Fabric filters filtration mechanism

Typical new equipment design efficiencies are between 99 and 99.9%. Older existing equipment have a range of actual operating efficiencies of 95 to 99.9%. Several factors determine fabric filter collection efficiency. These include gas filtration velocity, particle characteristics, fabric characteristics, and cleaning mechanism. In general, collection efficiency increases with increasing filtration velocity and particle size. 

One of the oldest, simplest, and most efficient methods for removing solid particulate contaminants from gas streams is by filtration through fabric media. The fabric filter is capable of providing high collection efficiencies for particles as small as 0.5 pm and will remove a substantial quantity of particles as small as 0.01 pm. In its simplest form, the industrial fabric filter consists of a woven or felted fabric through which dust-laden gases are forced. A combination of factors results in the collection of particles on the fabric filters. When woven fabrics arc used, a dust cake eventually forms. This, in turn, acts predominantly as a sieving mechanism. When felted fabrics are used, the dust cake is minimal or nonc.xistent. 

This chapter will provide the reader with a general introduction to the more common types of solid-gas (dust collection) and solid-liquid filtration mechanisms. The raw materials, polymers, fibres, and different types of fabric construction employed in filter media manufacture and some typical fabric finishing processes are discussed in detail. A brief look at filter market developments will also be considered. 

Filtration, in the most general sense, may be defined as the removal of particles from the aerosol. This occurs either by their attachment to nonaerosol media (walls, vegetation, "fabric filters", etc.) or to larger particles which are subsequently removed. Since particle transport in the gas is intimately involved, a characterization of the gas flow field and the detailed mechanisms of particle kinetic theory near a surface must be invoked. Classically, filtration was treated as the simple adhesion of a single particle to a surface. However, it is now known that after the first particles adhere, subsequent ones tend to be captured by the initial ones to form chains. Impaction of a large particle upon such a chain or other break-off processes can cause resuspension. Thus, filtration is dependent upon properties of the aerosol and gas as a whole [1.9,10]. 

Wood pulp fibres are frequently less than 1 pm in fibre size and dominate wet-laid nonwoven filter media they are able to provide higher filtrafion efficiency and are environmentally fiiendly. Synthetic fibres (eg, polyolefin, polyester, and polycarbonate (PC) fibres) could work as electret fibres to provide additional electrostatic filtration mechanism, and also could be made binder-fitee, thermobond nonwoven fabrics if they are made Ifom thermoplastic polymers. 

Filtration mechanism of nonwoven fabrics and their filter efficiency... 

The choice of the filter medium is often the most important consideration to ensure efficient operation of a filter. Its function is generally to act as a support for the filter cake, while the initial layers of cake provide the actual filter. The filter medium should be selected primarily on the basis of its ability to retain solids without binding. It should be mechanically strong and corrosion resistant, and should offer as little resistance as possible to the flow of the filtrate. The media are made from widely different materials such as cotton, wool, linen, nylon, jute, silk, glass fiber, porous carbon, metals, rayon and other synthetics, and miscellaneous materials like porous rubber. Cotton fabrics are most commonly used because they are available in a wide variety of weaves, and are cheap. 

If impurities cannot be removed with bag filters, filter cartridges or candles are used (Fig. 7.14). The filter cartridges are inserted in individual housings or, in large-scale equipment, up to 24 cartridges are inserted side by side or one above the other and held in place with quick-release mechanisms. Resin-bonded, sintered synthetic micro fibres and metals are used as filter material. The fineness of the filter fabric (5-100 pm) determines the filtration effect. 

Cake filtration this is undoubtedly the most widely encountered mechanism in industrial filtration and involves the accnmulation of particles that bridge together in a porous structure on the surface of the fabric. It follows from this that, once formed, the cake effectively becomes the filter medium, with the fabric thereafter acting simply as a support. In cases where it is difficult for the particles to form a naturally porous cake, the use of a special precoat or body feed may be employed to assist in this task. 

In similar manner, excessive stretch caused by the mass of filter cakes in filter press operations could result in the port holes in the fabric moving out of alignment with the corresponding holes in the filter plate, thereby restricting the flow of filtrate out of the press. Likewise, in other filtration systems, such as pressure leaf filters, the same stretch could result in the formation of creases and ultimately mechanical damage to the fabric. 

There is a range of important characteristics required in order to ensure correct function of filtration media. There are the primary characteristics of the filtration media concerning the ability to remove particles classified by filter rating and also the fabric characteristics having a direct influence over the filter performance. Whilst the filtration media must remove a specified amount of debris, they must also be resistant to various contaminants and mechanical effects. 

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