Cartridge filters are difficult to classify based on a definition. However, there are some distinct qualities of this filter, which include a cylindrical housing that is usually succumbed to pressure of the fluid being filtered. Within this is housed a replaceable filter element, commonly called the cartridge. They are generally made of a standard size to fit into a standard housing. More commonly, the cartridge filter is set up as a unit, wherein a single housing contains one single or multiple cartridges according to the blow requirements. Sometimes, however, when continuity of fluid flow is a requirement, a duplex system is deployed, with two cartridge filters that run parallel to each other. In this case, when one is being cleaned, the other runs for uninterrupted flow and vice versa.
Uses and Construction of Cartridge Filters:
These filters are primarily used for the purpose of clarification, which is the extraction of contamination from a fluid, which is generally liquid. The contamination levels ideal for cartridge filters should be less than 0.01% in weight. The filter is available in versatile types and filter mediums, but they can be broadly classified into two types, based on their construction models. The first type is the one made up of an integral unit of porous filter that is moulded into the desired cylindrical shape. The second one comprises of different sets of components that may not originally be porous, but transform into porosity when combined together.
On the other hand, there are a couple of replaceable element filters that may not fit the precise definition of a cartridge filter, but are included within this bracket due to the similarity of their function – namely the capsule filters. These are used majorly in laboratories and small production houses within the pharmaceutical and life science industries.
The filtration performance of cartridge filters is between 0.2 mm to 100mm. Until very recently, surface filters were graded on an absolute basis and depth filters on a nominal basis with levels of filtration efficiency. However, now, there is a rating basis called the absolute-rated depth filter media. Subsequently, there is no universally established system that can determine the removal ratings of cartridge filters in liquid mediums. The current most commonly used filter rating method that works with lubricating and hydraulic liquids, which is been adopted by a number of manufacturers is the OSUF-Z test developed at Oklahoma State University. While nominal filtration is described in percentage terms between 80%-90%, absolute rating ranges between 98% and 99.99% efficiency.
Types of Cartridge Filters:
Integral Media Cartridge:
These are cartridges that consist of a single piece of filter are created from a perforated cylindrical core that is made up of metal or still plastic, on which the material of the filter medium is fitted.
1. Thin Media :
Thin media are made up of any material that can be produced in a sheet form. However, a flat sheet that is wrapped around a central core could not make up an efficient filter element since the ratio of filtration area to housing volume would be minimal. This can be increased through the process of pleating the flat sheet in concertina folds that run parallel to the element’s axis. Any material that can withstand the stress of pleating, without cracking, can be used to manufacture pleated media cartridges. These are, therefore, available in paper (cellulose and glass), woven fabrics (mono and multifilament), thin felts, fleeces of nonwoven plastics, spun-bonded and melt-blown, and woven wire mesh, both plain and sintered. They can further be made with sheet membranes, with special attention paid to avoid cracking.
This is the type of cartridge that is pleated with a protective mesh, made up of wire or plastic, and is placed within a protective cylindrical perforated plate screen that is outside of its pleats. The end-caps of the cartridge are sealed along the top and bottom of the pleats in a coherent fashion. With every pleated medium, and especially one with cylindrical pleats around the central core, there is an optimal amount of pleats per unit length of core circumference that could be installed without compromising the parts closest to the core. This compromise occurs, either due to the pinching of the inter-pleat gaps or due to the increase in pressure drop to reach the bottom pleat. These factors then negate the whole purpose behind the pleating for extra filtration. Therefore, it is sometimes mandatory to interleave the pleats with strips of corrugated material at right angles that keep the pleats apart. Another means is by keeping the pleats at right angles to the axis of the cartridge and the finished element is made up from polypropylene with 60% more filtration area than a depth filter cartridge of the same volume.
These cartridges are designed to filter through the surface and sometimes, for this purpose, a thin cake is permitted to form atop the medium surface, but ensured that it does not interfere with the space between the pleats. The finite thickness of the medium allows for some amount of penetration that can be periodically cleaned through back flushing or routine cleaning.
