Filter materials

Filters of poly(phenylene sulfide) (PPS) fibers or fluorine resin fibers are widely used as dust filters for city garbage incinerators or coal boilers. The plants run at increasingly high inner temperatures for reducing the amount of dioxin that may be generated in these plants. Unfortunately, the particle size of dust generated becomes increasingly smaller with the increase of the inner temperatures of incinerators. Therefore, the filters 

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Bag filters. Bag filters, as discussed in Chap. 3 and illustrated in Fig. 3.66, are probably the most common method of separating particulate materials from gases. A cloth or felt filter material is used that is impervious to the particles. Bag filters are suitable for use in very high dust load conditions. They have an extremely high efficiency, but they suflFer from the disadvantage that the pressure drop across them may be high. ... 

To verify the modelling of the data eolleetion process, calculations of SAT 4, in the entrance window of the XRII was compared to measurements of RNR p oj in stored data as function of tube potential. The images object was a steel cylinder 5-mm) with a glass rod 1-mm) as defect. X-ray spectra were filtered with 0.6-mm copper. Tube current and exposure time were varied so that the signal beside the object. So, was kept constant for all tube potentials. Figure 8 shows measured and simulated SNR oproj, where both point out 100 kV as the tube potential that gives a maximum. Due to overestimation of the noise in calculations the maximum in the simulated values are normalised to the maximum in the measured values. Once the model was verified it was used to calculate optimal choice of filter materials and tube potentials, see figure 9. 

Extra coarse 170-220 Filtration of very coarse materials. Gas dispersion, gas washing, and extractor beds. Support of other filter materials. 

Size reference Filter Material Vims (size) Fluid Log titer reduction Reference... 

Depending on the chemical nature of the slurry to be filtered, materials of constmction include wood, plastics, earthenware, steel, lead-lined steel, and brick-lined cast iron. 

The primary advantages of this filter are its simple design and low maintenance costs. The main disadvantage is the difficulty of handling very fast-filtering materials on a large scale. Areas up to 93 m are available. 

The white cell adsorption filter layer is typically of a nonwoven fiber design. The biomaterials of the fiber media are surface modified to obtain an optimal avidity and selectivity for the different blood cells. Materials used include polyesters, eg, poly(ethylene terephthalate) and poly(butylene terephthalate), cellulose acetate, methacrylate, polyamides, and polyacrylonitrile. Filter materials are not cell specific and do not provide for specific filtration of lymphocytes out of the blood product rather than all leukocytes. 

Lipoproteins may denature on heating and if present during pasteurization can result in the formation of haze or turbidity in the final product. This material was removed traditionally by filtration through asbestos (qv) sheets (6) however, health hazards associated with asbestos have led to its replacement by alternative filter materials (23,37,193). These media have been less effective than asbestos and further measures have been required to ensure the visual clarity of albumin products, eg, further filtration developments for Hpid removal (194), preferential denaturation of contaminants using in-process heat treatment, and anion-exchange chromatography (49). 

Some additives have the ability to lower the pour point without lowering the cloud point. A number of laboratory scale flow tests have been developed to provide a better prediction of cold temperature operability. They include the cold filter plugging point (CFPP), used primarily in Europe, and the low temperature flow test (LTFT), used primarily in the United States. Both tests measure flow through filter materials under controlled conditions of temperature, pressure, etc, and are better predictors of cold temperature performance than either cloud or pour point for addithed fuels. 

Similar deposits of radiolaria or diatoms composed of siUceous skeletal remains are widespread in more temperate areas in deep water below 5000 m. The deposits maybe very pure. The diatoms recovered from deposits on land are used as fillers or filter materials or as a source of high quaUty carbonate or sihca (see Diatomite). 

Energy losses become high, and short circuiting passage of dirty water. Sand, mixed media, and diatomaceous earth are the most common filter materials. 

Large quantities of secondary cellulose acetate are used worldwide in the manufacture of filter material for cigarettes. Because of its excellent clarity and ease of processing, cellulose acetate film is widely used in display packaging and extmded plastic film for decorative signs (see Packaging materials). Injection-molded plastics of cellulose acetate are used in toothbmsh handles, computer bmshes, and a large variety of other appHcations (7). 

A filter removes particulate matter from the carrying gas stream because the particulate impinges on and then adheres to the filter material. As time passes, the deposit of particulate matter becomes greater and the deposit itself then acts as a filtering medium. When the deposit becomes so heavy that the pressure necessary to force the gas through the filter becomes excessive, or the flow reduction severely impairs the process, the filter must either be replaced or cleaned. 

