Information on
HEPA filter
HEPA filters are used to mechanically clean the air. Depending on the filter class, they are known as HEPA filters (High Efficiency Particulate Air filters), ULPA filters (Ultra Low Penetration Air) or SULPA filters (Super ULPA). They are used to filter out viruses, respirable dusts, mite eggs and excrement, pollen, smoke particles, asbestos, bacteria, various toxic dusts and aerosols from the air.
Areas of application
The HEPA filters are used in the medical sector, i.e. in operating theatres, intensive care units and laboratories as well as in clean rooms, in nuclear technology and in air purifiers. The respective application is decisive for the selection of the filter class.
Structure
HEPA filters are made of cellulose, glass fibres or other synthetic materials and have a fibre diameter of around 1 to 10 micrometres. To ensure that the air to be cleaned flows through as large an area as possible, several layers of the thin filter layers are stretched in a wave or serrated pattern in a plywood or metal frame. Special housings are used for contactless filter replacement. They are designed to prevent the carefully filtered pollutants from being touched or inhaled when the filter is changed. Depending on the material used, a HEPA filter can be washed out and reused.
How HEPA filters work
HEPA filters are close-meshed fibre nets with an irregular arrangement of fibres. In contrast to the way a sieve works, HEPA filters remove particles from the air that are significantly smaller than the spaces between the fibres. Filtering takes place as the air flows past the fibre mesh. The following modes of action are utilised for particle separation in the filter:
Inertia effect
Larger particles have a higher mass and therefore a higher inertia. Due to their inertia, the particles do not follow the air flow around a filter fibre in the event of a sudden change of direction, but instead collide with the fibre and stick to it.
Blocking effect
Small particles with a low mass follow the airflow as it moves around the fibres. If the particles come too close to a filter fibre, they are attracted by adhesion forces and stick to it.
Diffusion effect
The movement of particles smaller than 1 micrometre is influenced by collisions with other gas molecules. As a result, they do not follow the air flow, but have a trajectory similar to Brownian motion due to their collisions with the air molecules. Sooner or later they collide with the filter fibres and stick to them.
Particle filter classes
The European standard for the classification of HEPA filters is EN 1822-1:1998, which distinguishes between 17 different particle filter classes, with particle filter class 17 having the highest guaranteed filtration efficiency. HEPA filters are categorised in filter classes from H10 to H14, ULPA filters in classes U15 to U17. As particles between 0.1 and 0.3 micrometres in size are the most difficult to separate, HEPA and ULPA filters are classified according to their effectiveness in filtering out these particle sizes. Larger and smaller particles are better separated due to their physical properties. A distinction is made between the overall efficiency of the filter (overall filtration efficiency) and the worst localised location (local filtration efficiency).
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