2. Thick Media :
These cartridges also work through depth filtration, but their contaminant loading capacity is their distinctive usability feature. Thick media will be manufactured in a manner that the density is always increased toward the centre of the cartridge, which eventually improves the quality of depth filtration. Upon reaching its maximum load, the cartridge must be discarded. In rare cases a considerable amount of contaminants can be expelled through back flushing. There are multiple ways in which the thick media cartridges are formed. If the cartridge is sufficiently flexible, the medium is designed as a flat sheet, cut to size and wrapped around the core, with the sealing of the two edges together. The other way is where the medium is designed as a sleeve that is cut length-wise and placed over the core. Finally, there is a third type wherein the medium is placed on the core with the required thickness and then manufactured as required to provide the extent of filtration.
Irrelevant of the manufacturing style, the resulting cartridge will take on a cylindrical form of porous material, which will be sealed at two ends, and housed according to size. The thick medium can be made of almost every kind of material, such as, un-bonded fibre, felt, needle felt, woven multi-filament yarns of natural or synthetic fibre, wound as a sheet with several turns on the core to give a porous medium of the required thickness, resin (i.e. adhesive) bonded fibre, moulded to shape, in a mass that will need to be cured (to set the resin), employing fibres of cellulose, glass or plastic, thermally bonded (i.e. sintered) fibre, of plastic, metal, glass or ceramic, resin-bonded plastic granules, thermally bonded or sintered granules of plastic, metal or ceramic, and foams of metal or ceramic.
When the extruded plastic, whether it is spun-bonded, melt-blown or electro-spun, is dry-laid directly on the rotating cylindrical core, this makes the medium thicker and forms a distinctive group of polymeric fibre or filament media. This allows the fibre density to be varying in accordance with the depth of the fibre layer that provides a coarse format at the surface and the finer pores are toward the centre.
3. Sintered Media :
Sintered media, which mean thermally bonded materials, are usually designed in tubular shapes through the process of moulding or isostatic pressing, before sintering. These shapes end up being self-supportive that erases the need for a core. The only exception is when it is required in the forming process. The formed shapes appear more like candles than cartridges, however due to the single element in the housing being a cartridge it is procured under this category.
The porous plastic is sintered in a special manner through the use of high molecular weight thermo polymer powder. Special attention is paid to the selection of cryogenically ground powder that provides a particle retention range of 5-200micron as the pore size and filtration characteristic. Through a special design mechanism, the elements are made into individual moulds with variant shapes. These plastic elements are deployed for use in compressed air filtration or environmental use for temperatures below 80 degrees C.
Metal filters that are sintered create closer control of pore size (that can range from sub micrometer dimensions up to as much as 1micron), allowing for uniformity in shape and a resulting matrix that is strong, rigid and highly resistant to heat. In theory, a cut-off that is down to the finest filtering prerequisite can be provided through the use of a sintered metal powder element. The practical aspect, however, modifies this range of application through the increasing resistance to flow with diminishing pore size that usually decreases porosity. To maintain porosity and strength, there must be a reasonable balance between strength and permeability.
For filter elements, the porosity can be as high as 70% for low pressure drop elements. One of the biggest advantages of sintered metal filter elements is the high amount of strength and rigidity that makes them ideal for high pressure applications. They fall into two broad categories: sintering undertaken through loose powder in a mold and the ones manufactured through compaction. They are usually designed in spherical shapes that create more uniformity in pore sizes and are easier to produce and classify through spray automation and sieving. The coarser types of sintered metal filters are made from particles of 1mm diameters, which would yield a pore size of 150m in the material. However, these types of sintered metal filters are expensive to produce and hence not very practical. The ones with a pore size below 100m are more popularly used.Usage-wise, bronze filters are used for general purposes and for arduous duties with high pressure and temperature the use of stainless steel, Monel, pure nickel, hastily, titanium, and tungsten s best. Bronze and cupro-nickel sinter at low temperatures and can be used to create various shapes from metal powder in stainless steel or carbon moulds.
