Filtering with the help of fibrous filters
Fabric materials offer an effective means for the removal of dust from gas streams. The fabric is arranged in the equipment in such a way that the dust laden gas has to pass through the small openings of the fabric. Due to different physical phenomena the dust particles impinge on the surface of the fabric material and adhere to it.
Fig. 2.14 Fig. 2.15
Fig. 2.14 – Limestone particles adhering to a fiber; the diameter of the fiber is about 50 µm while the particle size ranges from 3 to 10 µm
Fig. 2.15 – Structure of needle felts which was exposed to tobacco smoke for a very short time
Fibres for filters can be used as unclose stuffing (as a fabric), or can be pressed as felt. Fibres can be the following kinds:
1) Metal. Properties: high corrosion stability and high porosity, they are successfully applied to filtering of very fine particles, such as particles of the catalyst.
2) Natural (cotton, flax, wool, silk, asbestos);
3) Chemical (polypropylene, polyether, polyamide, polyakrylnitryl, etc.). These fibres possess the higher resistance to chemical and thermal influence, than natural. With their help catch not only firm particles, but also drops of acid fog, as the surface of chemical fibres is hydrophobic and promotes flowing down of drops in a collector.
5) Glass. Thin glass fibres are differed the durability and flexibility even at high temperatures 150-3000, when properties almost of all natural and chemical fibres are worsened. Glass fabric possesses the excellent resistance to chemical influence and is not subjected to action of acids and alkalis. Even gaseous fluoric compounds can destroy glass only at the presence of a moisture, which is absent at working temperatures.
The choice of the certain material depends on the purpose of application. So, if concentration of particles is high (for example, waste gases of melting process) filters will be frequently exposed to clearing, therefore it is necessary to use the strong fabric, capable to maintain often washings. If concentration of particles low (for example, at filtering of air) clearing of the filter is made seldom. In this case the filtering layer from non-dansely packed fibrous material is used.
There are fibrous filters of special purpose:
1. The filters intended for work at temperatures from above 4000 (glass fiber on a steel grid);
2. Filters for catching of aerosols (drops flow down from filtering surfaces in the collector device);
3. The filters intended for deactivation of gaseous radioactive waste products (thick layers of glass and asbestine fibres).
4. Filters for air conditioning.
Modern filtering installations can process the great volumes of gases with high dust content and at high temperatures (up to 3500). The elementary filters represent the long filtering sleeves, which are suspended by lines and have been tied up from below. Bags are suspended inside a rectangular casing and symmetrize. Periodically sleeves are stirred up and released from firm particles.
Filtering sleeves used in filters are divided into two kinds: long round sleeves and flat filtering sleeves. Advantage of flat sleeves is that in smaller volume of a casing it is possible to place sleeves with the greater filtering surface. In filtering installations with round sleeves the stream of gases is directed, as a rule, from within to the external side of a sleeve. In case of application of flat sleeves the stream of gases is directed from lateral side of a sleeve inside of it.
When dusty gases pass through the filtering fabric, the caught particles are deposited on fibres, then the layer of the caught particles is formed. Pressure difference at passage through a fabric increases. When the layer achieves the certain thickness, it is necessary to remove it from a filtering fabric in the bunker for gathering of dust. For this purpose various methods are used: stirring, a purge of a fabric by return current of air, acoustic vibration, influence of low pressure shock waves, etc (fig. 2.18).
Fig. 2.16 – Arrangement for a bag filter and dust collection
Fig. 2.17 – Conventional design of a bag filter
Fig. 2.18 – Fluid-dynamic cleaning systems for fabric filters:
a) reverse flow cleaning system;
b) reverse pulse-jet cleaning system
Fig. 2.19 – Conventional design of a pocket filter
According to investigations fabric filters are the most effective dust removing devices for certain applications. The dust residues in cupola gases, after passing through various types of dust scrubbers, have been reported as follows:
|The type of devices||Dust residue, g/m3|
|Cyclone Venturi Dry electrostatic precipitator Wet electrostatic precipitator Fabric filter||0,502 0,349 0,146 0,122 0,041|
These important results not only prove the remarkably high collection efficiency but also the applicability of the fabric filter under extreme conditions.
Electrostatic precipitators. The method of electrodeposition consists in the following. Small droplets and particles all over again receive a charge from gas ions, which are formed in an electric field of a high voltage, and then they are moved to earth precipitating electrode. Having got on an earth catcher, particles stick and are unloaded. When the precipitating electrode acquires a layer of particles, they are shaken and gather in the bunker.
Fig. 2.20 – Schematic representation of a wire and pipe precipitator
Electro-filters are applied where it is necessary to clear the great volumes of gas and there is not danger of explosion. These installations are used for catching of flying ashes on power stations, for dust catching in the cement industry, in iron and steel industry for smoke catching, in the chemical industry for catching of particles and drops of a fog, for dust catching in systems of air-conditioning, etc.
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