Filtration

The filter ( colloquially the filter) holds, like a sieve, solids from a gas or liquid stream back. The solids-free phase is usually referred to in the filtration of liquids with the filtrate and gas filtration with clean gas. On the back surface of the filter is constant solid filter cake, however, the filtration takes place in particular in the gas phase, depending on the construction of the filter also or only in the interior, so that not a filter cake is formed in each case. The driving force before and after filtration, the filter has a differential pressure of the transport medium. The medium is either drawn through the filter (for example, cigarette filters, Nutsche ) and pressed by pressure through the filter.

Principle of operation

The filtration is done by:

• Cake filtration: On the filter medium, a filter cake with increasing thickness, which in turn acts as an increasingly dense filter media itself forms. The filter cake must be removed regularly. This gives both the solid and the purified carrier medium.
• Depth filtration The separation is carried out at depth or memory filters inside the filter medium. Here, the fluid medium of recyclables because the solids can be extracted only with difficulty from the filter.
• Cross-flow filtration: The filter medium is transverse to the direction of flow. By the shearing forces is prevented to form a filter cake, the surface is cleaned. In the flow direction, the suspension concentrate, the concentrate has to be removed ( cross-flow filtration).

The filtering effect based on the following physical properties:

• The particles are larger than the pores of the filter and thus can not enter the filter medium. This effect is referred to as a sieving effect and appeared predominantly in membrane surface coated filters and for the filtration of suspensions.
• Due to their inertia, the particles of the flow can not follow and they hit the filter material to which the medium flows past tangentially. As a measure of the inertial deposition, the Stokes number is being used.
• Due to the Brownian movement, the particle path varies randomly to the power line to which the particles move, which can cause the filter media to contact the particles. This is also referred to as a diffusion deposition effect is described by the Peclet number, and plays only for very small particles ( less than a few hundred nanometers ) play a role.
• In filter cake from surface filters is the blocking effect of the dominant deposition mechanism. The blocking effect, the trajectory of the center of gravity of the particles may be over leads to the filter material, the particles can not pass through the filter material due to its geometrical extension, and is deposited.
• With a sufficiently long waiting time, the particles separated by thermophoresis when they diffuse along a temperature gradient.
• In gas flow, small particles can be filtered out electrostatically. Used is non-woven filter, the fibers are electrically charged or polarized in the manufacture, so called electret fibers.

Filter media

The filter media can be used:

• Felts or generally oriented fiber webs
• Spunbondeds or generally random fiber webs
• Beds
• Porous solids and filter cartridges, such as sintered metals
• Wound filter cartridges with the yarns produced from different materials
• Tissue
• Paper

Here, depending on the application, the variety of materials are used, for example, synthetic fibers ( polyester, polyphenylene sulfide, polytetrafluoroethylene, etc. ), ceramic fibers / sintered body, glass or metals.

Added to chemical or physical treatment of the filter (for example, felts sengen and calenders ) and surface coatings (such as diaphragms ).

Characteristics of a filter

The following properties are used to assess and classification of filters:

• Describes the filtration efficiency, the efficiency with which the material to be separated is separated from the medium. 100% means that all of the material to be separated is filtered.
• On intercalation of Trennguts storage distinction is made between superficial ( surface filters ) and inner ( depth filters ).
• The pressure drop across the filter caused by the fact that the medium flows through the filter. Distinction must be made between the initial pressure drop of the filter material and the pressure drop by increasing storage and accumulation of Trennguts.
• The cleaning of the filter describes the extent to which the material to be separated can be taken out of the filter. In this case, there are filters that can be used only once, since the material to be separated and can not be taken out. This is also necessary to distinguish whether the material to be separated immediately used, or further processed or disposed of.

Examples

Overview filter materials

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

Paper filters are made of wet-strength paper. An application example is a vacuum cleaner bag.

Nonwoven filter can be found for example as a cigarette filter in cooker hoods or as an air filter, additional filters for vacuum cleaners, filter cartridges for industrial vacuum cleaner, laminar flow cabinets and filter systems of air conditioning systems and for clean rooms. When Japan mat is referred to special filters that are used in pools and garden ponds for filtering the water. Fuel and lubricants are cleaned by fuel and oil filters. Filter respirator included depending on the application of activated carbon, the large internal surface adsorbed complex gas molecules. Microfilter / particulate filter (HEPA ) filters, filter membrane ) hold back in medicine and in water treatment microbes.

Ceramic filter elements are made of open-cell ceramics. In foundry ceramic filter for cleaning molten metal can be used. Also water filters are partially made of ceramic. Diesel particulate filters can reduce particulate emissions of an automotive diesel engine significantly.

Porous sintered metal filter elements are used in addition to fabric filters, among others, in the manufacture of microelectronics for filtering gases and liquids.

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

Additional filters are hedging called police filter.

Backwash filter

The images show a continuous backwash filters. Here, the forming on the filtering surface is rinsed mass of solids with a small partial flow of the liquid. A wrap on the filter surface and acting like a vacuum cleaner rotor thereby promotes the solids out of the filter via a connected pipeline. The technology is used for filtration of large quantities of cooling water within steam power plants in order to keep the turbine condensers and tube heat exchanger fouling. A particular challenge in the construction is the long-fiber filtration soiling such as the monkey hair from the cooling water.

A further development in the field of backwash filters are the so-called active filter elements. These elements behave like normal operating filter surfaces, but during the Rückspülvorgangen they give an enlarged cross section free, so that the filtered solids can easily detach from the filter surface. Such filters are made for fine filtration of solids from 1,000 microns to 50 microns size.

Water filters in domestic technology

The waterworks supplies drinking water which meets the quality requirements of DIN 2000; in particular, it is then also free of solid and fine particles. On the way to the house wiring but can dissolve encrustations from the pipes and get into the water line, as it is also possible to enter new areas or when working on the public grid sand and other debris in the service line. In order for these bodies in the water cause no damage, according to DIN 1988 are installed for metallic pipes dutifully and for plastic pipes empfohlenermaßen filter according to DIN 19632. This filter, also known as micro-filter are installed in the house wiring space between the water meter and the pressure reducer. The metal tubes are protected from various corrosion such as pitting with possible following pipe rupture; the valves do not put to him and remain functional.

Essentially, there are two filter types, flushable and non- flushable filter, as well as those with automatic display due to the increasing differential pressure of up clogging the filter. For the other filters, is amended by opening a ball valve, the flow direction, the filter particles detach from the filter fabric and are washed away via a drain. For non - reversible flow filters, the filter insert, usually a filter sock must be replaced.

Both types of filters require regular inspection and maintenance, which is carried out either by the operator of the appliance or by an authorized installation company ( VIU ).

Technical water filter

Many different types of filters are used in the treatment of process water for industry and drinking water production. The main filter types are:

• Container with beds of filter media, also referred to as X-filter. X stands for the name of the filter material used such as sand, gravel, activated carbon, hydro- anthracite, limestone, basic materials such as Magno.
• Precoat
• Cartridge Filter
• Spatial filter
• Layer filter bed
• Magnetic filter

With these filters contained undissolved substances and harmful substances, turbidity in the raw water are reduced or removed. In the raw water dissolved iron and manganese compounds are converted by the addition of air or oxygen into insoluble hydrated rare oxidative chemicals, and can be separated in the filter. For better removal of suspended matter flocculant and / or flocculants are often also 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 carbonic acid is bound on a basic filter materials, further this under deacidification.