Types of Mechanical Air Purifiers
For cleaning air, two major types of mechanical air purifiers are used.
Generally, flat or panel filters are made up of either a low packing density of coarse glass fibres, animal hair, vegetable fibres, or synthetic fibres coated with a viscous substance, or slit and expanded
aluminum. The typical furnace filter installed in central heating and air-conditioning systems is the flat filter that is used in many homes. Flat filters remove only a small percentage of respirable size particles, though these may efficiently collect large particles.
"Electret" media, consisting of a permanently charged plastic film or fibre are also used for making flat filters. The charged material attracts polluting particles in the air.
Generally, pleated or extended surface filters trap more respirable size particles than flat filters. The use of smaller fibres allowed by the greater surface area leads to increased packing density of the filter without a substantial drop in the rate of airflow.
An electrical field is used to trap charged particles in electronic air purifiers. These may be installed, akin to mechanical purifier, in central heating and air-conditioning system ducts or portable units with fans. Electrostatic precipitators or charged-media filters are generally used in electronic air purifiers. Particles are collected on flat plates, arranged in series, in electrostatic precipitators. Charged-media filter devices are less common. In these, the fibres in a filter are used for collecting particles. The particles are ionised before the collection process, in order to obtain higher collection efficiency, in most electrostatic precipitators and some charged-media filters.
Static charges are used in ion generators to remove polluting particles from the air. These portable devices charge the particles in a room, so that the particles are attracted to walls, floors, tabletops, draperies, occupants, etc. To attract the charged particles back to the unit, these devices, in some cases, contain a collector.
The last two kinds of devices may produce ozone.
Some newer systems, referred to as "hybrid" devices, contain two or more of the particle removal devices described earlier.
Air purifiers may also contain adsorbents and reactive materials to make the removal of gaseous materials from indoor air easier, in addition to particle removal devices. Gaseous pollutants are not likely to be removed by air purifiers that do not contain these types of materials.
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