Drying equipment classification

Magnetic Separator Classification. Whereas a wide variety of magnetic equipment has evolved over the years, much of this equipment is specialized and has limited usage. A fundamental equipment classification can be made on the basis of feed condition. A wet condition involves the treatment of a slurry or slip a dry condition involves treatment where the particles are essentially free to move as independent particles. Both conditions depend on liberation of the magnetic particles. If liberation does not occur, the magnet traps the middling particles and reduces the concentration of... 

Drying equipment may be classified in several ways. The two most useful classifications are based on (1) the method of transferring heat to the wet solids or (2) the handling characteristics and physical properties of the wet material. The first method of classification reveals differences in dryer design and operation, while the second method is... 

A classification chart of drying equipment on the basis of heat transfer is shown in Fig. 12-45. This chart classifies dryers as direct or indirect, with subclasses of continuous or batchwise operation. 

Another useful classification is whether or not a dryer is a direct or indirect dryer. A direct contact dryer is one in which the material is dried by exposure to a hot gas, whereas in an indirect contact dryer, the heat required for evaporation is transferred from a heating medium through a metal wall to the material. Generally, direct heat dryers are more efficient. Dryer efficiency is defined by the fraction of energy supplied to the drying equipment which actually causes the evaporation of the liquid. As we shall see later in the chapter, heating is not always necessary to achieve drying. 

Drying equipment may be classified in several ways. Effective classification is vital in selection of the most appropriate dryer for the task and in understanding the key principles on which it operates. The main categories are as follows ... 

CLASSIFICATION OF DRYERS. There is no simple way of classifying drying equipment. Some dryers are continuous, and some operate batchwise some agitate the solids, and some are essentially unagitated. Operation under vacuum may be used to reduce the drying temperature. Some dryers can handle almost any kind of material, while others are severely limited in the type of feed they can accept. 

Mujumdar and Menon (1995) and Mujumdar (2000) provide detailed classification schemes for industrial dryers along with the numerous criteria that are important in making an appropriate selection. It is noted that one should select a drying system—including pre- and post-drying equipment—that can impact the choice of the dryer itself as well as its operating conditions. 

The fixed-carbon value is one of the values used in determining the efficiency of coal-burning equipment. It is a measure of the solid combustible material that remains after the volatile matter in coal has been removed. For this reason, it is also used as an indication of the yield of coke in a coking process. Fixed carbon plus ash essentially represents the yield of coke. Fixed-carbon values, corrected to a dry, mineral-matter-free basis, are used as parameters in the coal classification system (ASTM D-388). 

To use natural minerals it is necessary to grind them down to a desired particle size distribution. Grinding can be performed with the minerals dry or slurried in liquid. In most laboratories, this process is performed in a batch jar mill while on an industrial scale, continuous comminution equipment is used in conjunction with size classification equipment to recycle the coarse material. Figure 4.1 shows a typical comminution circuit with classification and recycle steps, as well as separation of the mineral from the conve3ring fluid. 

Dry classification equipment uses a gas stream to convey the solids. The gas used most oflen is air, and for that reason the term air classifiers is often used to describe this type of equipment (see Fig. 4.10). Air classifiers evolved from two sources, the original simple expansion chamber and the Mumford and Moodie separator, patented in 1885. 

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