Filter (fluid technology)

from Wikipedia, the free encyclopedia
Paper filter with filter cake

A filter (technically neuter , commonly masculine ), like a sieve , holds back solids from a gas or liquid flow.

The solid-free phase is referred to as the filtrate in the filtration of liquids and usually as the clean gas in the gas filtration . Solid matter remaining on the surface of the filter is called filter cake , however, depending on the structure of the filter, filtration also or exclusively takes place inside the filter, particularly in the gas phase ( depth filter ), so that a filter cake is not always created.

The driving force behind a filtration is the pressure difference between the transport medium before and after the filter. The medium is either sucked through the filter (e.g. cigarette filter , nutsche ) or pressed through the filter with overpressure .

Working principle

The filtration is done through:

  • Surface filtration
    • Cake filtration : the filtrate is drawn off in the direction of flow. A filter cake of increasing thickness forms on the filter medium, which itself acts as an increasingly dense filter medium. The filter cake must be removed regularly. Both the solid and the purified carrier medium are obtained.
    • Cross-flow filtration (also known as cross-flow filtration): The filtrate is drawn off across the direction of flow. The shear forces prevent a filter cake from forming and the surface is cleaned. The suspension is concentrated in the direction of flow , the concentrate must be discharged.
  • Depth filtration : In the case of depth or storage filters , the separation takes place inside (in the depth ) of the filter medium. In this case, the fluid medium is the valuable substance , as the solid matter can only be extracted with difficulty from the filter.

The filter effect is based on the following physical properties:

Classification and naming

Filters can be classified and named according to different criteria. Therefore, several terms from the following categories can apply to a filter, e.g. B. a cigarette filter is an air filter and a particle filter at the same time:

Filter media

Oil filter of an oil heater

Filter media can be divided into:

For filtration tasks, different materials can be used depending on the area of ​​application, for example synthetic fibers (made of polyester , polyphenylene sulfide , polytetrafluoroethylene etc.), ceramic fibers / sintered bodies, glass fibers or metals . These can be treated chemically or physically (for example, felts sengen and calenders ) and surface coatings have (for example, membranes ). Cabin air filters are often a combination of a textile filter and an adsorbent .

Properties of a filter

The following properties are used to assess and classify filters:

  • The filter effectiveness describes the effectiveness with which the separated material is separated from the medium. 100% means that all separated material is filtered out .
  • When storing the material to be separated, a distinction is made between superficial (surface filter) and internal (depth filter) storage.
  • The pressure loss on the filter arises from the fact that the medium flows through the filter. A distinction must be made between the initial pressure loss of the filter material and the pressure drop due to increasing storage and accumulation of the separated material.
  • The cleaning of the filter describes the extent to which the material to be separated can be taken out of the filter again. There are filters that can only be used once, as the material to be separated can no longer be removed. A distinction must also be made here as to whether the material to be separated is used immediately, processed further, processed or disposed of.


Overview of filter materials

A vacuum cleaner bag is also a filter

The simplest filter is the sieve . Reusable coffee filters consist of an etched metal screen or a synthetic fiber fabric.

Paper filter

Paper filters are made of wet-strength paper. Application examples are vacuum cleaner bags and disposable coffee filters .

Glass fiber fleece

Nonwoven filter found, for example as a cigarette filter , in range hoods or air filter , additional filters for vacuum cleaners, filter cartridges for industrial vacuum cleaner , laminar flow cabinets and filter systems of air conditioning and clean rooms .
As Japanese mat to special filters that are used in swimming pools and garden ponds for filtering the water designated. Fuel and lubricants are cleaned by
fuel and oil filters . Depending on the application, respiratory protection filters also contain activated carbon , the large inner surface of which adsorbs complex gas molecules.
Microfilters / fine dust filters (HEPA filters, filter membranes ) hold back microbes in medicine and in water treatment .


Ceramic filter elements consist of open-pored ceramic . Ceramic filters are used in the foundry to clean molten metal. Also, water filter are partially made of ceramic.
Diesel soot particle filters can significantly reduce the particle emissions of a motor vehicle
diesel engine .

Sintered metal

In addition to fleece filters, open- pore sintered metal filter elements are used in the manufacture of microelectronics for filtering gases and liquids (clean rooms).

Needle felt

Especially in the field of large-scale industrial gas filtration (for example in waste incineration plants , foundries etc.) needle felts are primarily used as cleanable filter media .

Additional filters for protection are called police filters.

