I’ve written about testing HEPA filters in previous articles, some about leakage testing after installation and some about integrity testing before leaving the factory. A few days ago, I summarized some conclusions while discussing the test criteria specified in some standards with a customer. Today, I will mainly write about a few key points of HEPA filter testing under different standards.
At present, there are three international standards for HEPA filters, namely, EN1822 of Europe, ISO14644 of the International Organization for Standardization, and IEST-RP of the U.S.A. Among them, we are most familiar with EN1822, which has the broadest scope of use and the highest degree of recognition. After researching and comparing, I have organized critical points about these tests in EN1822 and IEST.
Starting with HEPA
Nowadays, many people know about HEPA(High-efficiency particulate air) filters, and there are many definitions and explanations on the Internet.
In my conversations with some customers and engineers, I found that many people still have a superficial understanding of HEPA filters.
For example, in clean rooms, some engineers only know that the clean room should be installed with primary effect G4+medium effect F5+medium efficiency F9+high-efficiency H14 filters at the end. However, they still need to learn more about the characteristics of HEPA filters.
For example, the efficiency of the particles for the 0.3um concentration of not less than 99.97% for the United States of America’s A-class high-efficiency filters, the detailed meaning of this sentence only a few people think.
For example, the MPPS method introduces the H14 or U15 filter test particle size. What is the particle size?
Only a few people know the answer to these questions.
In the following section, I will discuss this aspect in detail.
1. HEPA filter test in IEST
The earliest use of HEPA came from the U.S. military for MIT-related standards. Subsequently, IEST slowly summarized and improved the standard.
IEST defines a HEPA filter as having an overall efficiency of not less than 99.97% for particles with an MMD=0.3um.
MMD
MMD-Mass Median Diameter in IEST refers to the particle mass median diameter, i.e., the value above which 50% of the mass of all aerosol size distributions are greater than and 50% of the mass of all aerosol size distributions are less than.
Therefore, do not take the IEST regulations @0.3um >=99.97% or @0.3um>=99.99%, which are only requirements for 0.3um.
This regulation requires an aerosol group with a mass diameter of 0.3um; the aerosol is a monodisperse phase or a polydisperse phase. It is an aerosol group within a range and is by no means a single particle size of 0.3um. aerosol
CMD
CMD-Count of number Median Diameter refers to the particle size median diameter; that is, 50% of the particles are larger than this value, and 50% are smaller than this value.
Particles with MMD=0.3um have a CMD of about 0.2um. This is because although the quantity weight of large particles is small, their mass weight is huge.
Therefore, it is said on IEST that 99.97%@0.3um is roughly equivalent to the particle diameter of MPPS in EN1882. It is only similar here, not completely equivalent.
2. IEST testing method and in an actual situation
For H13 and H14, IEST requires a sufficient mass concentration test.
A particle counter is required for the higher levels, U15 and above, and the more unfavorable of the two groups, 0.1um-0.2um or 0.2um-0.3um, is read directly.
However, it is not necessary to detect the MPPS value first.
For DOP testing in the field, many companies use TFI’s Cold Dust, a polydispersed phase, so the result is CMD at 0.4um and MMD at 0.7um, which is already greater than the requirement of 0.3um.
Therefore, requiring a field test mass concentration ratio of 99.99%@H13H14 is reasonable because the result is already lower than 0.3um for 99.99% in the lab.
So, it is safer for most projects to use HEPA filters above H14.
3. HEPA filter testing in EN1822
EN1882 stipulates that if the pass rate under the MPPS aerosol is greater than 99.95%, it can be HEPA, H13, or more.
MPPS
Determining the MPPS requires laboratory testing of a particular filter media, such as a particular glass fiber. At least five repetitions are needed.
Each test should be issued single dispersed phase or polydispersed phase particles, covered with the overall 0.1-0.5um interval. At the end, a curve is made to show the lowest point.
According to the current EN1822 statistics, most penetrating particles are in the range of 0.1um-0.25um.
From a commercial perspective, the aerosol particle size ejected from the aerosol generator or LASKIN nozzle cannot be guaranteed to be the only particle size. In terms of standard deviation, σ<1.15 can be considered monodisperse, and σ<1.5 is considered monodisperse. Quasi-monodisperse, σ>1.5 is polydisperse.
It should be noted that the standard deviation is not directly related to the particle size; that is, there is no relationship between the calculation results and the particle size.
Therefore, the particle size of MPPS is not for a specific particle size but for an interval during the test.
This is why the MPPS is required to test the sum of all the particles within an interval during the actual field test; it should also be noted that the MPPS is different for different filtering air speeds.
Therefore, for HEPA filter manufacturers, it is necessary to specify the rated air speed before leaving the factory.
CMD and MPPS
For actual MPPS testing, collecting the total number of particles within the range of MPPS/1.5—MPPS*1.5 is generally required.
