FAQs

Microfiltration is a pressure-driven membrane process that primarily retains particles and microorganisms on the basis of their size. Typical pore sizes range from approximately 0.1 to 10 µm; the distinction from ultrafiltration is not defined in exactly the same way in all specialist sources. The term 'ultrafiltration' is generally used when the pore sizes are below approximately 0.1 µm.

Depth filters trap particles within a porous, multi-layered structure and are often suitable for a wide range of particle sizes and gel-like contaminants. Surface filters primarily trap particles on a defined surface; pleated designs offer a large surface area while maintaining a compact design. The classification depends on the design, and not every pleated filter is automatically a surface-only filter.

The beta value βx is the ratio of the number of particles of size x µm and larger upstream of the filter to the number downstream of the filter. The separation efficiency is calculated as η = (βx − 1) / βx × 100 per cent. Examples: β10 = 10 corresponds to 90 per cent, β10 = 200 corresponds to 99.5 per cent and β10 = 5,000 corresponds to 99.98 per cent separation efficiency for particles 10 µm and larger – in each case under the specified test conditions.

A nominal filter fineness describes a manufacturer-defined, non-complete separation at the specified particle size. An absolute filter fineness indicates a specified, high separation efficiency; however, it must always be considered in conjunction with test procedures and the retention rate or beta value. Data from different manufacturers can therefore only be compared if the underlying test conditions are known.

The starting point is the largest particle size still permitted in the filtrate. In addition, particle distribution, the desired retention rate, process risk and downstream components must be taken into account. An unnecessarily fine design often leads to increased initial pressure loss, more frequent replacement and higher costs, without bringing about a corresponding improvement in process performance.

The filter fineness describes the particle size to which the specified filtration efficiency of a filter element refers. A lower µm value indicates finer filtration. However, the numerical value alone is not sufficient: what matters is whether the fineness is specified in nominal or absolute terms, and which test method or beta value is used to substantiate it.

The correct filter fineness depends on which particles need to be removed and the required purity of the filtered medium. When making a selection, therefore, particle size, process requirements, flow rate and permissible pressure drop should all be taken into account. It is advisable to make use of the advice offered by filter manufacturers.

The filter area required depends, in particular, on the flow rate, the filter medium, viscosity, particle load and the permissible pressure drop. The filter area should therefore be designed on the basis of the specific process data and the characteristic values of the respective filter element. It is advisable to make use of the advice offered by filter manufacturers.

In addition to the required filter fineness, the filter medium, temperature, operating pressure, flow rate and chemical resistance of the filter material are crucial. The dirt-holding capacity and the desired service life should also be taken into account.

The filter material must be chemically and thermally resistant to the medium to be filtered. Depending on the application, materials such as stainless steel, polypropylene, polyester, polyamide, PTFE or other suitable filter materials are used.

The differential pressure Δp is the difference in pressure between the filter inlet and the filter outlet. It consists of the initial pressure drop in the clean system and the resistance that builds up during operation due to accumulated contaminants. That is why Δp is an important criterion for condition monitoring, but not the only one.

As filtration progresses, particles accumulate on the filter element and increase the flow resistance. This increases the pressure difference between the filter inlet and outlet.

The key factors are the maximum differential pressure permitted by the manufacturer, the required flow rate and the required filtrate quality. The element must be replaced or cleaned at the latest when any of these criteria are no longer met. A universal conversion factor in bar is not reliable, as the design, medium, temperature and process conditions vary.

Appropriate measures include ensuring a sufficiently large filter area, multi-stage pre-filtration and fine filtration, depth or surface filtration appropriate to the type of particle, a low initial pressure drop, and avoiding unnecessarily fine filter elements. Process variations, precipitates and foreign matter should also be investigated. The most cost-effective solution is determined by service life, filtrate quality and total cost of operation.

The dirt-holding capacity describes the amount of contaminants that a filter element can retain up to a defined end point. This limit could, for example, be a specified differential pressure or a situation where the separation efficiency is no longer sufficient or the flow rate is no longer adequate.