Backwash filter

Schematic drawing of a backwash filter 1 Inlet housing 2 Outlet housing 3 Backwash rotor 4 Filter insert 5 Backwash amplifier 6 Gear motor 7 Flanges for differential pressure monitoring 8 Flange for the flushing water line

The picture shows a continuously working backwash filter . Here, the mass of solids that forms on the filter surface is rinsed off by a small partial flow of the liquid. A rotor rotating over the filter surface and acting like a vacuum cleaner conveys the solids out of the filter via a connected pipe . The technology is used for the filtration of large quantities of cooling water within steam power plants in order to keep turbine condensers and tube bundle heat exchangers free from fouling . A particular challenge in the construction is the filtration of long-fiber dirt such as monkey hair from the cooling water.

The so-called active filter elements are a further development in the area of ​​these backwash filters . These elements behave like normal filter surfaces during operation, but during the backwashing process they open up an enlarged cross-section so that the filtered solids can more easily detach from the filter surface. Such filters are made for the fine filtration of solids from 1000 µm to 50 µm in size.

Water filters in building services

The waterworks supplies drinking water that meets the quality requirements of DIN 2000 ; in particular, it is then also free of solid and very fine particles. On the way to the house installation, however, sinter incrustations can become detached from the pipes and get into the water pipe, just as it is also possible that sand and other dirt can get into the house connection pipe in new building areas or when working on the public network. To ensure that these floods do not cause any damage, filters according to DIN 19632 are mandatory for metallic pipes in accordance with DIN 1988 and recommended for plastic pipes . These filters, also called ultra-fine filters, are installed in the house connection room between the water meter and the pressure reducer . The metal pipes are thus protected against various signs of corrosion such as pitting with possible subsequent pipe rupture; the fittings do not clog and remain functional.

There are essentially two types of filter, backwashable and non-backwashable filters, as well as those with automatic display due to the increasing differential pressure of the clogging filter. In the case of backflushable filters, the direction of flow is changed by opening a ball valve, whereby the filter particles detach from the filter fabric and are washed away via a drain. In the case of filters that cannot be backwashed, the filter insert, usually a filter sock, must be replaced.

Both types of filter require regular inspection and maintenance, which is to be carried out either by the operator of the house system or by a contract installation company (VIU).

Technical water filters

Many different types of filters are used in the treatment of industrial water and drinking water production. The most important filter types are:

These filters are used to reduce or remove undissolved substances, pollutants and turbid substances contained in the raw water. Iron and manganese compounds dissolved in the raw water are converted into insoluble oxide hydrates by adding atmospheric oxygen or, more rarely, oxidative chemicals and can be separated in the filter. For the better removal of turbid substances, flocculants and / or flocculants are often added to the raw water before the filters. Chloramines , org. Halogen compounds and traces of pesticides can be removed by adsorption with activated carbon . Aggressive carbon dioxide is bound by alkaline filter media, more on this under deacidification .

Drip filter in alchemy

De-stillatio per filtrum (dripping through a filter)

Hieronymus Brunschwig described alchemical distillation techniques in his Small Distilling Book (1500) . This also included the “distillatio per filtrum” - distillation (draining) through a filter: Triangular sheep's wool felts were dipped three cross fingers deep with the broad end into an open, glazed vessel containing the liquid to be filtered (“distilled”). The pointed end of the felt dipped into a deeper set vessel ("viole") in which the distillate was collected. This process was primarily used to purify ("rectify") water that had been obtained with other processes. The process was similar to our filtration, but could also be used to separate immiscible liquids.

See also

Web links

Individual evidence

  1. Duden: Volume 1, The German orthography. 23rd edition. Dudenverlag, Mannheim 2004, ISBN 3-411-04013-0 , page 375
  2. a b c VDI 3677 sheet 1: 2010-11 Filtering separators; Surface filters (Filtering separators; Surface filters) . Beuth Verlag, Berlin. P. 13.
  3. 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.
  4. a b VDI 3677 sheet 1: 2010-11 Filtering separators; Surface filters (Filtering separators; Surface filters) . Beuth Verlag, Berlin. Pp. 23-24.
  5. VDI 3677 sheet 3: 2012-11 Filtering separators; Hot gas filtration (filtering separators; high-temperature gas filtration) . Beuth Verlag, Berlin. Pp. 15-18.
  6. ^ Frank Schmidt, Uta Sager, Eckhard Däuber: Dynamic Adsorption Behavior of Cabin Air Filters. Filtration & Separation, ISSN  0015-1882 , 39 (7) 2002, pp. 42-47.
  7. Hieronymus Brunschwig. Small distilling book. Grüninger, Strasbourg 1500, sheet 6v-7r digitized MDZ Munich
  8. Lawrence M. Principe : Working Methods . In: Claus Priesner and Karin Figala: Alchemy. Lexicon of a Hermetic Science. Beck, Munich 1998, pp. 51-57.