More precisely, these are mostly filtering separators that remove substances from the air in a filtering medium - a special form here are electrostatic precipitators, which are not classed as filtering separators in the narrower sense. Fibers or grains are usually used as the filter medium (collectors). A distinction is made between fiber bed filters , bulk bed filters and filters with a solid medium (more rarely, such as sintered layers , ceramics ). What all filters have in common is the relatively high amount of energy required to achieve the filter target.
This type of filter emerged in the 20th century and is still used in the automotive industry today. They are paper-like ring-shaped or rectangular flat structures that have zigzag-shaped folded fabric as a filter element. The folding increases the filter area and reduces the flow resistance while increasing the service life . In most cases of the ring cylinder, the contaminated air is sucked in outside the cylinder and the filtered, cleaned air is passed on inside. This type of filter is very reliable and hermetically sealed. This paper-like fabric is vulcanized or glued between two covers. These filter inserts are disposed of after a single use, they are so-called "disposable filters".
Flat air filters are also used as variants. Here, the filter element in an air filter box is represented by a flat, folded filter mat fixed in a frame. The air mostly flows from bottom to top through the horizontally aligned filter element. Separated larger foreign bodies can fall to the bottom of the filter box when the air flow stops and so do not additionally hinder the air flow.
Paper-based air filters are easy to manufacture and very inexpensive.
There are also foam dry filters. These are the same constructions as paper air filters, only that smooth foam layers are used instead of the folded paper. Advantages compared to paper filters are the higher air permeability and the possibility of cleaning (knocking out / washing out), which enables longer service lives. The disadvantage is the lower filter effect compared to paper filters, which require thicker foam layers. Also, not all of the accumulated dirt can be removed by cleaning, so that the filter slowly becomes clogged and eventually has to be replaced. In addition, foam is usually not durable; as a result, the filter elements begin to dissolve after a certain time, which then also requires replacement.
Both types of filter are often combined with one another: a pre-filter made of foam (for coarse dirt), then paper as the main filter. This principle is used in particular in areas of application with increased levels of dirt / dust, e.g. B. in agriculture or at home lawn mowers.
In motor vehicles that frequently work in a dusty environment (such as agricultural machinery or utility vehicles ), a variant of the centrifugal separator is used as a prefilter in front of the actual air filter. The so-called cyclone filter removes dust and water and thus relieves the actual air filter. This contributes to a significant increase in the service life of the entire filter system.
These filters are available in two versions:
- manual emptying of foreign bodies that are collected in a collecting container
- automatic emptying at standstill by valves or during operation by means of an impeller driven by the air flow
Liquid filter (oil bath air filter)
Liquids through which the air is passed are used less often for filtering, for example in the oil bath air filters of earlier cars and trucks. The air flow is directed vertically onto the oil bath and is strongly deflected there. Due to their inertia, the heavy airborne particles maintain their direction towards the oil bath, are bound by the liquid and collect in it. Cleaning is carried out by washing the steel sieve in the oil bath with benzine or cold cleaner . In addition, the oil must be changed after a certain interval, as the dirt particles that the steel sieve has retained have mixed with the oil. The use of oil bath air filter is carried out at to be expected high dirt accumulation and possible poor supply of spare parts (eg. As military). A disadvantage of this very robust filter construction is the high flow resistance caused by the deflection of the air flow and the associated loss of performance. In the passenger car sector, this technology is no longer used, apart from exceptional cases.
Another version is the oil-soaked steel mesh air filter, which is used on motorcycles such as. B. in the BMW R 25/3 , in older Vespa scooters or in older tractors. These filters consist of many layers of steel strips that are laid on top of each other like a net. Steel mesh air filters are soaked in engine oil, which means that the dust particles in the air flow stick to the oil when they are sucked in. This enables coarse to medium-fine filtering of the intake air. This oil-soaked steel sieve can be cleaned or washed out with benzine or the like and thus freed from the dust again. After cleaning, the filter must be soaked again with oil.
Some manufacturers such as K&N have special replacement air filters for many car and motorcycle makes, which replace the original air filters. The specialty is that these are so-called "permanent air filters" which, according to the manufacturer, can be used up to 100,000 km without cleaning and can be oiled again after cleaning and used again. The material is cotton soaked in special filter oil , which means that the filter, in contrast to the steel mesh air filter, also retains fine and finest particles. In order to take account of the change in the air flow rate, the mixture setting must be adjusted on older carburettor engines. In vehicles of more recent year of construction with injection, the air volume or mass is recorded via the air volume or mass meter. A manual mixture setting is then no longer necessary here. Checking the filter performance is essential, especially when converting engines.
