Filtration with woven fabrics

In the case of nonwoven fabrics, a reduction in pore size is achieved by compressing the fibres into a more dense structure (loads up to 300decaNm" may be necessary) and, by selection of the appropriate conditions, a more durable (Fig. 3.31) surface can also be obtained through partial fusion of the surface fibres. With woven fabrics, on the other hand, some deformation of the yams may be necessary to achieve the optimum filtration properties. This is particularly graphic in the case of fabrics woven from monofilament yams, as shown in Fig. 3.32 and Fig. 3.33. 

Compared with woven fabrics, needlefelts provide many more readily accessible pores per unit area, and hence the potential for greater rates of filtration. It should be noted that needlefelts have only limited use in liquid filtration largely due to their instability and tendency to bUnd. 

The frequency of cleaning depends on the nature and concentration of the dust. Typical cleaning intervals vary from about 2 to 15 minutes. However, the cleaning action of the pulse is so effective that the dust layer may be completely removed from the surface of the fabric. Consequently, the fabric itself must serve as the principal filter media for a substantial part of the filtration cycle, which decreases cleaning efficiency. Because of this, woven fabrics are unsuitable for use in these devices and felt-type fabrics are used instead. With felt filters, although the bulk of the dust is still removed, an adequate level of dust collection is provided by the fabric until the dust layer reforms. 

Asahi Medical Co. (Japan) developed Sepacell-PL filter as a device for removing white blood cells from whole blood [119, 120], Sepacell-PL is a non-woven fabric (PE or PET), coated with a copolymer of HEMA with die-thylaminoethyl methacrylate (e.g. in 5 mole%). It is noticeable that platelets in the blood can pass through the filter without suffering any damage during the filtration processes, owing to the presence of the amino group. 

Disc, pulp, and sheet filters accomplish extreme clarification. Not infrequently their mission is complete removal of particles above a stipulated cut size, which may be much less than 1 pm. They operate over a particle-size range of four to five orders of magnitude, contrasting with two orders of magnitude for most other filters. It is not surprising, therefore, that they involve a variety of kinds and grades of filter media, often in successive stages. In addition to packs or discs of cellulosic, polymeric, or asbestos fiber, sheets of pulp, paper, asbestos, carded fiber, woven fabrics, and porous cellophane or polymer are employed. Sandwich-pack composites of several materials have been used for viscous-dope filtration. 

The polyester fibers have high shrink properties, which make them useful in fiber applications where crimp retention or high bulk is desired. Typical applications are in carpet yams, hi-loft non-woven fabrics used as interlinings, cushioning media, and filtration media, as well as in specialty yams for weaving and knitting. The fibers have a lower melting temperature compared to PET, which makes them useful as binder fibers in non-wovens, particularly in combination with PET homopolymer fibers. 

Because they are capable of being produced in seamless tubular form, weft-knitted fabrics provide, in theory, an attractive and economic alternative to both woven and needled constructions. By inlaying appropriate yams into the knitted stracture, the elasticity normally associated with such fabrics can also be controlled and may be used to enhance the particle collection capability. On the down side, in critical appU-cations, the filtration efficiency will be inferior to a nonwoven constmction and further problems are likely to be found in respect of the large number of sleeve diameters which the industry requires. Limitations are also inevitable in respect of the number of physical and chemical finishes that are frequently administered to both woven and nonwoven constmctions. 

Although widely used in dust collection, nonwoven fabrics have found only limited use in liquid filtration because their thickness and density render them prone to blinding in many applications. One area where they have found some success, however, has been in the filtration of metal ore concentrates such as copper on horizontal vacuum belt filters. These applications tend to be very aggressive on the filter fabric, and hence a suitably designed and finished nonwoven fabric is often more cost effective than a considerably more expensive woven fabric, especially if required in lengths of around 80 m and widths up to 6 m. The solids which are captured in such applications quickly form a cake on the surface and, should some penetration occur, as with woollen spun yams, the bulky nature of the material provides scope for the particles to escape. For such arduous applications, nonwoven fabrics are generally in the area density range of 800-1000 gm- ... 

The size of a baghouse is primarily determined by the area of filter cloth required to filter the gases. The choice of a filtration velocity (or its equivalent, the air-to-cloth ratio, ACR, in cubic feet per minute of gas filtered per square foot of filter area) must take certain factors into consideration. The rule of thumb for ACR ratios for conventional fabric filter baghouses with woven cloth is 1.5-3.0 ft /min-ft. When dust is fine or loadings are... 

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