The pressure drop through the filter is a function of two separate effects. The clean filter has some initial pressure drop. This is a function of filter material, depth of the filter, the superficial gas velocity, which is the gas velocity perpendicular to the filter face, and the viscosity of the gas. Added to the clean filter resistance is the resistance that occurs when the adhering particles form a cake on the filter surface. This cake increases in thickness as approximately a linear function of time, and the pressure difference necessary to cause the same gas flow also becomes a linear function with time. Usually, the pressure available at the filter is limited so that as the cake builds up the flow decreases. Filter cleaning can be based, therefore, on (1) increased pressure drop across the filter, (2) decreased volume of gas flow, or (3) time elapsed since the last cleaning. 

The main principles of instrument design are summarized in Table 10.23. In filtration, e.g. for gravimetric analysis, selection of filter material (Table 10.22) requires careful consideration in terms of application, strength, collection efficiency, compatibility with pump, water uptake, etc. Humidity-controlled balance rooms, iTiicrobalances and careful handling techniques may be required. 

Table 10.22 Examples of filter material for collection of particulates...

Filtration operations are capable of handling suspensions of varying characteristics ranging from granular, incompressible, free-filtering materials to slime-like compositions, as well as finely divided colloidal suspensions in which the cakes are incompressible. These latter materials tend to contaminate or foul the filter medium. The interaction between the particles in suspension and the filter medium determines to a large extent the specific mechanisms responsible for filtration. 

In slow filtration, much of the effect is obtained by the formation of a filtration layer, including the substances that are extracted from the water. At the early stages of the operation, these substances contain microorganisms able to effect, beyond the filtration, biochemical degradation of the organic matter. This effect also depends on the total surface of the grains forming the filter material. The probability of contact between the undesirable constituents of the water and the surface of the filter medium increases in proportion to the size of the total surface... 

Such care includes washing of the filter material with the spray jets after every period of use, removal of grease and fats with warm soap solution if clogged, treatment with diluted hydrochloric acid for removal of lime encrustations, maintenance of scraper bade in careful adjustment to filter drum to prevent tearing of the filter material. 

The basic filter material is produced from glass fiber or synthetic plastic fibers (polyester, acrylic, polyamide). Separation is mainly of particles 5 pm and larger. 

As the filter accumulates dust, the pressure loss increases, and the dust removed improves the normal separation. Another effect can be seen with electrostatically charged filter material. During operation, the impurities neutralize the material, and the filter s capacity to separate is reduced. Figure 9.3" shows exam-... 

The lifetime of a filter is dependent on the concentration of dust, type of dust, airflow, and, of course, the selected final pressure loss. Filter material and filter construction are often a compromise or combination of filter effects and installation space. Low speed or large filter surface promotes efficiency, low pressure loss, but above all a longer lifetime. 

To prevent the surrounding air from mixing with the supply air, the air shower should use air-permeable filter material with a harder, load-bearing, nonflammable outer shell and an inner layer of softer material with a high air resistance. This design requires that the supplied air be filtered. 

Panel Any type of filter material mounted in a rigid frame. The frame must allow only the absolute minimum of particulates to pass between the frame and the sides of the filter material. 

Effective inlet air filtration is required to ensure satisfactory operation of the engine. The location of the unit determines the most appropriate filter system to use. Desert environments where a large amount of sand particles could be expected in the ambient air may use an automatic roll type of filter that allows new filter material to be rolled in front of the inlet without frequent shut-downs to change filters. Arctic or extremely cold locations may use pad type filters, snow hoods to prevent blockage, and exhaust recirculation to prevent icing. Filter assemblies for offshore marine environments may include weather louvers, demister pads, and barrier elements for salt and dirt removal. Screens may be u.sed for insect removal prior to filtration in areas with bug problems. 

Filter elements may be divided into two classes surface and depth. Surface filters are made of closely woven fabric or treated paper with a uniform pore size. Fluid flows through the pores of the filter material and contaminants are stropped on the filter s surface. This type of filter element is designed to prevent the passage of a high percentage of solids of a specific size. 

As previously indicated, most bag-type filters require a pre-coat of particulates before they can effectively remove airborne contaminates. However, particles can completely block air flow if the filter material becomes overloaded. Therefore, the primary operating criterion is to maintain the efficiency of the filter media by controlling the cleaning frequency. 

Increased dust-holding capacity can be obtained by making the filter material in a series of bags, which are normally about 400 mm deep, but also made up to 900 mm where maximum retaining capacity is required. Some bag filters are shown in Figure 27.15. 

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