Based on the U.S. Department of Energy and most international standards adopted by the industry, HEPA filter efficiency is 99.97% in filtering airborne particles with a diameter of 0.3 microns.
Hepa filter is one air filter type. There are also other air filters like pre-filter and others.
All air filters are made to remove particles and airborne contaminants to improve air quality.
Why 0.3 microns
Why does the DOE use a 0.3-micron diameter particle removal rate as a reference standard instead of a larger or smaller particle removal rate?
The reason is that 0.3-micron diameter particles are the Most Penetrating Particle Size (MPPS).
Larger particles do not pass through the fiber gaps, and smaller particles are more diffuse and tend to deviate from the airflow path and hit the fibers to be removed.
Only the 0.3-micron particles are the most mischievous and can escape with airflow.
Therefore, the removal rate of 0.3-micron particles becomes the touchstone of quality HEPA filters or true HEPA filters.
Hepa filter grades(European standard)
Euro EN1822 Efficiency
H13 >99.95%@0.3um
H14 >99.995%@0.3um
U15 >99.9995%@0.1-0.2um
U16 >99.99995%@0.1-0.2um
U17 >99.999995%@0.1-0.2um
Hepa filter Merv rating(minimum efficiency reporting value)
Merv rating Efficiency
Merv17 >99.97%@0.3um
Merv18 >99.99%@0.3um
Merv19 >99.999%@0.3um
Merv20 >99.999%@0.1-0.2um
Let’s talk about HEPA air filter efficiency.
Many people’s understanding of HEPA filters is still on the surface.
For example, people only know that the clean room should be installed with primary G4 + medium efficiency F5 + medium efficiency F9 + high-efficiency air filters H14 at the end. However, they still need to learn more about some characteristics of HEPA filters.
For example, what is the size of the test particles of the H14 or U15 filter in the MPPS method?
Let’s learn more about it.
The high-efficiency particulate air filter is a HEPA filter.
The earliest use of HEPA filters came from the U.S. military and was an MIT-related standard. Subsequently, Europe continued to optimize its specifications and standards.
Three major standards for HEPA filter
The three primary standards that prevail internationally are TEST-RP/EN1882/ISO14644.
TEST stipulates that the overall filtration efficiency of particles with MMD=0.3um is at least 99.97% for high efficiency.
EN1882 stipulates that a filter efficiency greater than 99.95% under the aerosol of MPPS can be a high-efficiency particulate air filter, that is, H13 or more.
For each standard, the analysis is as follows.
1. The MMD – Mass Median Diameter in TEST refers to the particle mass median, which is the value where 50% of the mass of all aerosols is greater than this value, and 50% of the mass is less than this value.
So don’t assume that the TEST regulations @0.3um>99.97% or @0.3um>99.99% are only for the 0.3um requirement.
This regulation requires an aerosol group of 0.3um in the middle diameter of the mass.
The aerosol is a monodisperse or polydisperse phase, a range of aerosol groups.
The aerosol is never a single particle size of 0.3um aerosol.
2. CMD -Count of number Median Diameter refers to the particle size median. That is, 50% of the particles are larger than this value. 50% of the particles are smaller than this value.
The particles with MMD=0.3um have a CMD of about 0.2um. This is because the particles of large particles occupy a small quantity weight but a significant mass weight.
Therefore, it is said in TEST that 99.97%@0.3um is, in fact, roughly equivalent to the particle diameter of the MPPs of EN1882. Here is also only similar and can not be fully equal.
3. The new EN1822-2009 classifies the former E10 E11 E12 as HEPA into high efficiency series, unlike the former EN1822-2000.
4. What is the value of MPPS?
It is necessary to do experimental tests on a certain kind of filter media, such as a certain kind of glass fiber. At least five replicate tests are required.
Each test should be sent monodisperse phase or polydisperse phase of the particles. Particle size should cover the overall 0.1-0.5um interval.
Finally, the lowest point is made with a curve.
According to the statistics of EN1822, most of the most easily penetrated particles are in a value between 0.1um-0.25um.
From a commercial point of view, the particle size of an aerosol emitted by an aerosol generator or a LASKIN nozzle cannot be guaranteed to be a unique particle size.
In terms of standard deviation. σ1.5 is polydispersed.
It is essential to be aware that the standard deviation is not directly related to the particle size, i.e., the result is unrelated to the particle size.
Therefore, we can assume that the MPPS particle size is not specific to a particular particle size but to an interval when testing.
This is why MPPS requires all particles in an interval to be tested during the actual field test.
It should also be noted that the MPPS is different for different filtration air speeds.
It is necessary to specify what the rated filter speed is before the high-efficiency particulate air filter leaves the factory.
5. For the actual MPPS test, it is generally required to collect the total number of all particles within the range MPPS/1.5 – MPPS*1.5.
The implied condition here is that the aerosol dust generation should fall precisely within this range,
The median CMD of the particle size should be exactly equal to the MPPS (idealized), and the deviation should not exceed 10%, according to EN 1822.
Reasons for the high efficiency of HEPA filters
Filter media structure
Observing the microstructure of the HEPA filter media with an electron microscope, it will be found that it is a fluffy network composed of interwoven fibers with a diameter of about 0.2~2.0um.
The diameter of the fibers is about the same as the particles, plus the layers are arranged in an intricate interweave.
The fine particles, which move with the airflow, hit a particular fiber and are adsorbed by it.
Van der Waals
In addition to intercepting the flowing particles by arranging a “net” like a sieve with a gap between the media smaller than the diameter of the particles, there is also the use of intermolecular van der Waals forces to “grab.”
