Secure support: Stainless steel filter for purity and hygiene

Filters and filter media made of stainless steel are indispensable in private, industrial, and large-scale technical environments. © WZV/Bopp

High flow rates and long service life characterize stainless steel filter elements like this filter cone. © © WZV/Bopp

Steel filter discs made of stainless steel ensure, in addition to the necessary corrosion resistance, the required mechanical robustness and wear resistance. © HAVER & BOECKER OHG

Filter presses are equipped with packages of frames and filter plates – often made of stainless steel – depending on the process and product. © WZV/MSE

Filter discs, reusable filter plates, and filter segments made of stainless steel, stainless, assume central functions in many industrial processes. © GKD

Filter fabric made of stainless steel, rust-free, is indispensable for the required process and product safety. © WZV/Bopp

The near-complete recyclability of stainless steel at the end of the lifespan of stainless steel filter elements contributes to improving the ecological footprint of companies and their products. © HAVER & BOECKER OHG

The range of filter candles made of stainless steel, which are used in various devices and components, reflects the complex spectrum of requirements. © WZV/Bopp

Disc filters made of stainless steel ensure, in addition to the necessary corrosion resistance, the required mechanical robustness and wear resistance. © GKD

Filters and filter media made of stainless steel are indispensable in the private, industrial, and large-scale technical environments. Whether for the filtration of steam, liquids, or gases: especially in hygiene-sensitive processes in the chemical, medical technology, food, pharmaceutical, water, or electronics industries, the non-corrosive material is essential for the required process and product safety.

In a wide variety of production processes, solid-liquid separation is fundamental. The associated challenges are as diverse as the procedures and filters used. The performance of filters is demonstrated by the retention rate – and here especially by the separation efficiency in challenging particle distributions – flow rate, differential pressure, and change interval. When optimally designed for the respective application, besides the micron class that defines the particle size the filter is designed for, the efficiency achieved with nominal and absolute filter fineness is decisive. While the efficiency with nominal filter fineness can only be 60 percent, it is 99 percent or more with specified absolute filter fineness.

Furthermore, a distinction is made between surface and depth filtration. In surface filtration with sieve, candle, or cross-flow filters, particles are retained on the surface of the filter medium. While sieve filters, for example, are used in clarifiers or power plants for pre-filtration and retention of larger particles, candle filters with filter media with pore sizes between 10 µm and 0.1 µm are used, for example, for microfiltration in beverage production. In cross-flow filters, the suspension to be filtered flows over the filter medium at high speed, preventing the formation of a filter cake. In depth filtration using candle or sedimentation filters, the filtration process takes place inside the filter medium. While a filter candle designed for depth filtration retains particles within the candle without forming a filter cake, sedimentation filtration requires the targeted buildup of a filter cake by adding a filter aid such as diatomaceous earth or perlite. A combination of these two filtration types is called cake filtration. Here, a filter cake builds up on the filter medium during the process, which retains ever finer particles as it thickens.

Wide range of systems and products

In industrial production processes, various system types are used to remove unwanted contaminants from liquids: These include filter presses, screw centrifuges – called decanters – as well as filter housings for filter discs or candles. Filter presses are equipped with packages of frames and filter plates – often made of stainless steel – depending on the process and product. Unlike filter presses, decanters operate continuously and are particularly used in solid-liquid separation with high solid content. They separate fine solids from the suspension using centrifugal force and transport the sediment accumulated on their wall with a screw to the discharge. All product-contacting components of decanters are usually made of high-quality, acid-resistant stainless steels.

A broad application field for disc filters, equipped with filter packs of discs with filter fabric in frame constructions, is the mechanical wastewater treatment in large-scale processing plants for process and service water. Here, disc filters and filter media made of stainless steel of grade 1.4404 are weldable and ensure, in addition to the necessary corrosion resistance, the required mechanical robustness and wear resistance. High flow rates and long service life qualify them, for example, also for plant oil, starch, sugar, beer, and wine filtration.

Filter candles made of stainless steel are resistant to corrosion and also withstand many acids, alkalis, ethers, and alcohols. They are therefore common filter elements in the food and pharmaceutical industries as well as in water management or gas filtration. For ballast water filtration in highly corrosive seawater environments, candle or disc filters made of duplex (1.4462) and super duplex steels (1.4410) are used. Due to their high temperature resistance up to 600 °C, stainless steel filter candles significantly increase process efficiency in hot gas filtration. An example of this is flue gas filtration in biogas plants.

