8.2 Two stage unit
In two stage precipitator charging takes place in one section which is followed by a section consisting of alternately charged plates. The collecting electric field is established independently of the corona field and such precipitator are termed two stage. In this unit the ground plates are about an inch apart and have an intermediate plate that is also charged. But instead of 40,000-60,000 volt DC supply, the two stage precipitator has a 13,000-15,000 volt supply with the intermediate supply at 7,500 volts. This type of collector usually developed for heating and ventilating service and it provides very efficient dust collection and is designed with self cleaning washing system. The washing system is a light duty until designed for 250 cycles. As the usual cleaning is only required monthly, this unit exceeds the life of other components of heating and ventilating system. The high voltage electrodes consist of very fine wire stretched across springs and it is required for ionization. The plates have to maintain it at more precise distances. In this kind of service, the air distribution is usually very even, since the dust collecting filtering device operated at the same velocities as the heating and cooling coils.
9. Hydrocyclone
Hydrocyclones are used to carry out sedimentation of particles in a liquid medium under enhanced separating force. It is a device in which the necessary force is generated by rotating the feed slurry rapidly in the cylindrical conical section. The cylindrical conical section is provided at the top and bottom of the cyclone so that fine particles and coarse particles can be withdrawn continuously. Because of the operational similarities hydrocyclone is confused with a centrifuge. Hydrocyclones are simple, robust separating device which can separate both coarse and fine particles in a range of 4- 500 µm. While centrifuges are normally used for particle sizes of 100 µm or finer particles. Centrifuges require much higher centrifugal force than hydro cyclone. Centrifuges are more expensive and require higher operating cost. Schematic of hydro cyclone and its flow pattern is shown in (figure 5.1)
In a hydrocyclone, feed slurry enters at the top part of the cylindrical body, the entry can be tangential or involute type. As soon as the slurry starts to rotate in the cylindrical body, as a result coarse or heavy particles are thrown outwards towards the wall that move downward in a spiral path towards the underflow or an open orifice where they are ejected. During the process fine particles remain near the center of the cyclone, close to an air core which is always present, and eventually move in an upward spiral towards the overflow orifice. The orifice consists of a cylindrical section called as vortex finder, its lower end extends below the level of the feed port. The separation in a hydrocyclone is made on the basis of settling velocities, particle size, density and particle shape. If the slurry contains a mixture of solid particles of different densities, it is possible that large light particles may settle along with the small heavy particles. A specific gravity difference must exist between the solid and liquid phase in order that a separation can take place.
Size of cyclone and the estimation of the number of stages required for a particular application depend upon the stream flow rate. Selection of cyclone should be based on the recovery of a particle of specific size. The limit is decided based on the size for example, d50 particle diameter is the diameter of the particle, 50 % of which will appear in the overflow and the 50% in the under flow. It is referred as cut size and denoted as d50c . Several correlations are available for determination of d50c.
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