Due to the existence of van der Waals forces, whether the particles are due to inertial motion or to Brownian movement (a phenomenon in which suspended particles never stop making an irregular motion, the smaller the particles, the more intense the Brownian activity) and the fibers they encounter, they will be firmly adsorbed and difficult to escape.
And these particles adsorbed on the fiber will continue to act as [sticky hook] under the action of van der Waals force, adsorbed around the passing particles (this is also called dust bridging).
Other features of HEPA filters
Large dust capacity of HEPA filter
In addition to the ability to capture particles, a large dust capacity is also a feature of HEPA filters.
Most people know that HEPA is pleated in a wave shape before and after.
Under the condition that the length and width of the filter are fixed, pleated HEPA filters help to maximize the filter area and thus expand the adsorption capacity of the filter.
It should be aware that the number of pleats and the pleat depth of the filter cannot be increased indefinitely.
There is an optimal value regardless of the number of pleats’ layers or deep. After this value, the purification effect begins to decline.
Here also involves the concept of HEPA filter resistance. The resistance of the HEPA filter material is the energy loss of the airflow through the filter material. The lower the HEPA filter resistance, the better the purification effect with the same energy consumption.
Pleats and resistance
In the case of a specific filter size, reducing the pleat spacing can increase the HEPA filter area, reduce the filter speed, and thus reduce the airflow resistance through the HEPA filter media.
However, with the increase of pleat spacing, the airflow channel becomes narrower.
The narrowing of the channel will increase airflow resistance through the filter media.
There is an ideal number of pleats: the resistance of the filter media is minimized with the rise of the number of pleats.
The same applies to pleat depth.
In cases where there is no requirement for filter depth dimensions, increasing the pleat depth can effectively improve the media area and thus reduce the resistance to airflow through the HEPA filter.
However, increasing pleat depth also increases frictional resistance in the airflow path.
HEPA filter materials
Not only will the number of pleats and pleat depth affect the filter’s resistance, but also the influence of the filter’s choice of filter material on the resistance of the filter.
Most of the current high-efficiency filters are made of glass fiber material, polypropylene (PP), polyester resin (PET), etc., through meltblown technology or PTFE (polytetrafluoroethylene) stretching.
Glass Fiber
Glass fibers are the most traditional and original filter media used in the nuclear industry.
Compared to synthetic fibers, glass fiber is resistant to high temperatures and has a high dust capacity, good stability, durability, and long life.
Its most important advantage, however, is that it can guarantee a high enough filtration efficiency in a single pass. Until today, the highest efficiency U15 ~ U17 filters still use glass fiber.
However, fiberglass also has disadvantages: high air resistance, high noise, high energy consumption, and fragility.
PP(polypropylene ) fibers
The filtration effect of PP fiber can achieve up to the H14 level (99.995%@0.3um)
PET fibers
PET has high hardness, good stiffness (good for the filter to keep its shape), stable performance, and large dust capacity.
However, PET is a coarse fiber of 10µm or more, with large voids, and the single filtration efficiency is relatively low, generally only E10-E11 level (85% to 95% efficiency).
PP-PET meltblown composites
Composite filter media (PP-PET), which is a meltblown layer of PP and then a meltblown layer of PET, combines the advantages of PP and PET: stiffness, ease to form, and filtration efficiency can reach H13 level (99.95%).
But the most significant feature is that the resistance is much smaller than pure PP filter media.
Summary of HEPA filter materials
The change of materials is a microcosm of the development of HEPA technology.
From its application in the scientific field in the 1940s to its popularization in the industrial area in the 1960s and the shift to the civilian field in the 1980s, HEPA has a history of nearly 80 years, with more mature development, significant purification effect, and fewer and smaller side effects.
Can a HEPA filter 0.1um particles?
A HEPA filter can only filter to 0.3um. Can it filter 0.1-micron ultra-fine particles?
The standard performance expression for all filters that meet the HEPA standard is 0.3-micron particle effective filtration of 99.95%.
This expression has led to thoughts and misunderstandings, which means that the HEPA filter can only filter to a minimum of 0.3 microns. Further down, the smaller particles, such as 0.1 microns, are not filtered. Will it leak from the holes in the filter?
It is not like that. Hepa filter can filter 0.1um particles.
For the motion of dust in the air, there is a tendency to maintain inertial motion, that is, straight-line motion, and the tendency to do “Brownian motion” by the irregular impact of gas molecules.
The tendency of linear motion is more evident for larger particles, and Brownian movement is more pronounced for smaller particles.
Larger particles that do a straight line movement and continue to go straight in a filter with a specific thickness immediately hit a particular fiber and are directly stopped or stuck.
Likewise, smaller particles in Brownian motion, not in the general direction of the airflow but in all orders, will soon hit other fibers laterally and end up in the same situation.
The only particles of a particular size are those that neither move in a perfectly straight line nor oscillate entirely around but go in the general direction of the airflow, yet wobble and zigzag.
Then it may be in the longitudinal, coincidentally, through one after another “mesh” gap, penetrating the entire filter and becoming a “leaky fish.”
Still MPPS
Such small particles are the most difficult to intercept. HEPA filter efficiency is the lowest for this particle, known as the Most Penetrating Particle Size (MPPS).
It varies from 0.10 to 0.45 microns depending on the filter media type, filtration rate, and airspeed.
But for HEPA filters, this MPPS size is about 0.3 microns.
So HEPA assessment standards are as long as the assessment of this level of particle filtration is enough.