Key factors include the medium, flow rate, operating and design pressure, temperature, viscosity, filter fineness, solids content, material and seal compatibility, connections, installation location and access for maintenance. In addition, relevant legal requirements and customer specifications must be checked.

As viscosity increases, flow resistance increases and the achievable flow rate at a constant pressure decreases. That is why viscosity and temperature must be considered together. Data relating to water cannot be applied to oils, paints or other viscous media without adjustment.

Yes, provided that all components in contact with the medium are suitable: housing, filter medium, support structure, adaptor, seals and, where applicable, coating. The permissible values of the weakest component are decisive. Chemical resistance must always be checked in relation to the medium, concentration, temperature and duration of exposure.

Yes, provided that the materials used and the design meet the relevant hygiene and conformity requirements. Depending on the process, other factors such as cleanability, sterilisability, minimal dead space, surface quality, traceability and microbiological retention may also be relevant. Authorisation must be granted on a case-by-case basis.

A bypass is defined as an undesirable flow path through which fluid flows past the filter medium. Possible causes include incorrect adaptors, unsuitable or damaged seals, dimensional deviations, incorrect assembly or a faulty filter element. A suitable component and careful installation are therefore just as important as the filter fineness.

Yes, suitable filter housings and filter elements are available for the food and beverage industry. In this context, materials, seals, design and cleanability must comply with the relevant hygiene requirements.

Activated carbon is a natural product based on natural carbonic raw materials. Its porous structure gives it a very large inner surface, allowing it to bind chemical compounds and molecules. 

Activated carbon filters are used to reduce dissolved organic contamination (color, odor, and taste contaminants, chlorine, pesticides and medication residue). It does not remove all pesticides, pharmaceutical residues or dissolved substances across the board. Suitability and capacity must be assessed for the specific substance and process.

For example:

• water treatment
• chlorine separation
• drinking water
• pesticide separation
• pharmaceutical products
• organic material separation
• food and cosmetics
• color and odor removal

Housings made of stainless steel are very resistant to temperature and pressure. So, where high pressures and high temperatures are present, stainless steel filter housings are the first choice.

Stainless steel filter housings are robust, durable and resistant to many media, high temperatures and high pressures. Depending on the grade of stainless steel – for example, 1.4301, 1.4401 or 1.4571 – they are suitable for a variety of industrial applications.

Stainless steel housings are suitable for many applications with heightened requirements in terms of strength, corrosion resistance, temperature, pressure or cleanability. However, suitability depends on the specific grade of stainless steel, the medium and the operating conditions; 'stainless steel' is not a blanket guarantee of resistance.

Stainless steel filter housings are primarily used in the industrial process filtration of liquids and gases. Typical sectors include the chemical industry, the food industry, water treatment, electroplating, metalworking, the paint and coatings industry, and the electrical industry.

No. A sufficiently smooth, professionally finished surface can reduce the build-up of deposits and make cleaning easier, but does not automatically prevent microbial growth. For hygienic applications, factors such as the material, surface roughness, minimal dead space, ease of drainage, seals, cleaning and sterilisation procedures, and process control must all be compatible.

Stainless steel housings generally offer greater mechanical strength and higher resistance to pressure and temperature than plastic housings. At the same time, different grades of stainless steel can be specifically tailored to the medium to be filtered. They can also be manufactured with defined surfaces that are easy to clean.

Depending on the filter element used, stainless steel filter housings can be used to filter, among other things, water, process fluids, oils, chemicals, varnishes, paints and other industrial fluids. Applications involving gas and compressed air filtration are also possible with suitable housings and filter elements.

Depending on the type of housing, filter bags, filter cartridges, sieve elements, metal filters or special high-performance elements can be used, for example. The choice of suitable filter elements depends, in particular, on the medium, the filter fineness, the flow rate, the pressure and the temperature.

