Protecting sensitive manufacturing processes, such as in microelectronics, preventing microbiological contamination in research laboratories, eliminating airborne infectious agents in the healthcare sector… HEPA filters are used across many fields, from the nuclear industry to the food, pharmaceutical, semiconductor, and aerospace sectors.
What are HEPA filters?
HEPA filters (High Efficiency Particulate Air) are used to remove airborne particles with very high efficiency. They are also known as THE filters (Très Haute Efficacité) or absolute filters. In practice, a filter is classified as HEPA if it can, in a single pass, remove at least 99.97% of particles with a diameter greater than or equal to 0.3 µm—a range of pollutants that is particularly difficult to eliminate. HEPA filters comply with a clearly defined standard (see below). They can capture extremely fine particles harmful to health: dust, dust mites, pollen, pet dander, and in some cases even bacteria and viruses.
How do HEPA filters work?
The filter media used in HEPA filters consists of intertwined fibers—typically glass fibers—that form a membrane capable of trapping particles. The diameter of a glass fiber ranges from 0.5 to 2 µm. By adjusting how the fibers are arranged within the filter, it is possible to capture particles smaller than 0.3 µm. Three distinct mechanisms are involved in particle filtration.
The largest particles (diameter > 1 µm) are stopped by inertial impaction on the glass fibers. Because of their high inertia, large particles cannot follow the airflow when it curves around a filter fiber. They continue along their trajectory and adhere to the upstream face of the fiber they collide with. The narrower the spacing between fibers and the higher the air velocity, the more frequent these collisions become.
The smallest particles (diameter < 0.1 µm) are captured through diffusion, a process driven by Brownian motion. Small particles do not follow the airflow streamlines around the fiber. They are constantly jostled by the Brownian motion of air molecules. When they encounter the filter fibers, they adhere to them and become trapped. The probability of these collisions increases when air velocity decreases and when both particle diameter and fiber diameter are reduced.
Finally, intermediate-size particles, the most difficult to capture, are trapped by interception. These particles follow the airflow around a filter fiber. If they travel along a streamline that passes sufficiently close to the fiber—closer than the particle’s radius—they are intercepted.
Interception does not depend on air velocity unless the velocity changes enough to alter the airflow pattern around the fiber. Interception increases with particle size, decreasing fiber diameter, and reduced spacing between fibers.
The HEPA standard
According to the European Standard EN1822:2009, HEPA filters are classified into different categories based on their minimum efficiency (%) : E10 to E12, H13 and H14, and U15 to U17 (ULPA).
| Class | Integral value (%) | Local value (%) |
|---|---|---|
| E10 | 85 | / |
| E11 | 95 | / |
| E12 | 99,5 | 97,5 |
| H13 | 99,95 | 99,75 |
| H14 | 99,995 | 99,975 |
| U15 | 99,9995 | 99,9975 |
| U16 | 99,99995 | 99,99975 |
| U17 | 99,999995 | 99,9999 |
HEPA filters: what are they used for?
HEPA filters are used in the handling of hazardous materials (asbestos, heavy metals, carcinogenic dust, lubricants…), in air-conditioning systems (operating rooms, laboratories, cleanrooms…), in sensitive industrial processes (pharmaceuticals, biotechnology, chemicals, optics, food processing, microelectronics…), and finally as a downstream safety filter in dust-collection systems.
Different types of HEPA filters
Depending on the application, different types of HEPA filters may be used. Beyond the required filtration efficiency, the goal is to minimize filter resistance in order to control operating energy costs.
Two main families of filters are distinguished:
Turbulent-flow HEPA filters are used in applications where air quality is essential, but where the airflow does not need to be laminar. These filters ensure a high flow rate thanks to a deep-pleat design. The filtration surface is 50 times larger than the front face of the filter. This structure provides low resistance and relatively low energy consumption.
In cleanrooms, where air purity is critical, laminar-flow HEPA filters are commonly used. Although they offer a lower airflow rate than turbulent-flow filters, they ensure impeccable cleanliness levels.
Some HEPA filters are designed for deployment under demanding conditions: high temperatures (up to 370 °C), high humidity, oxidizing environments, chemically corrosive streams, radioactive environments… For heavy particle loads that would quickly clog the inserts of standard HEPA filters, self-cleaning pulsed-jet HEPA filtration systems are available.
Learn more about:
HEPA filters in microelectronics
HEPA filters in cleanrooms
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