Non-corrosive steels of material grades 1.4301 and 1.4404 meet the strict requirements of the food, pharmaceutical, and life sciences industries for filter housings with stacked, process-specific modules or filter candles for solid-liquid separation. To comply with the guidelines of the European Hygienic Engineering and Design Group (EHEDG) applicable in the food industry, housings and filter elements must meet strict design and construction standards for easy cleaning and disinfection. In addition to using process-specific selected stainless steel and self-cleaning Clean-in-Place (CIP) systems, corners and edges must be designed according to EHEDG guidelines to avoid dead zones and ensure seamless batch changes. Furthermore, product-contacted electropolished surfaces and radii with roughness values between RA 0.1 µm and RA 0.6 µm are required.

Complex requirements for filter media used

The range of filter media made of non-corrosive steels used in various devices and components reflects the complex spectrum of industry requirements: In addition to fabrics and metal fiber webs, these include fabric laminates, slit sieves, perforated plates, and wire meshes. Stainless steel fabrics and fiber webs are particularly in demand due to their flexible shaping possibilities and easier regeneration, especially in critical processes and installation conditions, as well as under high load conditions such as high flow rates or frequent backflushing. Pleated constructions prove their worth in many applications by providing a larger filter surface and high dirt-holding capacity at the same time as low differential pressure. Regenerable pleated stainless steel filters made of acid-resistant, austenitic material 1.4435 are used, for example, in industrial vapor cleaning. This material features an increased molybdenum content, offering even higher corrosion resistance than chrome-nickel steels 1.4301 and 1.4404. It withstands reducing organic and inorganic acids as well as halogen-containing media even better. Additionally, it is less susceptible to pitting and intergranular corrosion. This allows the use of 1.4435 at temperatures up to 450 °C in continuous operation.

Indispensable in many industries

Process steam coming directly from the boiler must be cleaned by filtration before contact with the final product. In the food industry, steam is indispensable for pasteurization, sterilization of products, and decontamination of production facilities. The so-called culinary steam suitable for direct contact with food is process steam that is cleaned with a fine stainless steel filter with a typical pore size of five microns and then complies with standard 3A 609-03. Even stricter purity requirements apply to pure steam – the highest steam class for applications in the food and beverage industry – and for pure steam used specifically in the pharmaceutical industry. Pure steam is produced from highly pure water and filtered with a stainless steel filter with a fineness of one micron according to pharmaceutical standard 3A 609-01, for example, to remove possible metal particles from pipes. Also, in ventilation systems for hygiene and cleanroom applications, filters made of stainless steel with high indoor air quality contribute to process and product safety by removing particles and microorganisms (fungal spores, bacteria, and viruses). They thus prevent biological contamination and premature spoilage of products during manufacturing, ensuring stable production processes.

Filtration is an omnipresent process in the chemical and pharmaceutical industries to ensure the purity of products – filter cakes, filtrate, or chemicals – and to prevent production failures. Whether for filtering liquid process media, cleaning raw, process, cooling, or wastewater: stainless steel filters and filter media reliably meet the strictest requirements regarding precision, permeability, dirt-holding capacity, cleanability, and robustness. Also, in the petrochemical industry for crude oil and natural gas production, in the energy sector – for cooling water filtration in power plants, thermal oil filtration in solar thermal power plants, or hydrogen production via electrolysis – stainless steel filters are a guarantee for reliable processes, protection of the installed equipment, and the required quality of end products.

Many more examples could illustrate the importance and application spectrum of filters made of stainless steel. In industries such as electronics, semiconductors, biotechnology, plastics, automotive, or metalworking, they also perform central tasks. Whether filtering ink colors in inkjet printers, EMI shielding, cell culture media preparation, formulation and filling, extrusion of plastic melts, cleaning and distribution of fuel, gas, or hydraulic fluids, or conditioning process media: countless critical processes require reliable filter systems and media. Filter solutions made of stainless steel meet the strictest standards for hygiene, purity, and durability. Corrosion, temperature, and chemical resistance, strength, and weldability are prerequisites for long service life and process and product safety. The almost complete recyclability of stainless steel at the end of the lifespan of filter elements and media from non-corrosive steel additionally contributes to improving the ecological footprint of companies and their products.