Bag filter housings offer a high dirt-holding capacity at high flow rates and allow for a comparatively quick and easy filter change. They are particularly suitable for applications in which large quantities of solid particles need to be separated from liquids.

Stainless steel bag filter housings are mainly used for industrial liquid filtration. Typical applications can be found, for example, in the chemical industry, water treatment, the food industry, electroplating, and in the manufacture of paints and varnishes.

Stainless steel bag filter housings can filter liquids containing solid particles, such as process water, cooling water, oils, chemicals or industrial production media. The actual filter fineness achievable depends on the filter bag used.

Multi-bag housings accommodate several bags in parallel, thereby providing a larger filter area for high flow rates. Compared with several individual enclosures, the piping and space requirements may be reduced. Points to note include lid handling, emptying, a uniform flow of material and the safe replacement of all bags.

Plastic housings are a good option when low weight or chemical resistance to a specific medium is a priority. Their suitability must be assessed on the basis of the material, concentration, temperature, pressure and duration of exposure. Contec also offers special filter housings for the chemical and seawater areas.

SF cartridge filter housings can accommodate several filter cartridges, thereby providing a large filter area while maintaining a comparatively compact design. By selecting different filter cartridges, they can be used to meet various requirements in terms of filtration fineness and separation efficiency.

SF cartridge filter housings are resistant to, for example, hydrochloric and sulphuric acid, seawater, liquid hydrocarbons, alcohols and concentrated alkalis, and are predominantly used in the chemical industry or in seawater applications, where they are employed for fine and safety filtration of liquids. Typical applications include industrial process filtration, water treatment and the chemical industry.

The number of filter cartridges depends on the specific SF size and the design of the filter housing. To ensure a reliable selection, the housing type, cartridge length, filter flow rate and required filter area should therefore be considered in consultation with the manufacturer.

Single-place plastic filter housings are lightweight, compact and can offer high resistance to certain chemicals. They are particularly suitable for applications in which plastic materials such as polypropylene offer better resistance to process media than stainless steel.

Single-place plastic filter housings are suitable for the reliable pre-filtration and fine filtration of liquids and are used, among other things, in industrial process filtration, water treatment and chemical engineering. They are of particular interest when dealing with corrosive media against which metallic materials do not offer sufficient resistance.

Plastic bag-filter housings can offer high resistance to acids, alkalis and other corrosive media. At the same time, they are lighter than comparable stainless steel housings, although they are generally less resistant to pressure and temperature.

Plastic bag filter housings are used in particular with corrosive liquids, where stainless steel is not sufficiently resistant. Examples include applications involving hydrochloric or sulfuric acid, seawater, certain hydrocarbons, alcohols and concentrated alkalis.

Plastic bag filter housings can be used to filter liquids containing solid particles. Depending on the filter bag used, process water, chemicals or corrosive liquids, for example, can be filtered.

When selecting a filter cartridge, factors such as the design, length, connection, filter material and filter rating must be compatible with the filter housing and the intended application. In addition, the medium, temperature, pressure, flow rate and the desired service life are crucial.

'Filter element' is the general term for a component that retains particles or other contaminants from a medium. A filter cartridge is a specific type of filter element that is fitted into a suitable filter housing.

Common filter types include melt-blown, wound, pleated depth and surface filters, membrane filters, and metallic sieve and sintered elements. Depending on their design, they can be used for pre-filtration, fine filtration, microfiltration or sterile filtration.

Filter cartridges offer a large filtration area whilst maintaining a compact design, and enable controlled filtration using a variety of filter media. Depending on the design, a wide range of filter finenesses can be achieved, right up to depth filtration.

Filter cartridges are primarily used for the filtration of liquids, but with the appropriate design they can also be used for gases, air and steam. The appropriate filter material depends on the fluid, temperature, pressure and required filter fineness.

