Depending upon the collection efficiency, capacity and operation mode, the dust collection devices are divided in the following three categories: 1. Internal Separators 2. Wet Collection Devices 3. Electrostatic Precipitators.
1. Internal Separators:
Internal separator type dust collectors are manufactured in various shapes and sizes.
Following types are most common:
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(a) Louver collector,
(b) Fabric filters,
(c) Gravity settling chamber, and
(d) Cyclone.
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(a) Louver Collector:
When the direction of the gas stream is suddenly changed, because of greater inertia of dust particles, they cannot suddenly change their direction and are separated out. This principle is used in separating dust in Louver type dust collector. Fig. 6.1 shows the Louver collector.
This type of dust collector mainly consists of several blades set at angles to the flow path of the gas stream. The blades are set to force a quick sharp change in the direction for a large portion of the gas stream. At the point of sharp change of direction of gas stream, the dust particles are separated out and collected in the bed of the collector’s bottom.
If the blades are set at close spacing at sharp angles to the direction of gas flow, they will ensure optimum dust collection conditions.
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(b) Fabric Filters:
The method of dust collection through fabric filters is widely used in several collecting mechanisms such as inertial inspection, interception, diffusion, sedimentation and electrostatic separation. When the dusty gas passes over the fabric surface, the stream lines of gas diverges, whereas dust particles up to 0.01 size due to inertia are attached to the fabric.
Assuming that the particles are not diverging but follow the gas stream, they are intercepted to the fabric, when the centre of the particles at a distance of half of its diameter. Now when the particles of size 0.01 to 0.05 are moving in irregular way this Brownian motion causes them to diffuse to the fabric.
Sedimentation occurs when the gas is flowing at low velocities and it contains considerably larger particles (more than 1.097). Dust (particle size 0.05-5.0) collection due to electrostatic charges is also noticed, when the dirty gas passes over chemical fibre fabric.
The air to cloth ratio is one of the important parameters in the design of fibre filters. In the low ratio bug filter, the air to cloth ratio is between 7-25 mm/sec. Fig. 6.2 shows the bag filter. In this filter, the dust laden gas enters through the bottom of the hopper, where the heaviest particles are fallen out due to gravitational force.
Particles are deposited on the inner side of the fabric filter, when the gas is passed through them. For cleaning the bags, the compressed air is blown in opposite direction. At one plant rows of bags are used for dust collection.
When one row is in operation for collection of the dust, the other row of bags is in cleaning operations. The cleaning of the bags can also be done by reverted or by shakers and pulse. Normally the cloth to air ratio is kept between 7.5 to 12.5 mm/sec, but in high ratio bags it may be 20-50 mm/sec.
The filter medias of fabric filters may be divided into two groups depending upon the dust layer built up. These fabrics and felts are chemically and mechanically bonded.
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Table 6.1 gives the different types of filter fabrics and their temperature limitations.
(c) Gravity Settling Chambers:
Fig. 6.3 shows a gravity settling chamber. It mainly consists of an enclosed chamber, wherein the velocity of the dust laden gas is considerably reduced, which allow the dust particles to settle down by gravitational force.
In these chambers the horizontal gas velocity should be kept as low as possible, to allow stream line flow, for insuring optimum settling conditions. But practically due to the design of the chamber on the economically and physically prohibitive, the actual gas velocities are above the stream line conditions. The gas velocities are kept between 0.3 to 0.3 m/sec in the chambers, which allow only coarse particles of size 40 and above to be removed by these chambers.
(d) Cyclone:
Fig. 6.4 shows a cyclone for removing dust particles from the air. The cyclone works on the principle of separating the particles from the gas by transforming the inlet gas velocity into a double vortex. The entering gas spirals down at the inner surface and then spirals upward at the central portion of the cyclone.
Due to their inertia, the dust particles tend to concentrate on the surface of the cyclone wall, from where they are led to the receiver. These are cheaper in cost and are best suited to dry dust particles of size 10-40. The cyclones can handle a wide range of physical and chemical conditions of operations as compared with other collection equipment’s.