The elements of the sintered metal may be machined for higher tolerance in direct moulding, but machining should not be undertaken on the main areas of the filter element. A better option would be isostatic pressing, which can be done to produce a wider variety of shapes and sizes. Stainless steel elements are well-suited for higher strength, higher resistance to temperature and corrosion, and a widely used for plates and discs of filter element construction. When the solid metal is exposed to an attack, the corrosion of the porous metal will be higher due to the larger surface area exposure. Fine filtration of sintered metal elements, and the controlled level of porosity, guarantees a true absolute rating, which makes this filtration a popular choice for high power applications.
Sintered metal cartridges are also available in forms that are made from a combination of metal fibre and wire mesh, wherein both properties provide the balance and the wire mesh ensures the precision of the apertures. Wire meshes have the ability to be laminated and sintered to transform into a thick medium that offers precise variations in pore size. The sintered fibre elements contribute by providing higher flow rates and a higher dirt holding capacity and they can also be made thin to pleat. Finally, they can be easily cleaned in the traditional ways through back flushing or chemical cleaning.
Ceramic powders can also be used as sintered filters to produce an array of porous shapes that can be used as filter elements. Since early times, porous pottery was used as one of the earliest materials for filtration. Porous ceramic filters, as cartridges, are designed in a plain cylindrical form with a thick wall that provides the depth of the filter medium for retention of solids in the filtration process. Tubular elements, on the other hand, are plain cylinders or flanged candles. The size of sintered ceramic elements varies drastically, operating temperatures range between 900-1000 degrees C, pore size of 100m to 1mm, and average porosities of 35-45%. Ceramics, further, have a chemical resistance of 1-9 pH range.
4. Capsule Filters :
Small in-line capsule filters, because of being used until full of collected solids and then cleaned or discarded, are primarily used in laboratories and small-scale production processes of the life-science industries. They are entirely made of a polymer that is adaptable to the operating conditions, and take the shape of short cylindrical housings with the same length as the order of the diameter. They are closed at both ends, with the exception of a nipple or nozzle that can be attached to a plastic flow line. The capsule is housed inside the flow line, the process is undertaken, then the filter is removed and either discarded or the solids are emptied and retained, depending on the purpose of the filter.
5. Polymer Melt Filtration :
Quite different in construction and operating, but somewhat similar in operation, the polymer felt filtration is among the many filters whose purpose is to extract contaminants from molten polymer. When it comes to the filtering of plastic metals, it is quite challenging and has been overall unsatisfactory. Therefore, the filter media used for this task has been of polymer melt filtration through discs of sintered powder, fibre or mesh. To overcome the production interruptions, increasing costs and lengthy cleaning processes, continuous screen-changer disc filters have been created, wherein the filter disc rotates between two plates. This rotational technology has served as an ideal solution for the filtration of high or low viscosity plastic melts, such as thermoplastics, polycarbonates, PPS, PEK and PVC. Higher melt pressures and temperatures can be withstood and screen surfaces range from 8-500 cms in area.
6. Foams :
Foam is the subsequent creation of a mass of gas bubbles being evenly distributed throughout a liquid, in a manner where the gas occupies majority of the final volume, and is enveloped by the liquid in the walls of the bubbles. If this liquid then turns solid, a highly porous mass is created that makes a high quality filter medium. Due to the lack of interconnectedness of the bubbles in the frozen state, no fluid flow is possible through the foam. The solution to this is the reticulation of the foam through thermal and chemical processing that makes the material in the bubble walls retracts to the nodes of the original bubble network that produces a porous mask with interconnected pores. The pore size is determined by the original bubble size, and quite a considerable variation in this size is possible.
Through this, thick media cartridges can be prepared by fixing a layer of plastic foam to a perforated core. Another use is the formation of coarse ceramic foams, such as discs, which can be used as filters in the extraction of contaminants from molten metal.
