Filter (fluid technology)
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 .
The filtration is done through:
- 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:
- The particles are larger than the pores of the filter and therefore cannot penetrate the filter medium. This effect is known as the sieve effect and occurs primarily in membrane- coated surface filters and in the filtration of suspensions .
- Due to their inertia , the particles cannot follow the flow and hit the filter material, which the medium flows past tangentially. The Stokes number serves as a measure of the inertial separation .
- Due to Brownian molecular motion , the particle trajectory fluctuates statistically around the streamline on which the particle moves, which can lead to the particles coming into contact with the filter material. This effect, also known as diffusion separation, describes the Peclet number . It only plays a role for very small particles (smaller than a few hundred nanometers ).
- In the filter cake of surface filters , the barrier effect is the determining separation mechanism. The path of the center of mass of the particles leads past the filter material, but the particle cannot pass through the filter material due to its geometric dimensions and is separated out.
- If the waiting time is long enough, the particles separate by thermophoresis when they diffuse along a temperature gradient.
- Small particles can be electrostatically filtered out in the gas flow . Used to electret filter : nonwoven filter , the fibers in the manufacture of electrically polarized or loaded are ( electret fibers).
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:
- after the application or the application: cigarette filters , respirator filters , aquarium filters , cabin air filters , etc.
- according to the functional principle: surface filters , depth filters , membrane filters , electret filters , sieve filters , precoat filters , etc. (see above)
- according to the current to be cleaned: air filter , water filter , oil filter , coffee filter , circulating air filter , hot gas filter , etc.
- after the deposited materials: particulate filter , HEPA filter , carbon filter , grease filter , etc.
- after the execution or construction: bag filter , cartridge filter , bag filter , bulk-layer filters , etc.
- according to the fineness: microfilter , ultrafilter , nanofilter , HEPA filter , etc.
- according to the function or position in a multi-stage filter system: pre-filter, final filter, police filter, etc.
- according to the filter material or medium: paper filter , fabric filter , wire filter , gravel filter , etc. (see below)
Filter media can be divided into:
- flexible filter media, including fabrics , paper and nonwovens ( fiber-oriented nonwovens such as felts and tangled fiber nonwovens such as spunbonded nonwovens). The natural fibers used in earlier years have largely been replaced by synthetic fibers in industrial applications . In the case of fibers, there is usually a size distribution; therefore it can be assumed that filters made of fiber fleeces are inhomogeneous .
- Rigid filter media, which are characterized by high rigidity and inherent strength , usually consist of fibers or grains. The basic material that is usually sintered is plastic, metal or ceramic. Embodiments are porous solids and filter candles . Granular ceramic filter elements with expanded clay or silicon carbide as the base material can be used for hot gas filtration .
- In contrast to the other filter media, bulk bed filters are just a loose composite of granular material through which the fluid flows. They are made up of granular material, the type and size distribution of which can be adapted to the separation task. Often these are oxidic materials.
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
- Paper filter
- 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
- Needle felt
Additional filters for protection are called police filters.
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:
- Containers with beds of filter media, referred to as X-filters for short. X stands for the name of the filter material used, such as sand , gravel , activated carbon , hydroanthracite , limestone , basic materials such as Magno .
- Precoat filter
- Candle filter
- Spatial filter
- Stratified bed filter
- Magnetic filter
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
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.
- Leakage flow - calculation of flows in filter media
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