Inertia-type dedusters.
At sharp change of direction of a gas stream movement particles of dust under influence of inertial force will aspire to move in a former direction and further can be allocated from this stream. A lot of dedusters work on this principle. In these dedusters is provided the accelerated movement of particles aside their sedimentation due to change of a direction of a gas stream. Some devices are represented in figure 2.4.
Fig. 2.4 – Inertia-type dedusters: a – chamber with partition; б – chamber with fluent turning of gas stream; в – chamber with expanding cone; г – chamber with deep bunker.
The chamber with partition (2.4а) on efficiency of dust precipitation is similar to the usual sedimentation horizontal chamber, but it has the high hydraulic resistance. Smooth turn in the chamber (2.4б) allows to lower hydraulic resistance.
On fig. 2.4в is shown the chamber, in which the dusty gas stream goes in the beginning downwards on an expanding cone, and then turns on 1800 and it is deduced from above. As a result of it the dust particles are exposed to the additional effort directing them aside of the chamber bunker. The expanding cone allows to lower gradually speed of a gas stream and interferes with secondary dust ablation.
The jalousie-type deduster is represented on fig. 2.5. This device is widely applied to preliminary clearing of gases before cyclones or filters. In it about 90 % of gases partly are cleared of a dust at passage through jalousie, and other gas stream with the caught dust is allocated on clearing in a cyclone.
Fig. 2.5 – Jalousie-type dust separator with partial removal of dusty gas stream: 1 – jalousie lattice; 2 – purified gas (nearly 90 volume %); 3 – dusty gas (nearly 10 volume %).
At increasing of speed of gas submission to plates of a lattice the degree of dust catching in jalousie-type deduster in the beginning quickly grows; since speed about 10 m/s growth of speed is slowed. Usually speed of gases in a jalousie-type deduster makes 12-15 m/s. These devices are applied to catching of dust particles more 20 microns.
More effective clearing is achieved in the device on fig. 2.6. The gas stream is given in a wide part of the truncated cone. The surface of this cone has the special holes. At passage through these holes the gas stream changes direction and is released from dust particles, which together with a small part of gases are allocated in a cyclone. At clearing of plenty of gases several similar devices are established in parallel.
If the dusty gas stream collides with any body, the stream flows round it, and the dust particles, possessing the big inertia, hit about a surface of this body and settle on it. This principle is used in the device represented on fig. 2.7. In it some numbers of cores in the form of a half moon are established. Concave surfaces of cores are directed towards to a gas stream. The dust settled on cores is washed off by water.
Gas
Gas
Fig. 2.6 – Jalousie-type dust separator: 1 – cone with slots; 2 – fan; 3 – cyclone |
Gas
Fig. 2.7 – Inertia-type deduster with repulsing bars: 1 – case; 2 – bars; 3 – irrigation apparatus |
Centrifugal dedusters (cyclones)
At rotation of a gas stream there are centrifugal forces which are widely used in process of dust precipitation. Action of cyclones is based on this principle too.
A cyclone is an apparatus in which centrifugal forces are made available for the separation of dust particles from a gas stream. Three main problems arise in cyclone design.
1. How to make centrifugal forces of sufficient strength available? There is no other possibility but by rotational movement. If r is the radius of curvature of the particle trajectory, up – the tangential velocity, and mp – the mass of the particle, then the centrifugal force Kc is given by:
Kc = mp · up2/ r.
The centrifugal force increases with decreasing of radius r. This equation indicates that the separation efficiency of a small cyclone is better then that of a large cyclone.
2. How to generate a rotational motion of gas and particles? By tangential introduction of gas and particles into cylindrical element.
3. How to withdraw gas and dust particles after separation? By separate withdrawal at opposite ends of the cyclone, of the gas by pressure force and of the dust by gravity force.
Fig.2.8 schematically describes a cyclone of conventional design. It consists of a cylindrical entrance chamber, a conical dust separation or vortex chamber, an inlet channel, an outlet tube with extension into the cylindrical entrance chamber, a dust collection chamber, and a conical vortex spoiler which separates the separation chamber from the collection chamber, thereby preventing dust re-entrainement into the gas.