For example, as:

• pre-filtering in water treatment chemicals
• process filters for solvents
• electroplating
• surface engineering
• oils, greases
• painting and lacquering industry

The pleating provides a large effective filter area within a compact footprint. This can result in low initial pressure drop, high flow rates and a long service life. This is subject to the pleat geometry, filter medium and process conditions being properly matched.

Melt-blown filters consist of melt-blown, interwoven polymer fibres (melt-blown process), often made of polypropylene. Many versions have a pore structure that becomes finer from the outside in (multi-layer structure) and act as depth filters. The retention rate, support core, adaptor and material resistance vary depending on the product.

In the case of wound filter cartridges, yarn is wound around a support core in a defined pattern (yarn-winding process). They are regarded as robust depth filters and are available for a range of particle loads and media. Performance and fibre release depend largely on the yarn, the winding, the support core and the quality of manufacture.

Thanks to their wound structure, wound filter cartridges provide deep filtration and are therefore able to trap particles throughout the entire filter structure. They are available in a range of materials and filter grades and are considered to be very robust.

The dimensions and ring design must be compatible with the housing. In addition, the filter fineness and retention rate, the material and seal, the temperature, the differential pressure, the flow rate, the type of solid and the desired service life must be taken into account. In the case of critical processes, the bypass safety of the seal should also be assessed.

Bag filters are a robust and often cost-effective solution for coarse and fine filtration of liquids, particularly where there is a high solids load. They allow for easy element replacement and are available in various sizes, materials and filter grades.

Filter bags are mainly used to remove solid particles from liquids. Depending on the filter material and filter fineness, it is possible to filter, for example, water, oils, chemicals, paints and other industrial process fluids.

Bag filters are particularly suitable for liquids with a high solids content and where a high dirt-holding capacity is required. Cartridge filters are often the better choice when precise filtration and a large filter area are required in a compact design.

Sieve elements are robust surface filters designed for specific, generally coarser separation tasks. Many metal versions can be cleaned and reused. However, they generally have a lower dirt-holding capacity than depth filters and require suitable support to withstand differential pressure.

Sieve elements can, for example, be made from stainless steel or plastic. The choice of material depends in particular on the medium, temperature, pressure, corrosion resistance requirements and price.

The support body stabilises the sieve element and prevents it from collapsing or becoming damaged as a result of the differential pressure. This enables the filter element to retain its shape and filtering performance even under higher loads.

Sieve elements are particularly suitable for separating larger solid particles from liquids. Typical applications include, for example, process water, cooling water, oils and other industrial liquids.

Membrane filters enable the very fine and precise removal of particles and microorganisms. Depending on the type of membrane, they can be used for microfiltration, ultrafiltration or sterile filtration.

In membrane filtration, a porous membrane performs the specified separation function.

A distinction is made between

• Microfiltration (pore size approx. 0.1–10 µm) and
• Ultrafiltration (pore sizes approx. 0.001 – 0.1 µm)

Most of the separation takes place on the membrane surface. The average membrane pore size essentially determines the separation effect.

Membrane filters are always used when a high degree of purity is required for the suspension to be filtered. Membrane filters are most cost-effective when the particle load in the suspension to be filtered is very low. Thus there should be pre-filtering to protect the high-quality membrane filter elements from clogging too quickly. 

An integrity test checks whether the membrane and filter element are undamaged in accordance with specified criteria. Depending on the component and the application, tests such as bubble-point, diffusion or pressure-retention tests are used, for example. Test procedures and limit values must be specified by the component manufacturer and validated for the process.

Membrane filters have a wide range of applications.

They include the following:

• water treatment (ultra-pure water, demineralized water
• pharmaceutical products
• food
• beverages (wine, beer, spirits, sparkling wine, mineral water, etc.)
• cosmetics
• high-purity chemicals
• photoresist
• ink
• as a terminal filter

A split tube element – often designed as a split sieve or wedge wire element – has defined openings between profiled wires or components. It is mechanically robust, easy to clean and suitable for various solid-liquid separation tasks. The material, slit width, geometry and flow direction are adapted to the application.