Electrostatic precipitators are z. B. used in large stationary systems such as power plants (flue gas filter) or collieries (compressor protection filter). Furthermore, electrostatic precipitators of smaller design are used for separating aerosols, such as those found in e.g. B. in metalworking and metalworking caused by cooling lubricants (turning, grinding, pushing etc.) - about 50% of the separators used in metalworking and metalworking companies are electrostatic separators of various designs.
In electrostatic precipitators, dust particles or aerosols are electrostatically charged and deposited on the electrode surfaces. The pollution is z. B. eliminated by clocked knocking cycles and subsequent mechanical removal of the dust collected in hoppers using pipe screw conveyors. In the case of liquid components and often also non-toxic dusts, the electrode surfaces are regularly rinsed by machine. The disadvantage of electrostatic precipitators is the lack of operational reliability: if the high-voltage supply fails, large quantities of pollutants are usually passed through unfiltered, which must be taken into account in terms of process technology and, if necessary, avoided by other measures. The advantage of electrostatic precipitators is that they operate almost without consumables (e.g. filter inserts) compared to filtering separators (mechanical filters with filter inserts).
The manufacturers of large electrostatic precipitators are GEA Bischoff GmbH in Essen and ELEX AG in Schwerzenbach , Switzerland. Manufacturers of smaller electrostatic filters are United Air Specialists, Inc. (Bad Camberg) and ILT Industrie-Luftfiltertechnik GmbH (Ruppichteroth).
As an alternative to electrostatic precipitators, bag filters are used in industrial applications . These are filters that are equipped with a large number of individual filter bags stabilized by wire baskets. The filter bags hang vertically downwards; The dust collects on its cylindrical outside and a filter cake is formed . These filters are also cleaned either according to their degree of contamination (by monitoring the pressure difference, i.e. pressure-controlled ) or after a certain time ( time-controlled ) by introducing a short compressed air pulse into the filter bags against the flow direction (" pulse jet cleaning ") or the flow direction in the filter system is reversed (" reverse air cleaning "). As a result, the filter cake is thrown off the tubes and the filter material is permeable again for a longer period of time. The filter bags are wearing parts; their service life ranges from a few days to more than ten years, depending on the process and the filter material used. As a rule, the filter bags are cleaned several tens to hundreds of thousands of times during this time. The dust falling from the filter hoses is collected in funnels and removed from the system via tubular screw conveyors or rotary valves and then disposed of or added to the following processes (e.g. firing, cement production).
Air filters are classified according to particle size in accordance with DIN EN 779: 2012. Coarse dust class G1 to G4, fine dust class M5 to M6 and from F7 to F9.
- Depth filtration: particles are positioned in the filter medium by inertia , diffusion , electrostatics or the sieve effect (clogging phase)
- Cake / surface filtration: Construction of a filter cake , which itself acts as a highly efficient filter medium. In this phase, the separation takes place primarily through the blocking effect .
Air filters are used, for example
- in vehicles with gasoline or diesel engines , to clean the combustion air before it is drawn into the carburetor or the cylinders ,
- in the dust-emitting industry (e.g. paper, cement ) ( environmental protection ),
- in ventilation systems to keep pollutants away from apartments (see also controlled living space ventilation )
- in vacuum cleaners to purify the sucked in air as completely as possible,
- in the metal working and processing industry for removing oil and emulsion mist from machine tools ( oil mist separators ) and the like. v. m.
- Exhaust filter
- Diesel particulate filter
- Filter fan
- Oil mist separator
- Particle filter classes
- HEPA filters
- vacuum cleaner
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- BIA-Report 4/2004, Use of cooling lubricants in metal cutting, Main Association of Commercial Employer's Liability Insurance Associations (HVBG), Employer's Liability Insurance Association Institute for Occupational Safety and Health (BGIA), Sankt Augustin 2004, ISBN 3-88383-669-9 ( online )
- Until 2011 fine dust classes F5 to F9, according to DIN EN 779: 2003