Fig. 2.8 – Shape and dimension of a cyclone and velocities
Some of the important properties of the cyclone are as follows.
1. In the cyclone direct dust-gas separation is possible without introducing any separation agent such a liquid drops. The cyclone is a dry dust separator that does not generate new pollution problems.
2. The cyclone is designed for a continuous process with continuous withdrawal of gas and dust.
3. The technical attractiveness is a least partially due to its simplicity of design. There is no technical sophistication.
4. The physical phenomena of gas-dust separation in a cyclone are relatively well understood. This is a sound basis for the mathematical description of the process. Pressure drop and separation efficiency can be fairly well determined in advance.
5. The particle cut diameter is in the range from 2 to 5µm, with a fair chance to reduce it still further.
6. If the cut diameter does not meet the requirements, then the cyclone may be applied as a pre-separator.
7. The cyclone may be used either as a single unit or with many units operated in parallel.
8. Low cost for investment and for operation.
Cyclones are differed on a way of a supply of gases in the device. Movement of gases in a cyclone has complex character.
The basic circuit of a cyclone action:
Gases are given in a cylindrical part of a cyclone, got movement on a spiral (speed increases from periphery to the center inside), gone down on an external spiral, then they are risen on an internal spiral and left the cyclone through an exhaust pipe. Usually in cyclones centrifugal acceleration is much more than acceleration of gravity. Therefore even small particles of a dust under influence of centrifugal force are taken away from a curve of movement of gases in the direction of a wall.
Fig. 2.9 – Fluid and particle movements in a cyclone
Advantages of cyclones:
1) cheapness;
2) simplicity of the device and service (there are no moving parts in the device);
3) small hydraulic resistance;
4) the reliable action at high pressures and temperatures (up to 5000);
5) a dust is caught in a dry kind;
6) an opportunity of catching of abrasive materials at protection of internal surfaces by special coverings;
7) growth of a dust content of gases does not lead to reduction of efficiency of clearing;
8) high productivity.
Sometimes in group (battery) unite a plenty of small cyclonic elements, so-called multicyclones (fig. 2.11). The purpose: at decreasing of a cyclone diameter efficiency of clearing grows. Multicyclones are more complex in manufacturing and more expensive.
Particularly important in the design of single and polycyclones is an effective separation of the vortex chamber from the dust collection chamber by a vortex spoiler as shown in fig. The dust separated from the gas should move freely from the vortex to the collection chamber. For the gas, however, the collection chamber should be sealed off from the vortex chamber as hermetically as possible. This ideal situation can never be accomplished. The design of a cyclone is a compromise. But it should always be possible to design a separation element that prevents the vortex from penetrating the dust collection chamber. If penetration occurred the dust would be redispersed and carried back into the vortex chamber, and from there into the outlet tube of the cyclone. The proper design of the connection between vortex and dust collection chamber is indicated in fig. 2.10.
Fig. 2.10 – Dimensions of cyclones
The danger of dust redispersion is particular importance for polycyclones when they are connected to a joint dust collection chamber. The larger the size of the dust collection chamber, the greater the danger of redispersion.
The number of cyclones in parallel arrangement depends on the size of the cyclones. For big cyclones with a height between 2 to 6 meters, the number of cyclones will be restricted to about 6 to 12. In this case, the distribution of the raw gas poses no serious problems. In the case of very small cyclones a great number of cyclones can be arranged in parallel. In this case the distribution of the raw gas may become a very serious problem.
Fig. 2.11 – Arrangement of polycyclones
Fig. 2.11 gives a view on a polycyclone which consists of 40 small cyclones with axial inlet. The long tubes that connect the cyclones with the belt, which carries away the dust, prevent dust redispersion and gas by-pass. And absolutely uniform distribution of the gas on all cyclones is impossible. This must be taken into account when the number of cyclones is determined.