HEPA filters
HEPA filters are filters for the separation of particulate matter from the air. They belong to the depth filters and separate particles with an aerodynamic diameter of less than 1 µm , e.g. B. bacteria and viruses , pollen , mite eggs and excretions, dust , aerosols and smoke particles .
Depending on the decreasing separation efficiency , particulate filters are divided into:
- High-performance particulate filter (ULPA = Ultra-Low Penetration Air )
- Particulate filter (HEPA = High-Efficiency Particulate Air / Arrestance )
- High-performance particle filter (EPA = Efficient Particulate Air ).
technology
The filter mats used are in most cases mounted in plywood or metal frames so that they can be changed easily. As with most air filters, the filter medium consists of glass fiber mats with a fiber diameter of around 1 to 10 µm. Since the fiber diameter is subject to a size distribution, particulate filter media are inhomogeneous . To enlarge the filter surface , the mats are usually built into the frame in a wave or jagged shape.
When changing the filter, it must be avoided that separated pollutants can be released and inhaled or touched. For this reason, filter housings are used in many cases that allow a non-contact filter change ( protective bag change method ).
Mode of action
The process of particle separation in the particulate filter generally takes place in three different ways (see Fig.):
- Blocking effect (interception): Smaller particles, which follow the air flow around the filter fiber, stick if they come too close to the filter fiber.
- Inertia effect: larger particles do not follow the air flow around the fiber, but because of their inertia bounce against it and stick.
- Diffusion effect: Even very small particles (<1 µm) do not follow the air flow, but instead have a trajectory similar to Brownian motion due to their collision with the air molecules and thus collide with the filter fibers, whereby they stick.
history
This type of filter was developed in the 1940s as part of the Manhattan Project to remove dangerous radioactive particles from indoor air . After the Second World War , these filters were made accessible for other purposes and gradually classified.
Application area
They are used among others in the medical field, that is, in operating rooms , intensive care units and laboratories , as well as in clean rooms , the core technology and a number of air washers . The respective application is decisive for the selection of the filter class . EN ISO 14644 is used to classify rooms into different clean room classes .
A large number of filters are available, especially for non-professional applications (e.g. vacuum cleaners ), which have the designation HEPA in their name, but not the u. G. Guarantee specifications of the EN standards . The designation HEPA is also used outside the spatial scope of the EN standards . However, very different, sometimes not comparable test conditions are used for the designation.
Efficiency and standardization
European standardization
In Europe, particle filter classes from 1 to 17 are used to classify the different filter effectiveness : the higher the number, the higher the guaranteed degree of separation . The European standard for the classification of particulate filters is EN 1822-1: 2009.
According to the known filter effects, particles of 0.1 to 0.3 micrometers are the hardest to separate (MPPS = most penetrating particle size ). Larger and smaller particles are separated better due to their physical properties.
In the above According to the standard, EPA, HEPA and ULPA are classified according to their effectiveness for these grain sizes using a test aerosol made from di-2-ethylhexyl sebacate (DEHS). A distinction is made between the overall efficiency of the filter and the worst local point:
| Filter class | Degree of separation (total) | Degree of separation (local) | |
|---|---|---|---|
| EPA | E10 | > 85% | - |
| E11 | > 95% | - | |
| E12 | > 99.5% | - | |
| HEPA | H13 | > 99.95% | > 99.75% |
| H14 | > 99.995% | > 99.975% | |
| ULPA | U15 | > 99.9995% | > 99.9975% |
| U16 | > 99.99995% | > 99.99975% | |
| U17 | > 99.999995% | > 99.9999% |
United States
In contrast to the scope of the European standards, only the term HEPA with a fixed degree of separation exists in the United States . The degree of separation is comparable to that of filter class H13 according to EN 1822-1: 1998. According to DOE -STD-3020-97, it is 99.97 percent for particles with a size of 0.3 µm.
Test procedure
To test the filter medium , samples from it are exposed to a test aerosol and the particle number concentrations for different particle sizes are measured in the inflow and outflow.
To test the filter element , a test aerosol is also applied to it; A particle flow profile is recorded on the downstream side of the element, which can vary due to production-related irregularities and leaks .
The tests are carried out in the new condition, which is regarded as the most unfavorable condition .
Individual evidence
- ↑ VDI 3677 sheet 2: 2004-02 Filtering separators; Depth fiber filters (filtering separators) . Beuth Verlag, Berlin, p. 19
- ↑ VDI 3677 sheet 2: 2004-02 Filtering separators; Depth fiber filters (filtering separators) . Beuth Verlag, Berlin, p. 9
- ↑ Tobias Lücke, René Adam: Investigations into the separation efficiency of fiber filters for the filtration of suspended matter. In: Dust - cleanliness. Air . 54, No. 12, 1994, ISSN 0949-8036 , pp. 443-448
- ↑ DIN EN 1822 Part 1: 2011-01 Particulate matter filters (EPA, HEPA and ULPA) - Part 1: Classification, performance testing, labeling; German version EN 1822-1: 2009 . Beuth Verlag, Berlin
- ↑ DIN EN 1822 Part 3: 2011-01 HEPA filters (EPA, HEPA and ULPA) - Part 3: Testing the flat filter medium; German version EN 1822-3: 2009 . Beuth Verlag, Berlin
- ↑ DIN EN 1822 Part 4: 2011-01 HEPA filters (EPA, HEPA and ULPA) - Part 4: Testing the filter element (scan method); German version EN 1822-4: 2009 . Beuth Verlag, Berlin
- ↑ VDI 3677 sheet 2: 2004-02 Filtering separators; Depth fiber filters (filtering separators) . Beuth Verlag, Berlin, p. 25