assumes that these particles are Poisson distributed among the liquid tin droplets. The fit values are that the nucleation rate due to a single impurity particle is 0.02/s, and the average number of impurity particles per droplet is 1.2. Note that about 30% of

3136-500: The live bacteria and viruses trapped by the filter media. Some of the best-rated HEPA units have an efficiency rating of 99.995%, which assures a very high level of protection against airborne disease transmission . Many vacuum cleaners also use HEPA filters as part of their filtration systems. This is beneficial for asthma and allergy sufferers, because the HEPA filter traps the fine particles (such as pollen and house dust mite feces ) which trigger allergy and asthma symptoms. For

3200-447: The manufacturing of hard disk drives, medical devices, semiconductors, nuclear, food and pharmaceutical products, as well as in hospitals, homes, and vehicles. HEPA filters are composed of a mat of randomly arranged fibers . The fibers are typically composed of polypropylene or fiberglass with diameters between 0.5 and 2.0 micrometers. Most of the time, these filters are composed of tangled bundles of fine fibers . These fibers create

3264-545: The new red phase is so large it is more favourable for it to grow than to shrink back to nothing. This nucleus of the red phase then grows and converts the system to this phase. The standard theory that describes this behaviour for the nucleation of a new thermodynamic phase is called classical nucleation theory . However, the CNT fails in describing experimental results of vapour to liquid nucleation even for model substances like argon by several orders of magnitude. For nucleation of

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3328-415: The nucleation of crystals in that there is clear evidence for heterogeneous nucleation, and that nucleation is clearly stochastic. The freezing of small water droplets to ice is an important process, particularly in the formation and dynamics of clouds. Water (at atmospheric pressure) does not freeze at 0   °C, but rather at temperatures that tend to decrease as the volume of the water decreases and as

3392-570: The other 60 percent comes from outside the plane. Certified air filters block and capture 99.97 percent of airborne particles. In 2016, it was announced that the Tesla Model X would have the world's first HEPA-grade filter in a Tesla car. Following the release of the Model X, Tesla has updated the Model S to also have an optional HEPA air filter. The idea behind the development of the HEPA filter

3456-405: The rate of homogeneous nucleation is essentially zero, is often understood using classical nucleation theory . This predicts that the nucleation slows exponentially with the height of a free energy barrier ΔG*. This barrier comes from the free energy penalty of forming the surface of the growing nucleus. For homogeneous nucleation the nucleus is approximated by a sphere, but as we can see in

3520-456: The schematic of macroscopic droplets to the right, droplets on surfaces are not complete spheres and so the area of the interface between the droplet and the surrounding fluid is less than a sphere's 4 π r 2 {\displaystyle 4\pi r^{2}} . This reduction in surface area of the nucleus reduces the height of the barrier to nucleation and so speeds nucleation up exponentially. Nucleation can also start at

3584-447: The spread of airborne radioactive contaminants. The US Army Chemical Corps and National Defense Research Committee needed to develop a filter suitable for removing radioactive materials from the air. The Army Chemical Corps asked Nobel Laureate Irving Langmuir to recommend filter test methods and other general recommendations for creating the material to filter out these radioactive particles. He identified 0.3 micron size particles to be

3648-426: The surface of a liquid. For example, computer simulations of gold nanoparticles show that the crystal phase sometimes nucleates at the liquid-gold surface. Classical nucleation theory makes a number of assumptions, for example it treats a microscopic nucleus as if it is a macroscopic droplet with a well-defined surface whose free energy is estimated using an equilibrium property: the interfacial tension σ. For

3712-481: The system. Homogeneous nucleation occurs away from a surface. Nucleation is usually a stochastic (random) process, so even in two identical systems nucleation will occur at different times. A common mechanism is illustrated in the animation to the right. This shows nucleation of a new phase (shown in red) in an existing phase (white). In the existing phase microscopic fluctuations of the red phase appear and decay continuously, until an unusually large fluctuation of

3776-470: The time until the first crystal appears is usually defined to be the nucleation time. Calcium carbonate crystal nucleation depends not only on degree of supersaturation but also the ratio of calcium to carbonate ions in aqueous solutions. In larger volumes many nucleation events will occur. A simple model for crystallisation in that case, that combines nucleation and growth is the KJMA or Avrami model . Although

3840-535: The tin droplets never freeze; the data plateaus at a fraction of about 0.3. Within the model this is assumed to be because, by chance, these droplets do not have even one impurity particle and so there is no heterogeneous nucleation. Homogeneous nucleation is assumed to be negligible on the timescale of this experiment. The remaining droplets freeze in a stochastic way, at rates 0.02/s if they have one impurity particle, 0.04/s if they have two, and so on. These data are just one example, but they illustrate common features of

3904-408: The transition to 42 CFR 84 in 1995, use of the term HEPA has been deprecated except for powered air-purifying respirators . Some companies use the marketing term "True HEPA" to give consumers assurance that their air filters meet the HEPA standard, although this term has no legal or scientific meaning. Products that are marketed to be "HEPA-type," "HEPA-like," "HEPA-style" or "99% HEPA" do not satisfy

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3968-536: The use of an activated carbon (charcoal) or other type of filter instead of or in addition to a HEPA filter. Carbon cloth filters, claimed to be many times more efficient than the granular activated carbon form at adsorption of gaseous pollutants , are known as high efficiency gas adsorption filters (HEGA) and were originally developed by the British Armed Forces as a defense against chemical warfare . A HEPA bag filter can be used in conjunction with

4032-416: The volume flow rate (m/s) divided by the face area (m). The air space between HEPA filter fibers is typically much greater than 0.3 μm. HEPA filters in very high level for smallest particulate matter. Unlike sieves or membrane filters , where particles smaller than openings or pores can pass through, HEPA filters are designed to target a range of particle sizes. These particles are trapped (they stick to

4096-514: Was born from gas masks worn by soldiers fighting in World War II. A piece of paper found inserted into a German gas mask had a remarkably high capture efficiency for chemical smoke. The British Army Chemical Corps duplicated this and began to manufacture it in large quantities for their own service gas masks. They needed another solution for operational headquarters, where individual gas masks were impractical. The Army Chemical Corps developed

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