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, u_p – the tangential velocity, and m_p – the mass of the particle, then the centrifugal force K_c is given by:

K_c = m_p · u_p²/ 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-calledmulticyclones (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.