Split tube elements are used when solids need to be separated from liquids and a robust, mechanically resilient filter structure is required. They are particularly suitable for industrial applications involving a comparatively high solids load. When handled correctly, split tube elements are wear-free and easy to clean.

Split tube elements are made from 1.4404 as standard, but may also be made from other stainless steels such as Hastelloy ⁽¹⁾, Monel ⁽²⁾, duplex or super duplex, depending on the application. 

⁽¹⁾ Registered trademark of Hynes International
⁽²⁾ Registered trademark of inco Alloys

The choice of materials depends on the medium to be filtered, as well as on pressure, temperature and corrosion requirements.

Split tube elements are used to separate solid particles from liquids. Typical applications include industrial process media, high-viscosity media and other liquids.

The slit width refers to the distance between the profiles of a split or wedge wire filter element. It plays a key role in determining which particle sizes can be retained.

Wedge wire filters (split tube filters) are characterised by their simple and rapid filtration process. Their design allows for manual or automatic cleaning without interrupting the filtration process and with minimal product loss during solids discharge. 

The liquid flows through a split or sieve element, while solids are retained on its surface. A scraper cleans the surface through relative movement; the dislodged solids accumulate in the filter sump and are discharged either manually or automatically. 

Wedge wire filters are self-cleaning filters designed to reliably separate solids from liquids and highly viscous media. A key advantage is that they do not require the use of conventional disposable filter elements and enable the filtration process to continue without interruption. Wedge wire filters operate with virtually no wear.

Not necessarily. 'Split tube' primarily describes the design of the filter element; 'wedge wire filter' usually refers to a filter system whose surface is cleaned by scrapers or rakes. 

Depending on their design, wedge wire filters can be cleaned either manually or automatically. The dirt particles are discharged either manually via a valve on the housing or automatically. In fully automatic backwash filters, the contaminant particles are flushed out via a backwash mechanism in the filter housing and discharged as a result of the differential pressure.

They are frequently used for liquids containing a continuous suspension of solids, as well as for viscous media such as e.g.:

• paints and lacquers
• emulsion paints
• printing inks underbody protection
• tar products
• cooling lubricants
• adhesives
• solvents
• transmission oil, rolling oil, rapeseed oil
• emulsions for electrophoretic coating
• chocolate mass
• yeast
• plasticizer
• polyurethane

The control system monitors and automates the filter cleaning process. For example, it can initiate backflushing based on a set time, a differential pressure or another trigger signal.

Retrofitting is generally possible with suitable wedge wire filters, but depends on the existing filter design and the available connections. Before carrying out a retrofit, the filter type, actuator, differential pressure measurement and the required control function should be checked.

A backflush filter cleans its filter element while it is installed by deliberately reversing or redirecting the flow. The dislodged solids are discharged via a flush or sludge discharge system. The exact procedure and the required flushing pressure depend on the design.

During normal filtration, the solids are retained by the filter element. As the level of contamination increases, a backflush cycle is triggered, during which the filter element is flushed at high pressure in the opposite direction of flow at specific points, and the contaminants are removed from the filter.

Automatic filters can clean themselves, thereby reducing the need for manual maintenance and filter element replacement. They are particularly suitable for continuous processes involving high flow rates or a regular load of solids. Cleaning takes place without interrupting operation, with part of the liquid flow being diverted to clean the filter element.

A backflush filter is particularly useful when large volumes of liquid are being filtered continuously and solids are regularly being trapped. Thanks to the automatic cleaning system, the filter can continue to operate without the need for manual intervention or staff input.

Automatic filters are particularly suitable for continuous processes involving high flow rates and a regular influx of contaminants. Automatic cleaning reduces maintenance requirements and can improve the operational reliability of the filtration system.

With a disposable filter, the contaminated filter element is removed and replaced with a new one. A backflushable filter, on the other hand, cleans its filter element while it is installed and can therefore be operated for a longer period of time.