Conveyors pneumatic conveyor

Bulk alum can be stored in mild steel or concrete bins with dust collector vents located in, above, or adjacent to the equipment room. Recommended storage capacity is about 30 days. Dry alum in bulk form can be transferred or metered by means of screw conveyors, pneumatic conveyors, or bucket elevators made of mild steel. Pneumatic conveyor elbows should have a reinforced backing as the alum can contain abrasive impurities. 

CONVEYORS. Pneumatic conveyors for solids are described in Chap. 7. Other common devices for transportation include belt conveyors and bucket elevators, closed-belt conveyors with zipperlike fasteners, and various kinds of drag and flight conveyors. These all include a return leg that carries the empty belt or chain back from the discharge to the loading point. Vibrating conveyors and screw conveyors have no return leg but only operate over relatively short distances. Pneumatic conveyors also have no return leg and are not so limited as to distance of travel. Conveyors for solids are discussed in Ref. 9a. 

Sampling from pneumatic conveyors parallels gas sampling. The exception is that soflds loadings can be as high as 50 kg of soHds per kg of gas. Commercially available samplers extract particles directly from a transport line. Fixed position samplers are mounted directly on the pneumatic conveyor pipe. Devices are available which extract samples from the product stream by the projection of a sample tube iato the flow. Particles impact on the tube and fill the open cavity. The tube is then withdrawn, and an internal screw discharges the collected material (20). In another model, the RX Sampler (manufactured by Gustafson) (29), samples are withdrawn usiag compressed air. 

Particle size distribution determines surface-to-mass ratios and the distance internal moisture must travel to reach the surface. Large pieces thus have higher critical moisture contents than fine particles of the same material dried under the same conditions. Pneumatic-conveyor flash dryers work because very fine particles are produced during initial dispersion and these have low critical moisture contents. 

Fig. 12. Material residence time profiles in A, a pneumatic conveyor, B, a spray, and C, a rotary or fluid-bed dryer.

Fig. 16. Single-stage pneumatic conveyor dryer using a venturi for material acceleration and gas—soflds mixing.

The principal appHcations of pneumatic conveyors are for materials that are nonsticky and readily dispersible in the gas stream as drying must be entirely constant rate. Many are employed as predryers ahead of longer residence time fluid-bed and rotary dryers in polymer drying operations. 

Heat-transfer aspects and performance were studied and reported on by Depew and Farbar (ASME Pap. 62-HT-14, September 1962). Heat-transfer coefficient characteristics are similar to those shown in Sec. 11 for the indirectly heated fluid bed. Another frequent application on plastics is a sm, rather incidental but necessary amount of drying required for plastic pellets and powders on receipt when shipped in bulk to the users. Pneumatic conveyors modifiea for heat transfer can handle this readily. 

FIG. 12-33 Cocurrent gas-solids flow in a vertical-lift dilute-phase pneumatic conveyor. 

Pneumatic conveyors are frequently employed as both diy-product conveyors and coolers. Other types of coohng equipment often used are screw conveyors, vibrating conveyors, ana direct or indirec t rotating coolers. 

Pneumatic-Conveyor Dryers A pneumatic-conveyor dryer consists of a long tube or duct carrying a gas at high velocity, a fan to propel the gas, a suitable feeder for addition and dispersion of particulate solids in the gas stream, and a cyclone collector or other separation equipment for final recoveiy of sohds from the gas. 

In ordinaiy heating and coohng operations, during which there is no moisture pickup, continuous recirculation of the conveying gas is frequently employed. Also, solvent-recovery operations employing continuously recirculated inert gas with intercondensers and gas reheaters are carried out in pneumatic conveyors. 

FIG. 12-94 Two -stage air-stream and cage miU, pneumatic-conveyor dryer. (ABB Raymond. )... 

FIG. 12-95a Air-lift pneumatic-conveyor dryer includes partial recycle of dry product and expanding tube and cone sections to provide longer holdup for coarse particles. (Bepex Coip. )... 

Severaf typical products dried in pneumatic conveyors are described in Table 12-34. The air-stream type referred to is an ordi-naiy single-stage diyer hke the first stage of Fig. 12-94. 

TABLE 12-34 Typical Products Dried in Pneumatic-Conveyor Dryers ... 

Ring Dryers The ring diyer is a development of flash, or pneumatic-conveyor, diying technology, designed to increase the versatility of application of this technology and overcome many of its limitations. 

Special designs of spray dryers may provide for cooling air to enter around the chamber, closed systems for the recoveiy of solvents, and air sweepers or mechanical rakes to remove dry product from the walls and bottom of the chamber. Some are followed by pneumatic conveyors as depicted in Fig. 12-101, in which drying air is diluted with cool air for product cooling before separation. Spray dryers may... 

While successbil and economical system designs can be devised by experienced process engineers, the competent technical aid available from equipment supphers has led to a growing trend toward the purchase of complete systems, even on small jobs, rather than iu-plaut assembly from components on the basis of in-house designs. An idea of the change in capital investment for typical pneumatic-conveyor systems as a function of increasing transfer rates is given in Table 21-12. 

Types of Systems Generally, pneumatic conveyors are classified according to five basic types pressure, vacuum, combination pressure and vacuum, fluidizing, and the blow tank. 

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