The implicit condition here is that the dust emission of the aerosol should fall precisely within this range, and the median CMD of the particle size should be exactly equal to MPPS [idealized]. EN1822 stipulates that the deviation cannot exceed 10%.
4. Aerosol generator
Aerosol generators can be divided into several types: hot dust generation, cold dust generation, heterocondensation dust generation, and homocondensation dust generation. With the help of DPMS, monodisperse particle size can be ensured.
The particle of each aerosol generator
- Compressed air polydisperse heat and generates dust MMD=0.3UM CMD=0.2UM
- Air polydisperse cold dust MMD=0.6UM CMD=0.4UM
- Sinclair-LaMer Aerosol Generator Monodisperse Differential Condensation 1.05<σ<1.15
- Rapaport-Weinstork generator monodisperse phase heterocondensation 1.05<σ<1.15
- Free jet condensation aerosol generator quasi-monodisperse phase self-condensation σ=1.5
- Aerosol dust + DPMS polydisperse electromigration filtration is a monodisperse phase, σ=1.1 is unstable.
It can be seen from the IEST in the United States that they use the first and second dust generation methods and a photometer to test the mass concentration ratio, while the EU uses the latter four for testing.
However, the problem is that MPPS cannot directly measure a particle size value. It is a range. Many factories directly add up the number of particles between 0.1-0.2um as the number of MPPS, which should be in the range of MPPS/1.5-MPPS*1.5.
5. Polydisperse particle testing
According to EN1822, if monodisperse particles are used, the particle diameter will not be measured when testing filtration efficiency. Adding all particle numbers together can be used as the upstream and downstream particle counts.
However, what should be noted here is that the monodisperse phase is not truly a monodisperse phase, and it still has various particle size distributions. The standard deviation is less than 1.5, so it can be considered a monodisperse phase.
This is not the case for polydisperse phases. Because the standard deviation exceeds 1.5, the particle size distribution and number must be measured simultaneously during the test.
It should be noted that during upstream and downstream collection when MPPS is already known, only all or part of the particle numbers in the MPPS/2-MPPS*2 segment need to be measured.
In an actual situation
For example, EN1822 has now found that MPPS particles are generally located between 0.1um-0.25um. The ratio of the maximum value to the minimum value of this range is 2.5.
Therefore, under the requirements of EN1822, it is best to take the entire range or part of MPPS/2-MPPS*2. This is because actual optical particle counters have a measurement accuracy problem.
Everyone knows the measurement segments are generally 0.1um, 0.2um, 0.3um, 0.5um, etc.
Therefore, the final counting results of most manufacturers or testers use the total number between 0.1um and 0.2um as the basis for measurement.
How to choose the particle size in the test?
According to EN1822 requirements, the best measurement range is between mpps/1.5-mpps*1.5. However, the specification can also require that any group be selected as the dose range between mpps/2-mpps/1.5 or mpps*1.5-mpps*2. Therefore, the actual test can be
MPPS/1.5-MPPS*1.5
MPPS/2-MPPS*1.5
MPPS/1.5-MPPS*2
Measurements are made in these 3 situations.
Considering the actual optical particle analyzer, it should be possible to select any one of the three segments and select all particles or part of the particles. The choice of which one to use depends on the actual particle analyzer.
The actual result is to choose between 0.1-0.2um. Because most MPPS is at 0.18um, the second segmented method can select a part between 0.09um-0.27um, a value between 0.1um-0.2um as the dose standard.
6. Aerosol reception methods
In HEPA filter testing, there are 2 ways of receiving aerosols.
One is a photometer that measures mass concentration, which cannot count. Therefore, it can be used in DOP method testing H13 and H14, especially at the installation site.
The other is to use a particle counter or particle size analysis counter, which can measure both particle size and quantity.
However, it should be noted that it cannot be considered to be able to measure any particle size. It can generally only measure particle size in sections. For example, 0.1um, 0.2um, 0.3um, and 0.5um, instead of what everyone imagines, particle sizes such as 0.111um, 0.157um, and 0.18um can also be analyzed.
Therefore, the actual receiving particle MPPS is within a range of particle diameters. I’ve said it above.
This is because the lowest MPPS point for filter material testing in EN1822 is found from the curve, not measured. The regulations require measuring at least six different particle sizes and finding the lowest point MPPS by drawing a curve.
This method can only be tested with a monodisperse particle generator. Therefore, what is actually found is not a single particle size but an interval.
Last summary
Simply put, according to EN1822, MPPS needs to be tested for HEPA filters above H10. The test method has been explained above. Generally speaking, it is to measure the number of particle sizes in a section containing MPPS.
For the IEST test, we need to understand MMD and the dust generation method it chooses.
Personally speaking, in some projects with relatively high cleanliness requirements, choosing an H14 HEPA filter is relatively safer.
