Dilute-phase conveying system

Pneumatic conveying systems and in particular dilute phase conveying systems are known to create a high stress on particulate solids leading to significant attrition. In contrast to fluidized beds, it is not the material loss which is the main problem. Depending on the application, problems may rather occur in a number of different areas. Attrition may, for example,... 

Special care should be taken when selecting the mode of solids-gas flow. For example, flow separation and roping could occur even in very dilute-phase conveying systems (e.g., m < 1 for coal-fired boilers). Fluidized dense-phase also is possible for some systems and can offer many... 

Feeders for dilute-phase conveying systems (a) venture feeder and (b) star feeder. 

Figure 3.32 shows the four main types of dilute-phase conveying systems previously mentioned. The pressure system, also called positive-pressure, or push system operates at super-atmospheric pressure and is used for delivery to several outlets from one inlet (Figure 3.32a). Although most applications of these systems lie within the scope of dilute-phase conveying, under certain arrangements they can also operate as high-pressure, dense-phase...

Positive displacement blowers Positive displacement blowers are probably the most commonly used type of compressor for dilute phase conveying systems. They provide an ideal match, in terms of pressure capability, with the conventional low pressure rotary valve, and are a typical working combination on many plants. They can be used as vacuum pumps, or exhausters, as well as blowers. The principle of operation is illustrated in Figure 4.26. Twin rotors are mounted on parallel shafts within a casing, and they rotate in opposite directions. As the rotors turn, air is drawn into the spaces between the rotors and the casing wall, and is transported from the inlet to the outlet without compression. As the outlet port is reached, compression takes place when the air in the delivery pressure pipe flows back and meets the trapped air. 

Dilute Phase Conveying. Dilute conveying systems, sometimes called disperse conveying or stream conveying, operate as positive pressure systems at pressures up to 100 kPa (14.5 psig), or as negative pressure systems (vacuum conveying) at pressures up to —50 kPa (—500 mbar). 

A vacuum conv ing system was designed to dilute-phase convey crushed bath (median size... 

To escape aggregative fluidization and move to a circulating bed, the gas velocity is increased further. The fast-fluidization regime is reached where the soHds occupy only 5 to 20% of the bed volume. Gas velocities can easily be 100 times the terminal velocity of the bed particles. Increasing the gas velocity further results in a system so dilute that pneumatic conveying (qv), or dilute-phase transport, occurs. In this regime there is no actual bed in the column. 

M. V. Arastoopour, M. V. Modi, D. V. Punwani, and A. T. Talwalker,M Keview of Design Equations for Dilute Phase Gas-Solids Hori ntal Conveying Systems for Coal and Belated Material, Powder and Bulk SoHds Conference, Philadelphia, 1979. 

Salman, A. S., Verba, A., and Mills, D., Particle Degradation in Dilute Phase Pneumatic Conveying Systems, Proc. of the 1992 Powder Bulk Solids Conf. and Exhibition, Rosemont, USA (1992)... 

In addition Paul has prepared a chart listing the characteristics of the dense phase conveying in comparison to dilute and medium dense phase systems as shown in Fig. 17. 

Knowlton, T., The Effect of the System Pressure/Pipe-Diameter/Mass Flux Interaction on Pressure Drop in Dilute Phase Pneumatic Conveying, Proc. of Pneumatic Conveying Workshop, Powder Technol. Forum, AIChE, Denver (1994)... 

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