The efficiency of the cyclone increases with the increase in the following:
(i) Inlet velocity of the dust laden gas,
(ii) Diameter of the dust particle,
(iii) Density of the dust particle,
(iv) Dust concentration in the carrier gas, and
(v) Smoothness of the inner cyclone wall.
But it has been noted that increase in inlet velocity decreases the collection efficiency where agglomeration is significant.
2. Wet Collection Devices:
These devices remove the dust particles from gas by wetting the particles with a liquid droplet diffusion or condensation or by impinging the wetted or un-wetted particles on a collecting surface and cleaning them by a flush of liquid. In the diffusion mechanism Brownian motion becomes significant.
The dust particles by zig-zag motion in the neighbourhood of the collecting droplets diffuse to them and are intercepted. It has been noted that Brownian motion is also taking place in forming agglomerates and thereby simplifying the separation. There may be an interaction between a charges particle and droplet on which the droplet charge induces a given charge.
Following are the common wet collection devices:
(a) Cyclonic scrubbers.
(b) Spray chambers,
(c) Venturi scrubbers, and
(d) Packed towers.
3. Electrostatic Precipitators:
Frederic Gardner Cottrell invented the electrostatic precipitator in 1911. This method can be applied to great varieties of gas cleaning problems with a collection efficiency of 99.99% and capacities upto 150,000 litres/min at temperatures upto 600°C. There is very small pressure drop in this method only upto 6-10 mm of water.
Fig. 6.8 shows the electrostatic precipitation (ESP). The dirty gas is allowed to pass horizontally through narrow, vertical gas passages formed by parallel rows of grounded collecting electrodes. Electrically insulated high voltage wire of about 40-50 kV are spaced precisely on the centre lines of each gas passage thereby causing the dirty gas to pass through high voltage wires and grounded plates.
Operational principles of electrostatic precipitators are as follows:
(a) Ionization of Gas:
The high potential operation of this method at 40,000 V to 50,000 V causes production of billion of electrons to bombard with the gas molecules which in turn becomes as positive and negative ions. The visibility of blue corona represents the formation of gas ions.
(b) Dust Charging:
The positively charged ions returned back to the negative wire electrode and gain their electrons, whereas the negatively charged ions collide with the dust particles in the entrancing gas the thereby making the dust particles negatively charged.
(c) Precipitation of Dust:
The negatively charged dust particles are derived by powerful electric forces towards the grounded positively charged plate and these are held to them. In this way, the dust particles are collected on the collecting electrode and go on forming a thick layer.
This layer gradually bleeds their negative charge to the grounded electrode. This increasing dust layer thickness gives resistance to conduction of the negatively charged ion. which is known as ‘dust resistivity’. It has been noted that for the normal operation of an electrostatic Precipitator, this dust resistivity should be in between 107-1011 Q/cm.
(d) Collectrode Raping:
When the thickness of the dust layer increases more than 6 mm, the electrical attraction becomes weak. The recently deposited particles still hold the charge, because the collectrode was insulated by the dust layer. Under the above circumstances and because of the negatively charged particle on the collectrode, flash over between the wire electrode and collectrode occurs which decreases the efficiency of the Electrostatic Precipitator.
At this stage a sharp rap causes the dust layer to shear away from the collectrode. This rapping results in the dust to form agglomerates and is collected in the hoppers. The high potential field recharges any fine particles which are re-entrained during raping.
For obtaining maximum efficiencies, most of the electrostatic precipitators should be operated at gas velocities of 1 to 2 m/sec. and at 100°C to 150°C temperature. Efficiency of electrostatic precipitator can be expressed by the formula
n = 1 – (e)A W/V
Where n = efficiency of ESP
A = area of collecting plate in nr
V = flow rate of gas in Nm3/sec.
W = rate of precipitation in m/sec.
The precipitation rate parameter varies from 0.06 to 0.2 sec/m and it depends upon the particle size and characteristics of gas stream.
Electrostatic precipitators have many advantages such as less moving parts, can withstand temperatures up to 750°C, high gas holding capacity, high collection efficiency of very small particles and small pressure drops etc.
The disadvantages may include high initial cost, special trained persons to work at high voltage handling, pre-requirement of pre-cleaner like cyclones, limitation of application to liquid and solid phase pollutants.