This is the final group of surface filter cartridges and is made up of individual components that, when combined together in a cylindrical shape, create an externalize surface with a number of slots, the width of which defines the pore size of the filter. The simplest of such an assemblage is a set of circular discs that are stacked on top of each other around a central base that is hollow and perforated. Every disk has a few pimples on one side, so that upon clamping, there is a thin slot at the periphery between each adjacent pair of plates, made of plastic or metal. The cartridge is then housed on a cylindrical positioned that allows the plates to be held together, with a flow of liquid through the stack of discs toward the filtrate collection pipe.
Contaminants are extracted through a very precise dimension, which is set by the slot width. With the exception of larger particles that can get through. The same ‘metal edge’ effect is product through the use of a spring that operated in a continuous flat ribbon wound in a helical shape. The pimples on one side, the spring fully compressed in the cartridge housing, and one long continuous slot formed, offers the same precise cut-point. If the compression of the spring is loosened, however, the adjacent turns move and the accumulated dirt can be dislodged from the outside of the stacked ribbon and also from the turns.
In any case, the front of the wedge is put on the outer section of the cylinder to create the surface filtration. These discs are then replicated in another way where the discs are circles of paper forming the full diameter of the final element. Between each disc is another disc of paper of smaller diameter and high porosity through which the filtrate can flow, leaving behind any contaminant on the outside of the stack and blocked in the annular spaces between the main discs.
The spiral effect of this ribbon is displayed into the wire wound cartridge where a wire is wound in a close spiral around the core to form a filter medium that is working as a surface strainer. The wires are placed in grooves by turns, machined on the core, to create a precise slot between adjacent turns of the wire. These wires are made of metal or a single filament of polymer. This spiral is a property of the yarn-wound cartridge, wherein a multi-filament yarn is wound in a spiral fashion around a coarsely perforated core. Above 20 layers of yarn are put on the core, with regular turns at a distinct angle to the circumference of the forming cylinder, and with this angle reversed in successive layers. This results in the creation of a mass of diamond-shaped apertures and a tough path for the liquid in its flow through the wrapped yarn.
It works as an ideal depth filter, with multiple levels of filtration in the apertures and between the fibres in the yarn, which is made up of natural or synthetic fibres. The main advantage of this is compactness, with a high filtration area to cartridge volume ratio: 250mm long by 171 50mm diameter yarn-wound cartridge having the same filtration area as a surface filter of over 300mm diameter. It is among the first to be made in standard dimensions, fits into housings from multiple manufacturers, and is a distinct form of filter cartridge for a long period. Housings are available in plastic and metal, with lengths of 250, 500, 750, and 1000mm. These cartridges are created with a pore size that ranges from 1 to 100m and with coarse pore sizes that are capable of filtering down to lower limits due to depth filter action.
A much more varied filtration medium is created as another stacked disc filter. These discs are flat and held together through compressing springs. Within the flat surfaces of each disc are machined groove sets with triangular cross sections. The grooves run from the centre of the disc to its periphery, but at an angle to the radius. When the discs are clamped together, a large number of groove intersections appear. The irregular path of moving through a groove, then intersection, choosing which grooves to follow, and sharp turns in directions, causes the solid particles to be trapped at the regular intersections. The groove or radius angle determines the number of intersections. When the stack is full, the pressure holding the discs is relaxed, and then discs are then spun by incoming jets of liquid to release the trapped solids. These filters are used to strain processed and working fluids.
These are constructed cartridges, varied from others in the manner that the components of a single element are empowered to undertake filtration individually. This disc is a circular element made up of dual discs and joined outside the edge. They are separated at the centre through a filtrate off-take tube, which provides the disc with the shape of a lens. Even though the discs can be used independently, they are usually housed together, one on top of the other, to create a common filtrate pipe. Thereafter they are placed in a cylindrical housing as a cartridge.
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